Coal-mining

COAL-MINING.

The opening and laying out, or, as it is generally called, “winning,” of new collieries is rarely undertaken without a preliminary examination of the character of the strata by means of borings, either for the purpose of determining the number and nature of the coal-seams in new ground, or the position of the particular seam or seams which it is proposed to work in extensions of known coal-fields.

The principle of proving a mineral field by boring is illustrated by figure 3, which represents a line direct from the dip to the rise of the field, the inclination of the strata being one in eight. No. 1 bore is commenced at the dip, and reaches a seam of coal A, at 40 fathoms; at this depth it is considered proper to remove nearer to the outcrop, so that lower strata may be bored into at a less depth, and a second bore is commenced. To find the position of No. 2, so as to form a continuous section, it is necessary to reckon the inclination of the strata, which is 1 in 8: and as bore No. 1 was 40 fathoms in depth, we multiply the depth by the rate of inclination, 40 × 8 = 320 fathoms, which gives the point at which the coal seam A should reach the surface. But there is generally a certain depth of alluvial cover which requires to be deducted, and which we call 3 fathoms, then (40 - 3 = 37) × 8 = 296 fathoms ; or say 286 fathoms is the distance that the second bore should be placed to the rise of the first, so as to have for certain the seam of coal A in clear connection with the seam of coal B. In bore No. 3, where the seam B, according to the same system of arrangement, should have been found at or near the surface, another seam C is proved at a considerable depth, differing in character and thickness from either of the preceding. This derangement being carefully noted, another bore to the outcrop on the same principle is put down for the purpose of proving the seam C; the nature of the strata at first is found to agree with the latter part of that bored through in No. 3, but immediately on crossing the dislocation seen in the figure it is changed, and the deeper seam D is found.

The evidence therefore of these bores (3 and 4) indicates some material derangement, which is then proved by other bores, either towards the dip or the outcrop, according to the judgment of the borer, so as to ascertain the best position for sinking pits.

The methods of boring are similar to those adopted for deep wells, or in other departments of mining. For shallow bores, the boring is generally with wrought iron rods screwed together in lengths, armed with a cutting chisel, and working by percussion, the tool being lifted by hand and allowed to fall with its full weight upon the rock. The pounded material is removed at intervals, by substituting a shell pump or tube with valves at the bottom, whose action is similar to that of the foot valves of an ordinary lifting pump. The sludge brought to the surface indicates the nature of the ground passed through. In very deep borings, however, the use of rigid rods and fixed tools is found to present two serious evils, namely, the excessive weight on the tool caused by the increased length of the rods, and the great length of time required to withdraw the tool and remove the detritus. The first of these difficulties has been overcome by the use of the free falling cutters, where the tool, instead of being attached rigidly to the rod, moves in a guide-block in such a manner as to be lifted with the rods, falling freely when the top of the stroke is reached. The rods, when lowered, pick up the tool at the bottom of the hole in readiness for the next lift. By this means the momentum of the tool is kept constant whatever may be the weight of rods employed.

The use of a wire rope winding on a drum, instead of rods for suspending the boring tool, allows the latter to be withdrawn and replaced with much greater rapidity than can be done with rods. This method has been very successfully adopted by Messrs Mather & Platt of Salford. But perhaps the best methods of expeditious boring are those (Fauvelle’s) whereby the detritus is removed as it forms by continuously flushing out the hole with water, hollow rods being used down which the water flows while it rises through the annular space between the rod and the lining tube of the bore hole. This has the advantage of giving a clear surface for the tool to cut on, instead of its having to work through its own sludge, as is the case when the shell pump is only used at intervals. Of late years the value of boring for exploratory purposes has been much increased by the adoption of tubular or crown borers, which cut out an annular groove, leaving a core of unbroken rock in the centre, which is then brought out by a grapnel in a solid piece. One of. the most successful of these methods is that due to Leschot of Geneva, where a rotating cutter, armed with amorphous black diamond, the hardest known substance, is used, the detritus being continuously removed by water on Fauvelle’s plan. The machinery adopted for this purpose, as modified by Messrs Beaumont & Appleby, has been employed with great success to bore holes exceeding 2000 feet in depth.

The working of coal may be conducted either by means of levels or galleries driven from the outcrop in a valley, or by shafts or pits sunk from the surface. In the early days of coal mining, open working, or quarrying from the outcrop of the seams, was practised to a considerable extent; but there are now few if any places in England where this can be done. In 1873 there could be seen, in the thick coal seams of Bengal, near Raniganj, a seam about 50 feet thick laid bare, over an area of several acres, by stripping off a superficial covering varying from 10 to 30 feet, in order to remove the whole of the coal without loss by pillars. Such a case, however, is quite exceptional. The operations by which the coal is reached and laid out for removal are known as “winning,” the actual working or extraction of the coal being termed “getting.” In the accompanying figure, No. 4, A B is a cross cut-level, by which the seams of coal 1 and 2 are won, and C D a vertical shaft by which the seams 1, 2, and 3 are won. When the field is won by the former method, the coal lying above the level is said to be “level-free.” The mode of winning by level is of less general application than that by shafts° as the capacity for production is less, owing to the smaller size of roadways by which the coal must be brought to the surface, levels of large section being expensive and difficult to keep open when the mine has been for some time at work. Shafts, on the other hand, may be made of almost any capacity, owing to the high speed in drawing which is attainable with proper mechanism, and allow of the use of more perfect arrangements at the surface than can usually bo adopted at the mouth of a level on a hill side. A more cogent reason, however, is to be found in the fact that the principal coal-fields are in flat countries, and where the coal can only be reached by vertical sinking.

The methods adopted in driving levels for collieries are generally similar to those adopted in other mines. The ground is secured by timbering, or more usually by arching in masonry or brick-work. Levels like that in fig. 4, which are driven across the stratification, or generally anywhere not in coal, are known as “stone drifts.” The sinking of colliery shafts, however, differs considerably from that of other mines, owing to their generally large size, and the difficulties that are often encountered from water during the sinking. The actual coal measure strata, consisting mainly of shales and clays, are generally imperγious to water, but when strata of a permeable character are sunk through, such as the magnesian limestone of the north of England, the Permian sandstones of the central countries, or the chalk and greensand in the north of France and Westphalia, special methods are required in order to pass the waterbearing beds, and to protect the shaft and workings from the influx of water subsequently. Of these methods one of the chief is the plan of tubbing, or lining the excavation with an impermeable casing of wood or iron, generally the latter, which is built up in segments forming rings, that are piled upon each other throughout the whole depth of the water-bearing strata. This method necessitates the use of very considerable pumping power during the sinking, as the water has to be kept down in order to allow the sinkers to reach a water-tight stratum upon which the foundation of the tubbing can be placed. This consists in a heavy cast-iron ring, known as a wedging crib, or curb, also fitted together in segments, which is lodged in a square-edged groove cut for its reception, tightly caulked with moss, and wedged into position. Upon this the tubbing is built up in segments, usually from 10 to 12 being required for the entire cir cumference, the edges being made perfectly true. The thickness varies according to the pressure expected, but may be taken at from ¾ to 1∣ inches. The inner face is smooth, but the back is strengthened with angle brackets at the corners. A small hole is left in the centre of each segment, which is kept open during the fitting to prevent undue pressure upon any one, but is stopped as soon as the circle is completed. In the north of France and Belgium wooden tubbings, built of polygonal rings, were at one time in general use. The polygons adopted were of 20 or more sides approximating to a circular form.

The second principal method of sinking through waterbearing ground is that which was first adopted by Μ.

Triger, in France, and has also been used by civil engineers in putting down deep foundations for bridge piers, namely, by compressed air. The shaft is lined with a cylinder of "wrought iron, within which a tubular chamber, provided with doors above and below, known as an air-lock, is fitted by a telescopic joint, which is tightly packed so as to close the top of the shaft air-tight. Air is then forced into the inclosed space by means of a compressing engine, until the pressure is sufficient to oppose the flow of water into the excavation, and to drive out any that may collect i n the bottom of the shaft through a pipe which is carried through the air-sluice to the surface. The miners work in the bottom in the same manner as divers in an - ordinary diving-bell. Access to the surface is obtained through the double doors of the air sluice, the pressure being reduced to that of the external atmosphere when it is desired to open the upper door, and increased to that of the working space below when it is intended to communicate with the sinkers, or to raise the stuff broken in the bottom. This method has been adopted in various sinkings on the Continent. At Bracquenie, near Mons, the miners worked in an atmosphere up to 45 ft pressure on the square inch, without experiencing any great difficulty, but they were found to be more susceptible to pulmonary disorder upon changes of weather than those who worked under the ordinary conditions of pressure.

The third method of sinking through water-bearing strata is that of boring, adopted by Messrs Kind & Chaudron in Belgium and Germany. For this purpose a horizontal bar armed with vertical cutting chisels is used, which cuts out the whole section of the shaft simultaneously. In the first instance, a smaller cutting frame is used, boring a hole from 3 to 5 feet in diameter, which is kept some 50 or 60 feet in advance, so as to receive the detritus, which is removed by a shell pump of large size. The large trepan or cutter weighs about 16 tons, and cuts a hole of from 9 to 15 feet in diameter. The water-tight lining may be either a wrought iron tube, which is. pressed down by jack screws as the bore hole advances, or cast-iron tubbing put together in short complete rings, in contradistinction to the old plan of building them up of segments. The tubbing, which is considerably let« in diameter than the bore hole, is suspended by rods from the surface until a bed suitable for a foundation is reached, upon which a sliding length of tube, known as the moss box, bearing a shoulder, which is filled with dried moss, is placed. The whole weight of the tubbing is made to bear on the moss, which squeezes outwards, forming a completely water-tight joint. The interval between the back of the tubbing and the sides of the bore hole is then filled up with concrete, which on setting fixes the tubbing firmly in position.

The introduction of these special methods has considerably simplified the problem of sinking through water-bearing strata. Some of the earlier sinkings of this kind, when pumps had to be depended on for keeping down the water, were conducted at great cost, as, for instance, at South Hetton, and more recently Ryhope, near Sunderland, through the magnesian limestone of Durham.

The size and form of colliery shafts varies in different districts, but the tendency is now generally to make them round, and from 12 to 15 feet in diameter. In the Midland counties, from 7 to 9 feet is a very common size, but larger dimensions are adopted where a large production is required. At Bagillt, on the Dee, a shaft of 22 feet in diameter was commenced a few years ago, but was reduced in diameter a short distance down. Since the accident at Hartley colliery, caused by the breaking of the pumping engine beam, which fell into the shaft and blocked it up, whereby the whole company of men in the mine were starved to death—it has been made compulsory upon mine owners to have two pits for each working, in place of the single one divided by walls or brattices which was formerly thought sufficient. The use of two independent connections—whether separate pits or sections of the same pit, between the surface and the workings—is necessary for the service of the ventilation,—fresh air from the surface being carried down one, known as the “downcast,” while the foul or return air of the mine rises through the other or “upcast” pit back to the surface. Where the mine is heavily watered, it is often necessary to establish a special engine pit, with pumps permanently fixed, or a division of one of the pits may be devoted to this purpose. The use of direct-acting high-pressure pumping engines placed at the bottom of the shaft has become common during the last ten years. They have the advantage of doing away with the heavy reciprocating rod from the engine at the surface, and may be worked either by steam pipes carried down the pit, or, what is now more common, by boilers underground, which supply also steam for the underground hauling engines. Where the water does not accumulate very rapidly'it is a very common practice to allow it to" collect in a pit or sump below the working bottom of the shaft, and to draw it off in a water tub or bucket by the main engine, when the latter is not employed in raising coal.

The laying out of a colliery, after the coal has been won, by sinkings or levels, may be accomplished in various ways, according to the nature of the coal, its thickness and dip, and the extent of ground to be worked. In the South Staffordshire and other Midland coal-fields, where only shallow pits are required, and the coals are thick, a pair of pits may be sunk for a very few acres, while in the North of England, on the other hand, where sinking is expensive, an area of some thousands of acres may be commanded from the same number of pits. In the latter case, which represents the most approved practice, the sinking is usually placed about the centre of the ground, so that the workings may radiate in every direction from the pit bottom, with the view of employing the greatest number of hands to advantage. Where a large area cannot be commanded, it is best to sink to the lowest point of the field for the convenience of drawing the coal and water which become level-free in regard to the pit. Where properties are much divided, it is always necessary to maintain a thick barrier of unwrought coal between the boundary of the mine and the neighbouring workings, especially if the latter are to the dip. If a prominent line of fault crosses the area, it may usually be a convenient division of the field into sections or districts. The first process in laying out the workings consists in driving a gallery on the level along the course of the coal seam, which is known as a “dip head level,” and a lower parallel one, in which the water collects, known as a “lodgment level.” Galleries driven at right angles to these are known as “dip” or “rise headings,” according to their position above or below the pit bottom. In Staffordshire the main levels are also known as “gate roads.” To secure the perpendicularity of the shaft, it is necessary to leave a large mass or pillar of the seam untouched around the pit bottom. This pillar is known in Scotland as the “pit bottom stoop.” The junction of the levels with the pit is known as the “pit eye;” it is usually of an enlarged section, and lined with masonry or brickwork, so as to afford room for handling the waggons or trams of coal brought from the working faces. In this portion of the pit are generally placed the furnaces for ventilation, and the boilers required for working steam-engines underground, as well as the stables and lamp cabin.

Figs. 5 and 6 represent the pit bottom arrangements at Cambois colliery in Northumberland, which are of an extremely commodious character. There are four large Cornish boilers, supplying steam to the engines drawing coals from the workings, as well as to a direct-acting pumping engine, the flame and smoke being discharged by drifts into the upcast pit. For the purpose of handling large pieces of machinery and boilers, the level at the bottom is increased to a chamber 18 feet high, and roofed with rolled iron girders of a double T section. To protect the fillers working at the bottom, strong diagonal guard timbers are placed at S in order to deflect any materials falling down the shaft, and prevent them falling into the workings. This is an unusually large example, but is taken from a pit in the highest state of development, and making a very large daily outturn.

The removal of the coal after the roads have been driven may be effected in many different ways, according to the custom of the district, These may, however all be considered as modifications of two systems viz., pillar work and long-wall work, In the former, which is also known as “port and stall” or “bord and pillar” in the north of England, “pillar and stall” in South Wales, and “stoop and room” in Scotland, the field is divided into strips by numerous openings driven parallel to the main rise headings, called “bords” or “bord gates,” which are again divided by cutting through them at intervals, so as to leave a series of pillars arranged chequer-wise over the entire area. These pillars are left for the support of the roof as the workings advance, so as to keep the mine open and free from waste. Fig. 1, Plate III. represents the oldest form of this class of working as practised in Scotland, from which it will be seen that if the size of the pillar is equal to the width of the stall or excavation, about ¾ of the whole seam will be removed, the remainder being left in the pillars. A portion of this may be got by the process known as robbing the pillars, but the coal so obtained is liable to be very much crushed from the pressure of the superincumbent strata. This crushing may take place either from above or below, producing what are known as “creeps” or “sits.”

A coal seam with a soft pavement and a hard roof is the most subject to a “creep.” The first indication is a dull hollow sound heard when treading on the pavement or floor, probably occasioned by some of the individual layers parting from each other as shown at a fig. 7; the succeeding stages of creep are shown at δ, c, d, f, and g, in the same figure; the last being the final stage, when the coal begins to sustain the pressure from the overlying strata, in common with the disturbed pavement.

“Sits” are the reverse of creeps; in the one case the pavement is forced up, and in the other the roof is forced or falls down, for want of proper support or tenacity in itself. This accident generally arises from an improper size of pillars ; some roofs, however, are so difficult to support that sits take place where the half of the coal is left in pillars.

Fig. 8 will convey a general idea of the appearance of sits,— k, m, n showing different stages.

The modern method of pillar working is shown in Plate IV. In the Northumberland steam-coal district, where it is carried out in the most perfect manner, the boards are 5 to 6 yards in width, while the pillars are 22 yards broad and 30 yards long, which are subsequently got out on coming back. In the same figure is also shown the method of working whole coal and pillars at the same time, a barrier of two or three ranges of pillars or a rib of solid coal being left between the working in the solid and those in the pillars. The space from which the entire quantity of coal has been removed is known in different districts as the “goaf,” “gob,” or “waste.”

Fig. 9 represents the Lancashire system of pillarworking. The area is laid out by two pairs of level drifts, parallel to each other, about 150 yards apart, which are carried to the boundary. About 100 yards back from the boundary a communication is made between these levels, from which other levels are driven forward, dividing the coal into ribs of about 25 or 30 yards wide, which are then cut back by taking off the coal in slices from the level towards the rise in breadths of about six yards. By this method the whole of the coal is got backwards, the main roads being kept in solid coal; the intermediate levels not being driven till they are wanted, a greater amount cf support is given, and the pillars are less crushed than is usual in pillar working.

In the South Wales system of working, cross headings are driven from the main roads obliquely across the rise to get a sufficiently easy gradient for horse roads, and from these the stalls are opened out with a narrow entrance, in order to leave support on either side of the road, but afterwards widening to as great a breadth as the seam will allow, leaving pillars of a minimum thickness. The character of such workings is very irregular in plan, and as the ventilation is attended with considerable difficulty, it is now becoming generally superseded by more improved methods.

The second great principle of working is that known as long-wall or long-work, in which the coal is taken away either in broad faces from roads about 40 or 50 yards apart and parallel to each other, or along curved faces between roads radiating from the pit bottom—the essential feature in both cases being the removal of the whole of the coal at once, without first sub-dividing it into pillars, to be taken away at a second working. The roof is temporarily supported by wooden props or pack walling of stone, for a sufficient breadth along the face to protect the workmen, and allow them to work together behind. The general character of a long-wall working is shown in fig. 10, which represents an area of about 500 acres of the bottom hard steam coal at Shipley in Derbyshire. The principal road extends from the shafts southward; and on both sides of it the coal has been removed from the light-shaded area by cutting it back perpendicularly towards the boundaries, along faces about 50 yards in length, those nearest to the shaft being kept in advance of those farther away, producing a step-shaped outline to the face of the whole coal. It will be seen that by this method the whole of the seam, with the exception of the pillars left to protect the main roadways, is removed. The roads for drawing the coal from the working faces to the shaft are kept open by walling through the waste or goaf produced by the fall of the unsupported roof. The straight roads are the air-ways for carrying pure air from the down-cast shaft to the working faces, while the return air passes along the faces and back to the up-cast by the curved road. The above is the method of working long-wall forward, t.e., taking the coal in advance from the pit towards the boundary, with roads kept open through the gob. Another method consists in driving towards the boundary, and taking the coal backward towards the shafts, or working homeward, allowing the waste to close up without roads having to be kept open through it. This is of course preferable, but is only applicable where the owner of the mine can afford to expend the capital required to reach the limit of the field in excess of that necessary when the raising of coal proceeds pari passu with the extension of the main roads. Fig. 9 is substantially a modification of this kind of long-wall work. Plate III. fig 2, represents a method of working practised in the South Yorkshire district, known as bords and banks. The field is divided by levels and headings into rectangular banks, while from the main levels bords or wickets about 30“ yards wide, separated from each other by banks of about the same width, are carried forward in longwall work, as shown on the left side of the figure, the waste being carefully packed behind so as to secure the ventilation. When these have been worked up to the extremity, as shown on the right side, the intermediate bank is removed by working backward towards the level. This system, therefore, combines both methods of long-wall working, but is not generally applicable, owing to the difficulty of ventilation, due to the great length of air-way that has to be kept open around the waste on each bank.

The relative advantages of the different methods may be generally stated as follows. Long-wall work is best suited for thin coals, and those having a good roof, iae., one that gives way gradually and fills up the excavation made by removing the coal without scaling off suddenly and falling into the working faces, when practically the whole of the coal may be removed. Against these advantages must be placed the difficulties attending the maintenance of roads through the goaves, and in some cases the large proportion of slack to round or large coal obtained. Pillar working, in the whole coal, is generally reputed to give a more advantageous proportion of round coal to slack, the latter being more abundantly produced on the removal of the pillars, but as these form only a small portion of the whole seam, the general yield is more advantageous than in the former method. The ventilation of pillar working is often attended with difficulty, and the coal is longer exposed to the influence of the air, a point of importance in some coals, which deteriorate in quality when exposed to a hot damp atmosphere. The great increase in the size of the pillars in the best modern collieries worked upon this principle has, however, done much to approximate the two systems to an equality in other respects.

The working of very thick seams presents certain special peculiarities, owing to the difficulties of supporting the roof in the excavated portions, and supplying fresh air to the workings. The most typical example of this kind of working in England is afforded by the thick coal of South Staffordshire, which consists of a series of closely associated coal seams, varying from 8 to 12 or 13, divided from each other by their partings, but making together one great bed of from 25 to 40 feet or more in thickness. The partings together do not amount to more than 2 or 3 feet. The method of working which has been long in use is represented in fig. 11. The main level or gate road is driven in the benches coal, or lower part of the seam, while a smaller drift for ventilation, called an air heading, is carried above it in one of the upper beds called the slipper coal. From the gate road a heading called a bolt-hole is opened, and extended into a large rectangular chamber, known as a “side of work,” large pillars being left at regular intervals, besides smaller ones or cogs. The order in which the coal is cut is shown in the dotted and numbered squares in the figure. The coal is first cut to the top of the slipper coal from below, after which the upper portion is either broken down by wedging or falls of itself. The working of these upper portions is exceedingly dangerous, owing to the great height of the excavations, and fatal accidents from falls of roof are in consequence more common in South Staffordshire than in any other coal-field in this country. The air from the down-cast shaft enters from the gate road, and passes to the up-cast through the air heading above. About one-half of the total coal (or less) is obtained in the first working; the roof is then allowed to fall, and when the gob is sufficiently consolidated, fresh roads are driven through it to obtain the ribs and pillars left behind by a second or even, in some cases,.a third working. The loss of coal by this method is very considerable, besides great risk to life and danger from fire. It has, therefore, been to some extent superseded by the long-wall method, the upper half being taken at the first working, and removed as completely as possible, working backwards from the boundaries to the shaft. The lower half is then taken in the same manner, after the fallen roof has become sufficiently consolidated to allow the mine to be re-opened.

In the working of thick seams inclined at a high angle, such as those in the south of France, and in the lignite mines of Styria and Bohemia, the method of working in horizontal slices, about 12 or 15 feet thick, and filling up the excavation with broken rock and earth from the surface, is now generally adopted in preference to the systems formerly used. At Monceaux les Mines, in France, a seam 40 feet thick, and dipping at an angle of 20 degrees, is worked in the following manner. A level is driven in a sandstone forming the floor, along the course of the coal, into which communications are made by cross cuts at intervals of 16 yards, which are driven across to the roof, dividing up the area to be worked into panels· These are worked backwards, the coal being taken to a height of 20 feet, the opening being packed up with stone sent down from the surface. As each stage is worked out, the floor level is connected with that next below it by means of an incline, which facilitates the introduction of the packing material. Stuff containing a considerable amount of clay is found to be the best suited for the purpose of filling, as it consolidates readily under pressure.

The actual cutting of the coal is chiefly performed by manual labour, the tool employed being a sharp-pointed double-armed pick, which is nearly straight, except when required for use in hard rock, when the arms are made with an inclination or “anchored.” The terms pike, pick, mandril, and slitter are applied to the collier’s pick in different districts, the men being known as pikemen or fiewers. In driving levels it is necessary to cut grooves vertically parallel to the walls, a process known as shearing; but the most important operation is that known as holing or kirving, which consists in cutting a notch or groove in the floor of the seam to a depth of about 3 feet, measured back from the face, so as to leave the overhanging part unsupported, which then either falls of its own accord within a few hours, or is brought down either by driving wedges along the top, or by blasting with gunpowder. The process of holing in coal is one of the severest kinds of human labour. It has to be performed in a constrained position, and the miner lying on his side has to cut to a much greater height, in order to get room to carry the groove in to a sufficient depth, than is required to bring the coal down, giving rise to a great waste in slack as compared with machine work. This is sometimes obviated by holing in the beds below the coal, or in any portion of a seam of inferior quality that may not be worth working. This loss is proportionately greater in thin than in thick seams, the same quantity being cut to waste in either case. The method of cutting coal on the long-wall system is seen in fig. 12, representing the working at the Shipley colliery. The coal is 40 inches thick, with a seam of fire-clay and a roof of black shale , about 6 inches of the upper part, known as the roof coal, not being worth working, is left behind. A groove of triangular section of 30 inches base and 9 inches high is cut along the face, inclined timber props being placed at intervals to support the overhanging portion until the required length is cut. These are then removed, and the coal is allowed to fall, wedges or blasting being employed when necessary. The roof of the excavation is supported as the coal is removed, by packing up the waste material, and by a double row of props, two feet from each other, placed tern-porarily along the face. These are placed 5 feet apart, the props of the back row alternating with those in front. The p^rops used are preferably of small oak or English larch, but large quantities of fir props, cut to the right length, are also imported from the north of Europe. As the work proceeds onwards, the props are withdrawn and replaced in advance, except those that may be crushed by the pressure or buried by sudden falls of the roof.

In Yorkshire hollow square pillars, formed by piling up short blocks of wood or chocks, are often used instead of props formed of a single stem. Iron pit props have been proposed at different times, but their use has not become general. When the coal has been under-cut for a sufficient length, the struts are withdrawn, and the overhanging mass is allowed to fall during the time that the workmen are out of the pit, or it may be brought down by driving wedges, or if it be of a compact character a blast of gunpowder in a bore hole near the roof may be required. Sometimes, but rarely, it happens that it is necessary to cut vertical grooves in the face to determine the limit of the fall, such limits being usually dependent upon the cleet or divisional planes in the coal, especially when the work is carried perpendicular to them or on the end.

The substitution of machinery for hand labour in cutting coal has long been a favourite problem with inventors, the earliest plan being that of Menzies, in 1761, who proposed to work a heavy pick underground by power transmitted from an engine at the surface, through the agencies of spear-rods and chains passing over pulleys; but none of the methods suggested proved to be practically successful until the general introduction of compressed air into mines furnished a convenient motive power, susceptible of being carried to considerable distances without any great loss of pressure. This agent has of late years been applied in various ways, in machines which either imitate the action of the collier by cutting with a pick or make a groove by rotating cutters attached to an endless chain or a revolving disc or wheel. The most successful of the first class, or pick machines, is that of Mr William Firth of Sheffield, represented in fig. 13. It consists essen tially of a horizontal piston and cylinder engine fixed upon a platform carried upon four wheels, which are coupled together by side rods, so that on motion being communicated by means of a mitre wheel in the hind axle, it can be moved forward by hand. On the forward end of the frame are two bosses forming the centres for a pair of bell cranks or bent levers placed close to the ground, and facing in opposite directions, either one of which can be con nected with the piston rod. The outer arm of each lever carries a square socket, into which is fixed the pick, which has two cutting heads, one placed a little in front of the other so as to cut to the whole depth at one operation. In the older forms picks of different length were used, and it was necessary to go over the work a second or third time, in order to hole to the full depth. The cutting points are loose, being secured by cotters to the pick head, so that broken or blunted ones can be readily replaced without removing the pick arm. The power used is air, at about 40 to 60 ft> above atmospheric pressure. It is conducted from the reservoir connected with a compressing engine at the surface, through iron pipes fixed in the pit, and along the main roads to the working face, where thick vulcanized india-rubber pipes are used, sufficient length of pipe lying loose on the ground to allow the engine to move freely, the connection being made by a screwed joint at the back of the slide-valve chest. The valve is worked by tappets on the piston-rod, so as to be perfectly self-acting when properly adjusted; it can also bo moved by hand. The pick holders face in opposite directions, in order that the machine may be worked from either side. The size of the machine as ordinarily made is about 4 feet in length, 2 feet 2 inches high, and from 18 to 24 inches gauge of rails. The weight is about 15 cwt. The working speed is from 60 to 90 strokes per minute, corresponding to a length of from 10 to 20 yards, cut to a depth of 3 feet per hour. At the former rate, or 60 yards per shift of 6 hours, the work done corresponds to that of twelve average men. The width of the groove is from 2 to 3 inches at the face, diminishing to 1∣ inches at the back, the proportion of waste being very considerably diminished as compared with the system of holing by hand. The use of this machine has allowed a thin seam of cannel, from 10 to 14 inches in thickness, to be worked to profit, which had formerly been abandoned as too hard to be worked by hand-labour.

An earlier form of the second class of machine, in which the cutters have a continuous motion like those of a slotting machine, is that invented by Mr William Peace in the Wigan district, which is reproduced from the last edition as illustrating the principle which has since been carried out by other inventors in a more convenient and simplified form. It is represented in Plate V., figs. 1, 2, and 3. AAA is the frame, upon which are fixed one or more cylinders B, arranged so as to turn a crank shaft C, fixed to the frame, as is also another shaft D. This latter is capable of being turned by the former, by means of mitre or bevel wheels EEE; upon the lower end of the latter shaft D is placed a wheel termed the driving wheel, having upon its periphery a groove with suitable projections for working into and propelling a chain or band. Beneath or to the side of the frame (or both) is fixed temporarily or otherwise a lever, the extremity of which is constructed to carry a wheel called the terminal wheel, marked IIH; a chain or band is made to pass round the driving and terminal wheels, and by means of the driving wheel FF it is made to revolve. Into the chain are °fixed cutters of different forms (see the parts marked, figs. 4, 5, 6, and 7), which, when the machine is in action, revolve with it, and upon being pressed or drawn against the coal, erode and excavate the same. The distance of the excavation from the face of the coal is governed by the dimensions of the machine, and by the length of the lever and the distance between the driving and terminal wheels. The arrangements of the lever allow it to revolve, and to excavate any given range; see dotted lines fig. 1.

If found necessary, two or even three levers may be in operation at the same time, and arranged to cut in any direction. Other parts of the machine not particularly described are capable of elevating and depressing the front part of the machine, marked V, T, U, W; and those marked X, Y, Z, and K are capable of propelling the machine whilst at work, by acting against the prop.

The Gartsherrie machine of Messrs Baird is of the same character, but the chain of cutters works round a fixed frame or jib projecting at right angles from the engine carriage, instead of traversing upon a centre, an arrangement which makes it necessary to cut from the end of the block of coal to the full depth, instead of holing into it from the face. The forward feed is given by a chain winding upon a drum, which hauls upon a pulley fixed to a prop about 30 yards in advance. This is one of the most compact form of machines, the smaller size being only 20 inches high. With an air pressure of from 35 to 40 tt> per square inch, a length of from 300 to 350 feet of coal is holed, 2 ft, 9 in. deep, in the shift of from 8 to 10 hours.

One of the simplest forms of coal-cutting machines is that of Messrs Winstanly & Barker (fig. 14), which is driven by a pair of oscillating engines placed on a frame ru’n- ∏ing on rails in the usual way. The crank shaft carries a pinion which gears into a toothed wheel of a coarse pitch, carrying cutters at the ends of the teeth. This wheel is mounted on a carrier which, being movable about its centre by a screw gearing worked by hand, gives a radial sweep to the cutting edges, as in the machine figured in Plate V. When at work it is slowly turned until the carrier is at right angles to the frame, when the cut has attained the full depth. The forward motion is given by a chain-winding upon a crab placed in front, which is worked by a boy who hauls it slowly forward. With 25 lb pressure it will hole 3 feet deep, at the rate of 30 yards per hour, the cut being only 2¾ in. high, but it will only work on one side of the carriage.

Another kind of application of machinery to coal mining-is that of Messrs Bidder & Jones, which is intended to replace the use of blasting with gunpowder for bringing down the coal, a practice which in fiery collieries is often attended with considerable danger from the flash of the explosion firing the gas given off the coal. It consists of a small hydraulic press, which forces a set of expanding bits or wedges into a bore-hole previously bored by a long screw augur or drill, worked by hand, the action of the press being continued until a sufficient strain is obtained to bring down the coal. The arrangement is, in fact, a modification of the plug and feather system used in stone quarrying for obtaining large blocks, but with the substitution of the powerful rending force of the hydraulic press for hand-power in driving up the wedges. This apparatus has been used at Harecastle in North Staffordshire, and found to work well, but with the disadvantage of bringing down the coal in unmanageably large masses. The use of gunpowder in very fiery mines is always attended with danger, and a method of wedging down coal sufficiently perfected to be of general application would add greatly to the security of the colliers in working such mines.

The removal of the coal when broken from the working faces to the pit bottom or to the main levels is effected mainly by hand labour when the mine is small, and the distances to be traversed inconsiderable, and in mines of greater extent by horse or steam traction. The simplest method is that of loading the broken coal on to a sledge, which is dragged along the floor to the level, but now the practice of carrying railways to the face is almost universal. The old form of flat rail or tram is still largely used, the waggons having sharp-edged disc wheels, but probably edge rails and flanged wheels are now more general. The class of rail used is generally a flat-bottomed or bridge section, weighing from 15 to 25 tt> per yard, laid upon cross sleepers, which, in roads that are intended to be kept open for some time, are fixed down firmly, but are laid in a temporary manner along the working faces, and in similar positions where it is necessary to be continually shifting them, as, for instance, wheie coal-cutting machines are used. The arrangement of the drawing roads at the face of a long-wall colliery is seen in the plan fig. 12, where the rails are brought to the face upon a smooth’ iron plate, upon which the trams can be easily handled by turning on the flanges of the wheels. The names applied to the vehicles in which the coal is carried vary considerably, as do also their size and capacity. The word “corf” or “corve,” representing the old basket sledge, is one of the most generally used, as are “tram,” signifying a tram waggon, and “tub,” of the same signification as the last, but a representative of the old method of drawing in wooden buckets. In South Staffordshire and other Midland districts, a contrivance called a “skip” is the representative method of conveyance; this consists of a platform with tram wheels, upon which the coal is built up to a considerable height, the large pieces round the sides being kept together by loose rings of sheet iron, and the intermediate spaces packed full with small coal,—the whole arrangement representing a kind of cask. This, however, like most of the similar primitive methods, is giving way to the more improved system of tubs or trams. These are small railway trucks, generally with flanged wheels and squaresided bodies, either of wood or wrought iron, varying in capacity from 4 cwt. in thin seams to 10 or 12 cwt. in thicker seams.

In the removal of the coal from the workings the first portion of the journey is generally performed by hand power, boys being employed to push the trams before them to the main roads. In the thin seams of South Yorkshire and other places, considerable journeys are often performed in this way, the boys known, as “hurriers” or “putters” being obliged to crawl at full length, owing to the lowness of the excavation. As a general rule boys are not allowed to work in collieries when below 12 years of age, but in these thin mines special exemptions are granted, permitting the use of younger boys as putters when required. Where the levels are large, horse traction is in common use; tho trams are formed up into trains, and from 6 to 15 vehicles are drawn by one horse. A considerable number of ponies are imported into the northern ports of this country from Norway and Iceland for this purpose every year. The supply of horses is, however, becoming scarcer, and the price higher, so that the use of underground engines is generally adopted where the output is sufficiently large to justify the expenditure. This is done by hauling or, as it is calledin the North of England, leading the trains of tubs by rope traction. In a large colliery where the shafts are situated near the centre of the field, and the workings extend on all sides, both to the dip and rise, the drawing roads for the coal may be of three different kinds,—(1) levels driven at right angles to the dip, suitable for horse roads, (2) rise ways, known as jinny roads, jig-brows, or up-brows, which, when of sufficient slope, may be used as self-acting planes, f.e., the loaded waggons may be made to pull back the empty ones to the working faces, and (3) dip or down-brows, requiring engine power. A road may be used as a self-acting or gravitating incline when the gradient is 1 in 30 or steeper, in which case the train is lowered by a rope passing over a pulley or brake drum at the upper end, the return empty train being attached to the opposite end of the rope and hauled up by the descending load. The arrangements for this purpose vary, of course, with the amount of work to be done with one fixing of the machinery; where it is likely to be used for a considerable time, the drum and brake are solidly constructed, and the ropes of steel or iron wiτe carefully guided over friction rollers, placed at intervals between the rails to prevent them from chafing and wearing out on the ground. Where the load has to be hauled up a rising gradient, underground engines, driven by steam or compressed air, are now generally used. In some cases steam generated in boilers at the surface is carried in pipes to the engines below, but this can be done with less loss of power by sending down compressed air in the same way. The use of underground boilers placed near the upcast pit, as in fig. 6, so that the smoke and gases help the ventilating furnace, is most convenient in the majority of cases. Water-pressure engines, driven by a column of water equal to the depth of the pit, have also been employed for hauling. These can, however, only be used advantageously where there are fixed pumps, the fall of water generating the power resulting in a load to be removed by the expenditure of an equivalent amount of power in the pumping engine above that necessary for keeping down the mine water.

There are four principal methods in which steam power can be applied to underground traction. These, which have been discussed in the fullest manner in the Report of the North of England Institute of Mining Engineers for 1867- 68, are as follows :—

1. Tail rope system.

2. Endless chain system.

3. Endless rope system on the ground.

4. Endless rope system overhead.

The three last may be considered as modifications of the same principle. In the first, which is that generally used in Northumberland and Durham, a single line of rails is used, the loaded tubs beiug drawn “out bye,” t'.e., towards the shaft, and the empty ones returned “in bye,” or towards the working faces, by reversing the engine; while in the other systems, double lines, with the rope travelling continuously in the same direction, are the rule. On the tail rope plan the engine has two drums worked by spur gearing, which can be connected with, or cast loose from, the driving shaft at pleasure. The main rope, which draws out the loaded tubs, coils upon one drum, and passes near the floor over guide sheaves placed about 20 feet apart. The tail rope, which is of lighter section than the main one, is coiled on the second drum, passes over similar guide sheaves placed near the roof or side of the gallery round a pulley at the bottom of the plane, and is fixed to the end of the train or set of tubs. When the load is being drawn out, the engine pulls directly on the main rope, coiling it on to its own drum, while the tail drum runs loose paying out its rope, a slight brake pressure being used to prevent its running out too fast. When the set arrives ont bye, the main rope will be wound up, and the tail rope pass out from the drum to the end and back, i.e., twice the length of the way; thq set is returned in bye, by reversing the engine, casting loose the main, and coupling up the tail drum, so that the tail rope is wound up, and the main rope paid out. This method, which is the oldest, having been in use for twenty-five years or more in the North of England, is best adapted for ways that are nearly level, or when many branches are intended to be worked from one engine, and can be carried round curves of small radius without deranging the trains; but as it is intermittent in action, considerable engine-power is required in order to get up the required speed, which is from 8 to 10 miles per hour. From 8 to 10 tubs are usually drawn in a set, the ways being often from 2000 to 3000 yards long. In dip workings the tail rope is often made to work a pump connected with the bottom pulley, which forces the water back to the cistern of the main pumping engine in the pit.

For the endless chain system, which is much used in the Wigan district a double line of w ay is necessary, one line for full and the other for empty tubs. The chain passes over a pulley driven by the engine, placed at such a height as to allow it to rest upon the tops of the tubs, and round a similar pulley at the far end of the plane. The forward edge of the tub carries a projecting pin or horn, with a notch into which the chain falls which drags the tub forward. The road at the outer end is made of a less slope than the chain, so that on arrival the tub is lowered, clears the pin, and so becomes detached from the chain. The tubs are placed on at intervals of about 20 yards, the chain moving continuously at a speed of from 2∣ to 4 miles per hour. This system presents the greatest advantages in point of economy of driving power, especially where the gradients are variable, but is expensive in first cost, and is not well suited for curves, and branch roads cannot be worked continuously, as a fresh set of pulleys worked by bevel gearing is required for each branch.

The endless rope system may be used with either a single or double line of w ay, but the latter is more generally advantageous. The rope, which is guided upon sheaves between the rails, is taken twice round the head pulley; or a Fowler’s clip pulley may be used. It is also customary to use a stretching pulley to keep the rope strained when the pull of the load diminishes. This is done by passing a loop at the upper end round a pulley mounted in a travelling frame, to which is attached a weight of about 15 cwt. hanging by a chain. This weight pulls directly against the rope; so if the latter slacks, the weight pulls out the pulley frame and tightens it up again. The tubs are usually formed into sets of from 2 to 12, the front one being coupled up by a short length of chain to a clamping hook formed of two jaws moulded to the curve of the rope which are attached by the “run rider,’ as the driver accompanying the train is called. This system in many respects resembles the tail rope, but has the advantage of working with one-third less length of rope for the same length of way.

The endless rope system overhead is substantially similar to the endless chain. The waggons are attached at intervals by short lengths of chain lapped twice round the rope and hooked into one of the links, or in some cases the chains are hooked into hempen loops on the main rope.

One of the most important branches of colliery work is the management of the ventilation, involving as it does the supply of fresh air to the men working in the pit, as well as the removal of inflammable gases that may be given off by the coal. This is effected by carrying through the workings a large volume of air which is kept continually moving in the same direction, descending from the surface by one or more pits known as intake or downcast pits, and leaving the mine by a return or upcast pit. Such a circulation of air can only be effected by mechanical means when the workings are of any extent, as will be apparent from the following considerations :—

If the shafts A and B, fig. 15, were of equal depth from the horizontal plane, and connected by the mine C, the air would fill the openings and remain quiescent. If the one were to the dip of the other, but communicating with the surface at a higher level, as by fig. 16, it would sometimes happen, in summer, that D would be the downcast, and E the upcast, and in winter, E the downcast, and D the upcast. These conditions are induced by the temperature of the earth at a certain depth being nearly constant, while the atmosphere is changeable,—the column of air in D d being at a lower temperature in summer than the column of air E e, and the reverse in winter.

The methods actually adopted are—(1 ) The rarefaction of the air in the upcast pit by a furnace placed at the bottom; and (2) Exhaustion by machinery at the surface. The former plan, although hitherto most generally used, is in many places becoming replaced by some form of machine.

The usual form of ventilating furnace is a plain firegrate placed under an arch, and communicating with the upcast shaft by an inclined drift. It is separated from the coal by a narrow passage walled and arched in brickwork on both sides. The size of the grate varies with the requirements of the ventilation, but from 6 to 10 feet broad and from 6 to 8 feet long are usual dimensions. At Shireoaks Colliery, in Nottinghamshire, a furnace consuming 6 tons of slack per 24 hours upon a grate surface of 72 square feet maintains a circulation of about 120,000 cubic feet per minute. At Hetton Colliery, Durham, the grate is a long, narrow rectangle, 25 feet by 5 feet, with numerous furnace-doors on the long side, so arranged that the surface fired may be varied according to the amount of draught required. There are two bunker-ìioles for coals, and a stoking passage, 7 feet wide, in front of the furnace. The fire should be kept as thin and bright as possible, to reduce the amount of smoke in the upcast. When the mine is free from gas, the furnace may be worked by the return air, but it is better to take fresh air directly from the downcast by a scale, or split, from the main current. The return air from fiery workings is never allowed to approach the furnace, but is carried into the upcast by a special channel, called a dumb drift, some distance above the furnace drift, so as not to come in contact with the products of combustion until they have been cooled below the igniting point of fire-damp. Where the upcast pit is used for drawing coal, it is usual to discharge the smoke and gases through a short lateral drift near the surface into a tall chimney, so as to keep the pit-top as clear as possible for working. Otherwise the chimney is built directly over the mouth of the pit.

Various kinds of machines for ventilation, both by direct exhaustion and centrifugal displacement, have been tried both in England and in Belgium. Of the former class are the great bell machines, resembling gasometers, 12 feet to 22 feet in diameter, and 9 feet high, moving in a water-tank with balanced flap valves for alternately admitting and exhausting the air. These were used at Marihaye, near Liège, and at Cwm Avon in South Wales, by Mr Struvé. Perhaps the largest of the class of piston machines is that at Nixon’s Navigation Pit, near Aberdare, which has rectangular pistons, 30 feet by 22 feet, moving horizontally through a stroke of 7 feet, the lower edge being supported by rollers running on rails. The great weight of the moving parts in this class of machine makes them incapable of acting at any very high speed, and consequently expensive for the amount of work done. This is in some degree obviated in the rotary piston machines of Fabry and Lemielle, the former resembling in principle Root’s blower, now so much used in blowing foundry and smiths’ fires, but on a larger scale. Lemielle’s ventilator is a vertical drum revolving eccentrically within a cylindrical casing. The drum carries three jointed blades, which are drawn in or out by radius bars as it revolves, so as to enclose and sweep out at each revolution the body of air included between the two cylinders. This is one of the best machines of its class, producing a comparatively high effect for the power expended. An American machine of this kind is described and figured in the article Bellows, vol. iii. p. 552, fig. 5.

Of late years, various kinds of centrifugal machines, or fans, have come into use instead of ventilating furnaces. One of the most successful of these is that invented by Mr Guibal of Liège, represented in fig. 17. The fan has eight arms, framed together of wrought-iron bars, with diagonal struts, so as to obtain rigidity with comparative lightness, carrying flat close-boarded blades at their extremities. It revolves with the smallest possible clearance in a chamber of masonry, one of the side walls being perforated by a large round hole, through which the air from the mine is admitted to the centre of the fan. The lower quadrant of the casing is enlarged spirally, so as to leave a narrow rectangular opening at the bottom, through which the air is discharged into a chimney of gradually increasing section carried to a height of about 25 feet. The size of the discharge aperture can be varied by means of a flexible wooden shutter sliding in a groove in a cast-iron plate, curved to the slope of the casing. By the use of the spiral guide casing and the chimney, the velocity of the effluent air is gradually reduced up to the point of final discharge into the atmosphere, whereby a greater useful effect is realized than is the case when the air streams freely from the circumference with a velocity equal to that of the rotating fan. The power is applied by steam acting directly on a crank at one end of the axle. In most of the newer examples, which are generally of large size, the power is divided, an engine being placed on each side. At Washington Colliery, Durham, a machine of 36 feet diameter, 12 feet breadth of face, and 13 feet diameter of intake passage, draws 120,000 cubic feet of air per minute, when making 38 revolutions. Another at Us worth, 48 feet diameter and 12 feet breadth of face, driven by two high-pressure engines, with cylinders 3 feet in diameter and 3 feet stroke, equal to about 280 horse-power, exhausts 200,000 cubic feet per minute. The useful effect realized under the most favourable conditions is as much as 50 per cent, of that of the steam power employed.

Waddle’s fan, represented in fig. 18, is an example of another class of centrifugal ventilator, in which a close casing is not used, the air exhausted being discharged from the circumference directly into the atmosphere. It consists of a hollow sheet-iron drum formed by two conoidal tubes, united together by numerous guide blades, dividing it up into a series of rectangular tubes of diminishing section, attached to a horizontal axle by cast-iron bosses and wrought-iron arms. The tubes at their smallest part are connected to a cast-iron ring, 10 feet in diameter, but at their outer circumference they are only 2 feet apart. The extreme diameter is 25 feet. A fan of these dimensions at Brownhills in Staffordshire, in making 50 revolutions per minute, circulates 47,000 cubic feet of air through the workings. It has also been in use for some years in South Wales, and is found to work well; it is less expensive in first cost than Guibal’s, although proportionally less economical from the smaller effect realized for the power expended.

Another method of colliery ventilation is that by jets of steam blowing off at a high velocity into the upcast shaft, and producing a draught similar to that of the exhaust blast in the chimney of a locomotive. This plan found several advocates some years since, and was the subject of numerous comparative trials against the ventilating furnace in the North of England, but the results were unfavourable, the amount of air circulation produced being exceedingly small for the fuel expended. It seems probable, however, that this want of success was in great part due to the defective character of the apparatus applied, and that, with properly-constructed aspirators and discharge passages, the steam jet may prove to be a very efficient means of ventilation.

The comparative merits of furnace and machine ventilation have long been discussed without any definite result. The former was at one time regarded in England as practically superior in every respect, but this opinion has been modified since the introduction of the improved forms of fans which have been worked to a considerable extent. In France and Belgium, on the contrary, machine ventilation has been more generally in favour. For a deep and extensive mine where the coal is not fiery, the furnace is undoubtedly the simplest and most efficacious method of producing a large circulation of air; but for moderate depths, especially with fiery return air, a ventilating machine at the surface is in many cases to be preferred. There is also an important advantage procured by the latter, namely, that of reserve power, so that a larger circulation may be obtained immediately in case of need, e.g., when the barometer falls suddenly, by merely increasing the speed of rotation, which cannot so readily be done with the furnace, which has a tendency to slacken at the time when the increased work is wanted.

The quantity of air required for a large colliery depends upon the number of men employed, as for actual respiration from 100 to 200 cubic feet per minute should be allowed. In fiery mines, however, a very much larger amount must be provided in order to dilute the gas to the point of safety. Even with the best arrangements a dangerous increase in the amount of gas is not unfrequent from the sudden release of stored up masses in the coal, which, overpowering the ventilation, produce magazines of explosive material ready for ignition when brought in contact with the flame of a lamp or the blast of a shot. The management of such places, therefore, requires the most constant vigilance on the part of the workmen, especially in the examination of the working places that have been standing empty during the night, in which gas may have accumulated, to see that they are properly cleared before the new shift commences.

The actual conveyance or coursing of the air from the intake to the working faces is effected by splitting or dividing the current at different points in its course, so as to carry it as directly as possible to the places where it is required. In laying out the mine, it is customary to drive the levels or roads in pairs, communication being made between them at intervals by cutting through the intermediate pillar, the air then passes along one, and returns by the other. As the roads advance other pillars are driven through in the same manner, the passages first made being closed by stoppings of broken rock, or built up with brick and mortar walls, or both. When it is desired to preserve a way from one road or similar class of working to another, double doors placed at sufficient intervals apart to take in one or more trams between them when closed are used, forming a kind of lock or sluice. These are made to shut air-tight against their frames, so as to prevent the air from taking a short cut back to the upcast, while preserving free access between the different districts without following the whole round of the airways. The ventilation of ends is effected by means of brattices or temporary partitions of thin boards placed midway in the drift, and extending to within a few feet of the face. The air passes along one side of the brattice, courses round the free end, and returns on the other side. In many cases a light but air-proof cloth, specially made for the purpose, is used instead of wood for brattices, as being more handy and more easily removed. In large mines where the air-ways are numerous and complicated, it often happens that currents travelling in opposite directions are brought together at one point In these cases it is necessary to cross them in the manner shown in fig. 2, Plate III. The return air is usually made to pass over the intake by a curved drift carried some distance above in the solid measures, both ways being arched in brickwork, or even in some cases lined with sheet-iron so as to ensure a separation not likely to be destroyed in case of an explosion. The relation of the ventilation to the workings under the different systems is indicated on the several plates by arrows and other signs, from which the general character of the arrangements adopted can be made out without further description.

The lighting of underground workings in collieries is closely connected with the subject of ventilation. In many of the smaller pits in the Midland districts, and generally in South Staffordshire, the coals are sufficiently free from gas, or rather the gases are not liable to become explosive when mixed with air, to allow the use of naked lights, candles being generally used. Oil lamps are employed in many of the Scotch collieries, and are almost universally used in Belgium and other Continental countries. The buildings near the pit bottom, such as the stables and lamp cabin, and even the main roads for some distance, are often in large collieries lighted with gas brought from the surface, or in some cases the gas given off by the coal is used for the same purpose. Where the gases are fiery, the use of protected lights or safety lamps becomes a necessity.

The nature of the gases evolved by coal when freshly exposed to the atmosphere has been investigated by several chemists, more particularly by Playfair and Meyer. The latter observer found the gases given off by coal from the district of Newcastle and Durham to contain carbonic acid (anhydride), marsh gas or light carburetted hydrogen (the fire-damp of the miner), oxygen, and nitrogen. A newer investigation, by Mr J. W. Thomas, of the gases dissolved or occluded in coals from South Wales basin shows them to vary considerably with the class of coal. The results given below, which are selected from a much larger series published in the Journal of the Chemical Society, were obtained by heating samples of the different coals in vacuo for several hours at the temperature of boiling water. [table]

In one instance, about 1 per cent, of hydride of ethyl was found in the gas from a blower in a pit in the Rhondda district, which was collected in a tube and brought to the surface to be used in lighting the engine-room and pit-bank. The gases from the bituminous house coals of South Wales are comparatively free lrom marsh gas, as compared with those from the steam coal and anthracite pits. The latter class of coal contains the largest proportion of this dangerous gas, but holds it more tenaciously than do the steam coals, thus rendering the workings comparatively safer. It was found that, of the entire volume of occluded gas in an anthracite, only one-third could be expelled at the temperature of boiling water, and that the whole quantity, amounting to 650 cubic feet per ton, was only to be driven out by a heat of 300° C. Steam coals being softer and more porous give off enormous volumes of gas from the working face in most of the deep pits, many of which have been the scene of disastrous explosions.

The gases evolved from the sudden outbursts or blowers in coal, which are often given off at a considerable tension, are the most dangerous enemy that the collier has to contend with. They consist almost entirely of marsh gas, with only a small quantity of carbonic acid, usually under 1 per cent., and from 1 to 4 percent, of nitrogen.

Fire-damp when mixed with from four to twelve times its volume of atmospheric air is explosive; but when the proportion is above or below these limits, it is inflammable, burning quietly with a pale blue flame. When a lighted candle is exposed in a non-explosive mixture of this gas, the flame gradually elongates, forming a conical cap, floating above the wick, which may be extinguished by cautious withdrawal without communicating the fire to the surrounding atmosphere. This method of testing for gas in the working places and wastes, which is obviously only to be trusted in skilled hands, used to be commonly practised, but since the introduction of safety lamps it has fallen into disuse.

The principle involved in the construction of safetylamps consists in surrounding the flame of a lamp by a protecting metal case, perforated with numerous small holes, through which the air for feeding the flame may freely enter, and the products of combustion pass out, while the passage of flame, or gases sufficiently heated to cause the ignition of the external air when laden with explosive gases, is prevented. In 1816 Sir Humphrey Davy made the great discovery that these conditions are fulfilled by the use of tubes reduced to a mere section, such as the apertures in wire gauze, when the substance of the wire is rightly proportioned to the size of the aperture. The standard adopted as the limit for safety at that time was a gauze of 28 iron wires to the linear inch, having 784 apertures per square inch, which has been used ever since. The common safety or Davy lamp consists of a small cylindrical oil lamp, covered with a cylinder of wire gauze about 6 inches long and 1J inches in diameter, with a flat gauze top. The upper part of the gauze is doubled to prevent its being worn into holes by the products of combustion, and the air for feeding the flame enters round the wick. The gauze is mounted in a cage, consisting of three upright wires, screwed into a flat brass ring at each end. A handle is attached to the upper ring, while the lower one screws on to a collar on the oil-vessel of the lamp. When the two parts are screwed together the lamp is locked by a bolt passing through both parts, which is screwed down flush with or below the surface of the outer ring, so that the gauze cannot be removed without the use of a key.

In Stephenson’s safety-lamp, generally known as the “Geordie,” from the inventor George Stephenson, the light is covered by a glass chimney, surrounded by an outer casing and top of wire gauze. The feed air is admitted through numerous small holes in a copper ring a little below the level of the wick. This is one of the safest forms of lamp, but requires considerable care in use, especially in keeping the small feed holes clear from dust and oil; the glass protects the gauze from becoming overheated.

and when the air is dangerously charged with gas the light is extinguished.

Various forms of safety-lamps have been introduced at different times, for the purpose of increasing the amount of light by substituting a glass cylinder for the lower portion of the wire gauze. The oldest of these is that of Dr Clanny, contemporary with those of Davy and Stephenson. The air for supplying the flame, entering at the bottom of the gauze, and passing down the inner side of the glass, protects the latter to some extent from becoming overheated, but a large amount of light is lost by absorption in the glass, so that there is no great advantage over the ordinary Davy lamp to compensate for the extra weight and cost, especially as the safety property of the lamp depends upon the glass cylinder, which may be readily broken when subjected to the ordinary accidents of working. A more perfect form of lamp of the same character is that of Museler, which is extensively used in Belgium. It differs from Clanny’s lamp by the addition of a conical chimney above the flame, which produces a rapid draught, and consequently a more perfect cooling of the glass cylinder by the down-draught of feed air for the flame.

Boty’s lamp, which was recommended by a commission of the Belgian Government as being safe in use, is essentially that of Dr Clanny with Stephenson’s perforated ring for admitting air at the level of the wick. Another Belgian variety is that of Eloin, in which the glass is shaped to the surface produced by the revolution of a parabolic arc, so as to disperse the light in parallel lines. The air is admitted by a Stephenson ring, combined with an Argand cap, the glass being surrounded by a brass chimney with a gauze top. In another form of the same lamp Museler’s chimney is added.

The locking of safety-lamps, so as to render them incapable of being opened by the miners when at work, is a point that has given play to a large amount of ingenuity. One of the most favourite devices is a combination of the wick-holder with the locking bolt, so that the latter cannot be withdrawn without lowering the wick and extinguishing the flame. Another method consists in the use of a lead rivet, uniting the two parts of the lamp, impressed with a seal, which cannot be removed without defacing the device. All this class of contrivances have the defect of only being efficacious when the miners are not provided with matches, or other means of obtaining a light. A more physically perfect method is that adopted by Bidder, where the locking bolt is magnetized and held in place by a force which can only be overcome by the application of a battery of heavy and powerful steel magnets. These are kept in the lamp cabin at the pit bottom, where the lamps are cleaned and served out lighted to the miners at the commencement of the shift, and are collected before they return to the surface.

When a Davy lamp is exposed to an atmosphere containing less than 8 per cent, of marsh gas, the flame lengthens and becomes smoky; when that amount is reached the flame returns to its usual size, but a column of blue flame rises to the top of the gauze. With 10 per cent, the flame of the wick is extinguished, the whole of the space within the gauze being filled with a blue flame of burning gas. If the lamp is allowed to remain too long in a fiery atmosphere it becomes dangerous, as the gauze being heated to redness may fire the gas. The safety of the lamp is also endangered by an exposure to a current of gas moving at the rate of more than 6 or 8 feet per second, as the flame can then be readily driven through the gauze. It is therefore usual to protect the flame by a sliding shield of tin plate, horn, or mica from the direct action of any sudden outburst of gas in the workings. Lamps with glass cylinders are generally very safe, except from the risk of accidental breakage, which, however, is less frequent than might be imagined, and those taking air through a feed ring, such as Stephenson’s, are readily extinguished in a foul atmosphere.

The danger arising from gas in the workings may be considerably increased by the presence of coal dust in the air. This point has been the subject of investigation by Galloway, who found that an explosion may be produced by ignited particles of coal dust through the agency of a safety-lamp which under ordinary circumstances would be perfectly trustworthy. At Blanzy, in France, several fatal explosions have been traced to the firing of coal dust from the flame of a shot, even in cases where no fire-damp was present in the workings.

An electric lamp, where the light is obtained from the discharge in a Geissler vacuum tube, has been proposed by Benoit-Dumas, instead of the ordinary safety lamps, or for use in exploring after explosions or in bad air ways. This consists of a box containing a galvanic battery, consisting of two Bunsen cells, and a small induction coil, with connecting wires which convey the current to the lamp. The Bunsen cells may be conveniently replaced by a single bottle-shaped bichromate battery. The cost and complication of this apparatus must necessarily limit its use.

Apparatus, originating in France, known as aerophores, which enable the miner to carry sufficient fresh air for his own respiration, and to keep a lamp alight for a short time in a totally irrespirable atmosphere, have of late years come into use for the purposes of saving life after explosions, and repairing shafts and pit-work under water. There are two principal patterns, those of Galibert and Denayrouze. The former, which is the simplest, consists of an air-tight bag of about 12 cubic feet capacity, containing air at a little above atmospheric pressure, which is carried on the miner’s back like a knapsack. The air, after being used, is returned with the products of respiration into the bag, and can be used over again until it becomes too impure for further use. It is obvious, therefore, that such an apparatus must be of very limited application, but its simplicity and cheapness are points in its favour for use in sudden emergencies. The Denayrouze apparatus consists of a series of sheet metal cylinders, containing air compressed to 300 or 350 lb to the square inch, which can be carried on the back, and served out at a pressure very slightly above that of the atmosphere by means of a reducing valve, whose construction is essentially the same in principle as that of the ordinary pressure regulator used in gas-works, iae., a conical plug dosed against its seat by the pressure of the air in the reservoir, which is constantly opposed by an external force tending to open it. This force is supplied by a disc of vulcanized india-rubber, w hich opens the valve at each inspiration, and allows a fresh supply of air to escape into the chamber of the regulator through the small aperture of the valve. Of course, all communication with the external air must be cut off, so that respiration can only take place through the mouth, the air-tube being attached by an india-rubber mask called a mouth-closer, and the nostrils closed by a spring clip. A similar regulator valve, so constructed as to keep the india-rubber spring under a slight excess pressure in order to maintain a flow of air, is in connection with the lamp. This is of the ordinary Museler construction, with the addition of a chamber outside the gauze to receive the products of combustion, which are discharged through a conical valve at the top, a reflux of the exterior gases being prevented by the pressure of a counter spring. The air is carried to the lamp by an india-rubber tube, which is sufficiently flexible to allow a certain freedom of motion. The distance that an explorer can penetrate with this apparatus is obviously limited by the capacity of the air-cylinders.

These have been made large enough to supply air to a man with a lamp for an hour, but this is an inconvenient size, being too large to be carried on the back.

Underground fires are not uncommon accidents in coalmines. In the thick coal workings in South Staffordshire the slack left behind in the sides of work is especially liable to fire from so-called spontaneous combustion, due to the rapid oxidization that is set up, when finely-divided coal is brought in contact with air. The best remedy in such cases is to prevent the air from gaining access to the coal by building a wall round the burning portion, which can in this way be isolated from the remainder of the working, and the fire prevented from spreading, even if it cannot be extinguished. When the coal is fired by the blast of an explosion it is often necessary to completely isolate the mine by stopping up the mouths of the pits with earth, or in extreme cases it must be flooded with water or carbonic acid before the fire can be brought under. There have been several instances of this being done in the fiery pits in the Barnsley district, notably at the great explosion at the Oaks colliery in 1866, when 360 lives were lost.

The drawing or winding of the coal from the pit bottom to the surface is one of the most important operations in coal mining, and probably the department in which mechanical appliances have been brought to the highest state of development. In the simplest case, where the mine is worked by levels, the trains of coal may be drawn from the working faces directly to the level mouth by horse power, or in some exceptional cases locomotives worked by compressed air are used. In South Wales the power for lifting the load in the shaft is still in some small workings furnished by a water balance, that is, a box which is filled with water at a high level, and in descending raises the loaded trucks by a rope passing over a pulley at the surface. This method is only available when there is a free drainage level for the water to run off when the box reaches the lowest point. Other hydraulic motors, such as wheels, pressure engines, &c., are used in different localities as well as animal power, where the amount of coal to be drawn is small, but as a general rule it is necessary to have recourse to steam power to maintain an adequate output. The old custom of drawing the coals in tubs or hutches (cuffat of the French miner), swinging freely from the end of the drawing rope, is now almost entirely superseded by the adoption of cages sliding between fixed guides, which allow the load to move freely up and down while checking lateral oscillation. This improvement, which is due to Mr John Curr of Sheffield, was originally introduced in 1798, but made surprisingly little progress for nearly half a century. It was first brought into general use in the North of England, but in many of the smaller pits of the Midland counties the older custom prevailed until recently.

The different elements making up the drawing arrangements of a colliery are—(1) the cage, (2) the shaft or pit fittings, (3) the drawing-rope, (4) the engine, and (5) the surface arrangements. The cage, as its name implies, consists of one or more platforms connected by an open framework of vertical bars of wrought iron or steel, with a top bar to which the drawing-rope is attached. It is customary to have a curved sheet-iron roof or bonnet when the cage is used for raising or lowering the miners, to prevent them from injury by falling materials. The number of platforms or decks varies considerably; in small mines only a single one may be used, but in the larger modern pits two, three, or even four-decked cages are used. The use of several decks is necessary in old pits of small section, where only a single tram can be carried on each. In the large shafts of the Northern and Wigan districts the cages are made about 8 feet long and 3∣ feet broad, being sufficient to carry two large trams on one deck. These are received upon a railway made of two strips of angle iron of the proper gauge for the wheels, and are locked fast by a latch falling over their ends.

The guides or conductors in the pit may be constructed of wood, in which case rectangular fir beams, about 3 by 4 inches, are used, attached at intervals of a few feet to buntons or cross-beams, built into the lining of the pit. Two guides are required for each cage; they may be placed opposite to each other, either on the long or short sides—the latter being preferable. The cage is guided by shoes of wrought iron, a few inches long and bell-mouthed at the ends, attached to the horizontal bars of the framing, which pass loosely over the guides on three sides. In some of the large collieries in Northumberland wrought iron guides have been adopted with advantage. They are applied on one side of the cage only, forming a complete vertical railway,—light flange rails such as are used for the roadways underground being used instead of wooden rods and iron cross sleepers, with proper seats for the rails instead of wooden buntons; the cage is guided by curved shoes of a proper section to cover the heads of the rails. Rigid guides connected with the walling of the pit are probably the best and safest, but they have the disadvantage of being liable to distortion, in case of the pit altering its form, owing to irregular movements of the ground, or other causes. Wooden guides being of considerable size, block up a certain portion of the area of the pit, and thus offer an impediment to the ventilation, especially in upcast shafts, where the high temperature, when furnace ventilation is used, is also against their use. In the Wigan district, wire-rope guides have been introduced to a very considerable extent, with a view of meeting the above objections. These are simply wire-ropes, from ∣ to 1∣ inches in diameter, hanging from a cross-bar connected with the pit-head framing at the surface, and attached to a similar bar at the bottom, which are kept straight by a stretching weight of from 30 cwt. to 4 tons attached to the lower bar. In some cases four guides are used—two to each of the long sides of the cage; but a more general arrangement is to have three—two on one side, and the third in an intermediate position on the opposite side. Many colliery managers, however, prefer to have only two opposite guides, as being safer. The cage is connected by tubular clips, made in two pieces and bolted together, which slide over the ropes. In addition to this, it is necessary to have an extra system of fixed guides at the surface and at the bottom, where it is necessary to keep the cage steady during the operations of loading and landing, there being a much greater amount of oscillation during the passage of the cage than with fixed guides. For the same reason it is necessary to give a considerable clearance between the two lines of guides, which are kept from 15 to 18 inches apart, to prevent the possibility of the two cages striking each other in passing. With proper precautions, however, wire guides are perfectly safe for use at the highest travelling speed.

The cage is connected with the drawing-rope by short lengths of chain from the corners known as tackling chains, gathered into a central ring, to which the rope is attached. Round steel wire-ropes, about 2 inches in diameter, are now commonly used; but in very deep pits they are sometimes tapered in section to reduce the dead weight lifted. Flat ropes of steel or iron wire were and are still used to a great extent, but round ones are now generally preferred. In Belgium flat ropes of aloe fibre are in high repute, being considered preferable by many colliery managers to wire, in spite of their great weight. In South Staffordshire, flat link chains made with three or more parallel links, with a stud of wood filling up the hollow, are or were in general use in the numerous shallow pits working the thick coal in the neighbourhood of Dudley, &c.

The best modern engines for drawing in collieries are usually direct-acting, with either horizontal or vertical cylinders. In the north of England a single engine with a heavy fly-wheel is often used, but the more general arrangement is to have two engines coupled to the opposite ends of the winding drum-shaft. In almost all cases steam is used at high pressure without condensation.

The drum, when round ropes are used, is a plain broad cylinder, w ith flanged rims, and cased with soft wood packing, upon which the rope is coiled; the breadth is made sufficient to take the whole length of the rope at two laps. One drum is usually fixed to the shaft, while the other is loose, with a screw link or other means of coupling, in order to be able to adjust the two ropes to exactly the same length, so that one cage may be at the surface when the other is at the bottom, without having to pay out or take up any slack rope by the engine.

For flat ropes, the drum or bobbin consists of a solid disc, of the width of the rope fixed upon the shaft, with numerous parallel pairs of arms or horns, arranged radially on both sides, the space between being just sufficient to allow the rope to enter and coil regularly upon the preced ing lap. This method has the advantage of equalizing the work of the engine throughout the journey, for when the load is greatest, with the full cage at the bottom and the whole length of rope out, the duty required iu the first revolution of the engine is measured by the length of the smallest circumference; while the assistance derived from gravitating action of the descending cage in the same period is equal to the weight of the falling mass through a height corresponding to the length of the largest lap, and so on, the speed being increased as the weight diminishes, and vice versa.

The same thing can be effected in a more perfect manner by the use of spiral or scroll drums, in which the rope is made to coil in a spiral groove upon the surface of the drum, which is formed by the frusta of two obtuse cones placed wslth their smaller diameters outwards. This plan, though mechanically a very good one, has certain defects, especially in the possibility of danger resulting from the rope slipping sideways, if the grooves in the bed are not perfectly true. The great size, and weight of such drums are also disadvantages, as giving rather unmanageable dimensions in a very deep pit.

The use of a counterbalance chain for the winding engines is common in the collieries of the Midland districts of England. In this method a third drum is used to receive a heavy flat link chain, shorter than the main drawing-ropes, the end of which hangs down a special or balance pit. At starting, when the full load is to be lifted, the balance chain uncoils, and continues to do so until the desired equilibrium between the working loads is attained, when it is coiled up again in the reverse direction, to be again given out on the return trip.

The surface arrangements of a modern colliery are often of considerable extent and complexity, the most important feature being the pit-frame carrying the guide-pulleys or rope-rolls which lead the drawing-ropes from the vertical line of the pit to the engine-drum. This consists essentially of an upright framework, carefully braced together, and strutted by diagonal beams against the wall of the engine-house, or other solid abutment. It is generally necessary to have a clear head-room, 10 or 20 feet or more, for the working arrangements at the surface above the level of the ground, especially in flat countries; the pitframes are made of considerable height, from 50 to 70 feet being not uncommon; and when, as is generally the case, they are made of wood, they afford opportunities for the exercise of skilful carpentry. Of late years, however, wrought iron pit-frames have been adopted to some extent, which allows of a comparatively simpler construction being used, the main elements of the frame consisting of hollow latticed pillars and beams, similar to the construction now generally adopted for the pillars of railway signals, but of course of a more solid construction. They have one great advantage over wooden frames, in not being liable to destruction by fire, an accident which has occasionally happened with the latter. The guide-pulleys for iron or steèl wire-ropes are made of very large dimensions, to avoid strain upon the wires by sudden change of direction when moving at a high speed. The usual construction is a deep channeled rim or tire of cast-iron, from 7 to 20 feet in diameter, supported by numerous thin wrought iron arms, inclining inwards from a central cast-iron boss,—a form combining rigidity with comparative lightness. They are in fact very similar to the driving wheels of the large modern bicycles, supposing a channeled rim to be substituted for the indiarubber tire.

To prevent accidents from the breakage of the rope on the shaft, or from overwinding when the engine is not stopped at the right moment, whereby the cage may be dragged up to the head pulleys (both which kinds of accident are unhappily not uncommon), various forms of safety catch and disengaging hooks have been proposed. These consist of variously-constructed toothed levers, cams, or eccentrics, mounted upon transverse axes, attached to the top of the cage, whose function is to take hold of the guides, and support the cage in the event of its becoming detached from the rope. They are generally applied by means of springs acting against the pull of the rope. Figs. 19-21 represent a form of safety catch, introduced some years since by Messrs White and Grant of Glasgow. The catches BB consist of partially toothed eccentrics, which when released are forced inwards against the wooden guide a by the coiled springs d d i as shown in fig. 21.

When the rope is drawing, the catches are lifted by the pull of the chains attached to the pulleys c c, which turn the broad toothed portions outwards, and away from the guides. The connection with the rope is made by the slide bar C and spring catch h having a projecting trigger, which, if the cage is lifted too high, strikes against the cross-bar of the framing k, and detaches the cage, which is then left hanging by the catches to the guides in the pit. The use of safety catches is more common in the collieries of France, Belgium, and Germany than in England, where they are not generally popular, owing to their uncertainty in action, as they are often found to fail when most wanted. The constant drag of the catches on the guides when the rope slacks is also objectionable, but this has been overcome to a great extent in a very ingenious contrivance invented by Mr Calow, where the catches are not brought into action unless the cage is actually falling clear of the rope, with a certain acquired momentum of its own. The only real safeguards against accidents in winding are to be found in constant vigilance, in maintaining the ropes in working efficiency, and in the use of proper signals and brake power in the engine house.

The speed attained by the load in the shaft in the best-appointed English collieries is very considerable, and may be paralleled with that of a fast railway train. At Shireoaks Colliery, Nottinghamshire, the cage with a load of 34 cwts. of coal in five tubs, and weighing in all 60 cwts., or with the rope at the bottom 92∣ cwts., is raised from a depth of 516 yards in 45 seconds, corresponding to an average of 35 feet per second, or 24 miles per hour, the maximum speed when the load is mid-way being 50 feet per second, or nearly 35 miles an hour. The ropes used are round, of steel wire, weighing 13 lbs. to the yard, winding on to a spiral drum, increasing from 17 to 20 feet in diameter. Thera are two engines with vertical cylinders, 32 inches diameter and 6 feet stroke, developing a useful effect of about 320 horse-power. The guide pulleys are 12 feet in diameter.

The above may be taken as a good example of the modern class of winding engines, such as are required to draw from 600 to 1200 tons in the shift of 10 hours. When the pits are of small depth it is better to increase the weight of the load than to draw at a very high speed, as the loss of time in filling and unloading or striking the cages is the same for a short as for a long journey, so that it becomes advantageous to diminish the number of journeys for a given quantity of coal drawn.

The great amount of dead weight required to be raised in the ordinary system of winding (e.g., in the instance given above, the total weight moved is nearly four times that of the nett load drawn, that of the ropes being nearly 1 ⅛ times as much as the latter), has led to the proposal of various plans to obtain a more mechanically economical method, but none of these have at present been brought into successful use. One of the latest is that of Μ. Blanchet, who proposes to draw a number of tubs linked together into a long vertical train in a closed tube about 5⅛ feet in diameter, by exhausting the air above them in the manner adopted in the pneumatic tubes used for the transmission of parcels. An experimental apparatus of this class has been recently constructed at Creusot, in France, designed to lift a cage with 9 tubs, attached to a piston, weighing in all about 12 j tons.

When the cage arrives at the surface, or rather the platform forming the working top above the mouth of the pit, it is received upon the keeps, a pair of hinged gratings which are kept in an inclined position over the pit-top by counterbalance weights, so that they are pushed aside to allow the cage to pass upwards, but fall back and receive it when the engine is reversed. The tubs are then removed or struck by the landers, who pull them forward on to the platform, which is covered with cast-iron plates; at the same time empty ones are pushed in from the opposite side. The cage is then lifted by the engine clear of the keeps, which are opened by a lever worked by hand, and the empty tubs start on the return trip. When the cage has several decks, it is necessary to repeat this operation for each, unless there is a special provision made for load-ipg and discharging the tubs at different levels. An arrangement of this kind for shifting the load from a large cage at one operation has recently been introduced by Mr Fowler at Hucknall, in Leicestershire, where the trains are received into a framework with a number of platforms corresponding to those of the cage, carried on the head of a plunger movable by hydraulic pressure in a vertical cylinder. The empty tubs are carried by a corresponding arrangement on the opposite side. By this means the time of stoppage is reduced to a minimum, 8 seconds for a three-decked cage as against 28 seconds, as the operations of lowering the tubs to the level of the pit-top, discharging, and replacing them are performed during the time that the following load is being drawn up the pit.

The tub when brought to the surface, after passing over a weigh-bridge, where it is weighed and tallied by a weigher specially appointed for the purpose by the men and the owner jointly, is run into a tipping cage, and the contents are discharged into an inclined screen with bars about 1 inch to 1∣ inches apart. The large coal remaining passes through a spout into a railway waggon placed below, the discharge being regulated by a valve at the lower end. The small coal passing through is either sold as such, or inay be lifted by an elevator to a second series of screens, either fixed or rotating, with half-inch apertures. These make a further separation of larger pieces, which are sold as “nuts,” while the small, or slack, passing through is sent to the coke ovens, if the quality of the coal is suitable. Asa rule, noncaking coals are not very closely screened, as the small is of comparatively little value, and therefore must have a proportion of larger sizes mixed with it to form saleable slack.

Figs. 22-24, representing the surface arrangements adopted at a pair of pits in the Wigan district, may be taken as fairly representative of the fittings of a large modern colliery, where a considerable output of coal has to be screened and loaded in an ordinary working day of less than twelve hours. The details, of course, will vary, according to the nature of the outlet or vend, which may be by retailing into carts sent by purchasers, or by canals or railways, or by a combination of all three. In the example selected, the coal is loaded directly from the screens into full-sized trucks, each carrying from 6 to 8 tons, on a main line of railway. Of the two pits, one is an upcast, and is surmounted by a chimney at the surface,—the drawing being confined to the downcast, which is 310 yards deep and 10∣ feet in diameter. 600 tons of coal are drawn from this depth in 10 hours by a pair of direct-acting engines, with vertical cylinders working a spiral drum, increasing from 13 j feet to 17⅛ feet in diameter. The pit-head frame is of wood, with guide pulleys 7 feet in diameter,— a much smaller size than is now usually adopted; the iron wire drawing-ropes are round, weighing 5 lb to the yard. Double-decked cages of a light construction in wrought iron are used, carrying four tubs at a time. The landing platform is raised upon pillars 20 feet above the surface of the ground, and covered with iron plates. As soon as the cage arrives at the surface, the tubs are run into tumbling cages, which discharge their contents on to fixed screens, with bars of 1 to l¾ inch aperture. The large coal passes by a shoot directly into the railway waggon, while the first screenings fall into a channel below, which is traversed by a series of scrapers attached to an endless chain, and are carried to an elevator or Jacob’s ladder, and discharged into rotatory drum sieves of about ∣-inch aperture, producing a second size of saleable coal, known as nuts, and slack, which is sent away to the coke ovens attached to the colliery. The whole of the labour required in the screening the output of 600 tons in the day of ten hours is performed by one engineman, who has charge of all the mechanical arrangements, and nine boys, who pick out any large lumps of stone from the coal as it passes the first screens. The engine driving the screens and elevators is in charge of a special engineman.

Fig, 25 ιepresents one of a pair of pits at Pemberton Colliery, near Wigan, having the pit frames constructed in wrought iron lattice truss-work instead of wood. The screens for large coal (S) are arranged symmetrically on the landing platform, three on each side of the pit top, and discharge directly into waggons on the railway below. The small coal from these screens is passed by a screw creeper C, like those used in flour mills, to a bucket elevator E, which delivers it at the top of the second set of screens R, where the nuts and slack are separated. The platform, as in most of the new collieries in this district, is roofed over to protect the workmen from the weather. The second pit, which occupies a corresponding position on the opposite side of the engine-house, is in every respect similar.

The large collieries in the steam-coal district of Northumberland are among the most productive; thus, at Bed-lington, near Morpeth, 1200 tons are raised daily, and at North Seaton from 1500 to 1800 tons.

When the coal is very much mixed with shale, the slack often contains so much mineral matter as to be quite worthless, until at least a partial separation has beeκ⅛ effected. This is now done by means of coal-washing machines, which were first adopted in France, but have now become general in other countries. There are many different forms, but the most usual is a fixed sieve plate, upon which the slack is received and subjected to the action of a current of water forced through the holes by the action of a fast-moving short-stroke plunger pump, which puts the whole of the materials into suspension, and allows them to fall through the water at each stroke. By this means the coal, being the lighter material, travels to the surface, and the heavier shale and stone going to the bottom are discharged through a valve there. The apparatus is in fact a form of the hydraulic jigging hutch used for the dressing of lead and other ores, except that in this case the lighter and not the heavier part is the valuable mineral. In another form of coal-dressing machine introduced by Mr Evrard, the jigging action is produced by a jet of steam acting directly upon the water instead of a plunger piston. Washed slack when suitable is used for conversion into coke, but in France and Belgium it is now generally employed in the production of agglomerated fuel, or briquettes, or what is usually known in England as patent fuel. These consist of coal dust mixed with a sufficient amount of gas-pitch to be moulded into coherent bricks or cylinders, which are afterwards dried at a high temperature, but below the point of carbonization. The consolidation of the slack may also be effected by the use of starch or dextrine, or even by cement or clay. This class of fuel is mueh used upon the French railways, being convenient for stowage and economical in use; but as a rule it is disagreeable to the passengers from the large amount of coal-dust carried off by the exhaust steam, and the unpleasant vapours produced by the burning pitch. The principal production of patent fuel in Britain is in South Wales.

The anthracite coal of Pennsylvania is subjected to the exceptional treatment of breaking between toothed rollers, and an elaborate system of screening before it is fit for sale. The largest or lump coal is that which remains upon a riddle having the bars four inches apart; the second, or steamboat coal, is above 3 inches; broken coal includes sizes above 2¾ or 2∣ inches ; egg coal, pieces above 2¾ inches square; large stove coal, l¾ inches; small stove, 1 to 1∣ or l¾ inches; chestnut coal, ¾ to ¾ inch; pea coal, ¾ inch; and buckwheat coal, ⅜ inch. The most valuable of these are the egg and stove sizes, which are broken to the proper dimensions for household use, the larger lumps being unfit for burning in open fire-places.

The proportion of coal utilized in the working, as compared with the total contents of the seam, varies very considerably in different districts, being greatest in seams of moderate thickness, from 3 to 5 feet, which on the long-wall system can be entirely removed. In thick coals, such as the ten-yard seam of South Staffordshire, the waste is very considerable. In Cheshire and Lancashire about 1330 tons of saleable coal are obtained from an acre for each foot of thickness in the seam, only 8 per cent, of the total being left behind in the workings.

At Dowlais, on the north of the South Wales coal-field, the yield is 1190 tons to the foot by long-wall, but only 866 tons when the same seam was worked by the pillar and stall system; but on the south side of the basin, where the seams lie at a steep slope, the loss is often much greater, being from 20 to 50 per cent, on pillar and stall workings. In the Barnsley district, the yield is from 1150 to 1280 tons in thick seams, and a maximum of 1417 tons has been obtained in a thin seam, the solid contents of the whole coal being estimated at 1556 tons per foot per acre. In Northumberland about 1200 tons are got out of a total of 1300. In the thick coal of South Staffordshire, from 12,000 to 16,000 tons per acre are got at the first working on an average thickness of 25⅜ feet, or about 640 tons to the foot, or from 50 to 60 per cent, of the whole, which is increased by the second and third working to 70 or 75 per cent., making a loss of from 25 to 30 per cent. This amount is reduced, however, by the long-wall method of working.

Probably from 10 to 15 per cent, may be taken as the unavoidable loss in working under the most favourable conditions, but in many cases the proportion is considerably higher.

In the United Kingdom the ownership of coal, like that of other minerals, is in the proprietor of the soil, and passes with it, except when specially reserved in the sale. The greater number of collieries are worked upon leases, the rents or royalties being variously charged in different localities. A minimum reserved rent to cover a certain output, with a rate per ton on any quantity in excess, is the most general practice; but in Lancashire and Yorkshire the royalties are charged at a fixed rate per acre per annum upon each seam worked, and in South Staffordshire at a proportion (from ⅜ to - 1 ⅛) of the coal at the pit’s mouth.

Coal lying under the sea below low-water mark belongs to the Crown, and can only be worked upon payment of royalties, even when it is approached from shafts sunk upon land in private ownership.

In the Forest of Dean, which is the property of the Crown as a royal forest, there are certain curious rights held by a portion of the inhabitants known as the Free Miners of the Forest, who are entitled to mine for coal and iron ore, under leases, known as gales, granted by the principal agent or gaveller representing the Crown, in tracts not otherwise occupied. This is the only instance in Great Britain of the custom of free mining under a Government grant or concession, which is the rule in almost every country on the Continent.

The working of collieries in the United Kingdom is subject to the provisions of the Coal Mines Regulation Act of 1872, 35 and 36 Vict. cap. 76, which is administered by inspectors appointed by the Home Office, and forms a complete disciplinary code in all matters connected with coal-mining. Among the chief provisions of the Act are the following :—

1. Females and boys under 10 are not allowed to work underground.

2. Boys between 10 and 12 are not allowed to work except in thin mines.

3. No boy under 12 to drive a gin horse, or under 18 a steam-engine.

4. Wages not to be paid at public-houses.

5. Working of mines by a single shaft prohibited.

6. Managers to be certificated as competent by a board of examiners.

7. Annual return of coal wrought to be made to Inspectors.

8. Notice of accidents to be sent to Inspector.

9. Openings of abandoned workings to be fenced.

10. Plans to be kept up to within six months of date.

11. Plans of abandoned mines to be deposited with Home Office.

12. General rules for the safety of miners in fiery mines, management of ventilation, safety lamps, and gunpowder, protection against accidents in shafts and levels, &c.

13. Power to frame special rules subject to approval of the Secretary of State.

Breaches of the provisions of the Act are punishable by fine and imprisonment by a court of summary jurisdiction, subject to appeal to the Quarter Sessions, or to the Circuit Court in Scotland.

The relation between the number of hands employed and the output of collieries varies considerably in different districts, being highest in those where the coal is moderately thick, soft, easily cut, regularly shaped, and with a good roof, and least in faulted and disturbed seams, and those with a bad roof, where the accessory operations of timbering and driving stone drifts require the employment of a large proportion of the working staff on nonproductive work, iae., other than cutting coal. The following figures give the relative force employed above and below ground in two large steam-coal collieries in South Wales, each producing about 500 tons per day :— [table]

showing in the one case an average of about 1 ton, in the other about l¾ ton per hand per day, but if the hands cutting coal be alone considered, the amount is about the same in both cases, or a little over two tons per day.

The annual output per man on the total force employed in several of the principal European coal-fields has been computed as follows :— [table]

These figures refer to some years back, and are probably not quite accurate at the present date, as the amount of work done by the individual collier has sensibly decreased in most countries. It will be seen that the output is smallest in the thin disturbed measures of the Franco-Belgian coal-field.

In Prussia in 1874, with an output of 33,000,000 tons of coal and 8,000,000 tons of lignite, the average per underground hand was about 243 tons for the former and about 600 tons for the latter. The larger comparative yield in lignite mines is due to the fact that a very large proportion are worked as quarries.

The annual production of coal throughout the world may be roughly estimated at about 260 millions of tons for 1874, which quantity includes about 17 million tons of lignite and coal from formations newer than the coal measures in Europe. Nearly one-half of the total is raised in the United Kingdom, the approximate quantities of the different countries being as follows :— [table]

There is no trustworthy information as to the produce of China and Japan, but these probably do not exceed 100,000 tons. In the larger coal-producing European countries the output was very high in 1873, the following year having shown a slight falling off, but in America the annual increase was maintained.

According to the official mineral statistics, the produce of coal in the United Kingdom for the years 1873, 1874, 1875, classified according to districts, was as shown in the following table, from which it will be seen that the checkin 187 4 ∙ was followed by great increase of production in 1875 :— [table]

The quantities of coal consumed by the different branches of manufacturing industry as well as for lighting, heating, and other purposes, was investigated by the Royal Commission on Coal, from vol. iii. of whose Report, published in 1870, the following summary is taken. The figures refer to the year 1869. [table]

The above quantities may be proportionally classified as follows :— [table]

Coal-mining is unfortunately a dangerous occupation, more than a thousand deaths from accident being reported annually by the inspectors of mines as occurring iu the collieries of the United Kingdom. The following table shows the number of lives lost during the last five years, classified according to the inspectors’ returns :— [table]

The principal sources of danger to the collier, as distinguished from other miners, are explosion of fire-damp and falls of roof in getting coal,—these together make up about 70 per cent, of the whole number of deaths. It will be seen that the former class of accidents, though attended with great loss of life at one time, are less fatal than the latter. The great increase in the deaths from explosion in 1875, over the preceding year, is to be attributed to the Swaithe Main explosion at Barnsley on December 6th, when 143 lives were lost.

The following return expresses the relation between the fatal accidents and the total number of miners employed, and the amount of coal raised for each death. The latter quantities are in some degree conjectural, being dependent upon estimated returns of produce, and are probably somewhat too large. [table]

In Prussia, in the year 1874, there were 484 deaths from accidents, which corresponds to about three deaths per thousand hands employed, or, according to the above classification, 1 in 334, with a produce of about 65,000 tons of coal for each death. It would appear, therefore, that the proportional loss of life, in the collieries of the United Kingdom, is less than that in foreign countries.

Assay and Analysis. — The chemical examination of a coal may be either complete or partial. When it is desired to obtain information as to the exact composition, the analysis is conducted in the same manner as the analysis of organic compounds by combustion with oxide of copper or chromate of lead in a hard glass tube, the carbonic acid and water formed being absorbed by solution of hydrate of potassium and dry chloride of calcium respectively, and the proportion of carbon and hydrogen being calculated from the increase of weight in the tubes containing the absorbing media. It is usual to operate upon a small sample (about 5 grains), which is very finely powdered and placed in a small trough or boat of platinum in the tube, the combustion being aided by a stream of oxygen from a gasholder. By this means the incombustible residue or ash is left in a condition for weighing, being free from admixture of foreign substances. Sulphur is determined by the fusion of a weighed quantity with a mixture of salt and nitrate of potassium in a platinum vessel, producing sulphate of potassium, which, on the addition of a salt of barium, is precipitated as sulphate of barium. Care must be taken to perform the operation over a flame free from the vapour of sulphur compounds, which may vitiate the result by apparently increas ing the amount of sulphur present. For this reason, the flame of a spirit lamp is to be preferred in making the fusion to that of coal gas, which is rarely free from sulphur compounds. Sulphur existing in the form of gypsum or sulphate of calcium may be removed by washing a sample with boiling water, and determining the sulphuric acid in the solution. The washed sample is then fused with nitre in the usual way to determine the proportion of sulphur existing as iron pyrites. This distinction is of importance in the examination of coals intended for iron smelting, as the sulphates of the earthy metals are reduced by the gases of the furnace to sulphides, which pass into the slag without affecting the quality of the iron produced, while the sulphur of the metallic sulphides in the ash acts prejudicially upon the metal.

The difference between the original weight of the sample and that of the carbon, hydrogen, sulphur, and ash, after making allowance for hygroscopic water, is attributed to oxygen and nitrogen, which are not directly determined.

The character of the ash affords some guide to the quality of the coal from which it is derived. Thus, a red tint is generally indicative of the presence of iron pyrites, and a light or white colour of its absence. Phosphorus if present will be found in the ash, and may be determined by the ordinary processes of analysis. A useful approximate method of determining the character of a coal is by exposing a coarsely powdered sample of known weight, in a covered crucible, to a strong red heat as long as inflammable vapours are given off, when it is cooled and weighed. The loss of weight represents the volatile constituents—hydrogen, oxygen, and hydrocarbon gases, produced by destructive distillation, while the residual coke includes the ash, and is called fixed carbon. The character of the button of coke obtained is a good indication as to the caking or non-caking quality of the coal from which it is derived, and the amount of ash may be determined by burning it in a muffle or over the flame of a Bunsen burner. The fitness of a coal for gas-making is usually determined by operating upon a sample of a few pounds’ weight in a special apparatus which reproduces the processes of manufacture upon a small scale.

One of the most important factors in the economic valuation of a coal, is the so-called calorific power or value, by which is usually understood the number of pounds of water at boiling point that can be evaporated by the complete combustion of one pound of coal. This may be obtained theoretically, when the composition of the coal is known, by computing the heating effect of the carbon and the disposable hydrogen; but in the absence of an analysis, it may also be determined directly by several approximate methods. One of the most convenient instruments for this purpose is Thompson’s calorimeter. This consists of a copper cylinder in which a weighed quantity of coal intimately mixed with chlorate or nitrate of potassium is deflagrated under a copper case like a diving-bell, placed at the bottom of a deep glass jar filled with a known weight of water. The gases produced by the combustion rising through the water are cooled, with a corresponding increase of temperature in the latter, so that the difference between the temperature observed before and after the experiment furnishes a measure of the evaporative power desired. The instrument is so constructed that 30 grains of coal are burnt in 29,010 grains of water, or in the proportion of 1 to 937, these numbers being selected that the observed rise of temperature in Fahrenheit degrees corresponds to the required evaporative value in pounds, subject only to a correction for the amount of heat absorbed by the mass of the instruments, for which a special co-efficient is required, and must be experimentally determined. Another approximate method, due to Berthier, is based upon the reduction of oxide of lead by the carbon and hydrogen of the coal, the amount of lead reduced affording a measure of the oxygen expended, whence the heating power may be calculated, 1 part of pure carbon being capable of producing 34⅜ times its weight of lead. The operation is performed by mixing the weighed sample with a large excess - bf litharge in a crucible, and exposing it to a bright red heat for a short time. After cooling, the crucible is broken and the reduced button of lead is cleaned and weighed. The results obtained by this method are less accurate with coals containing much disposable hydrogen and iron pyrites than with those approximating to anthracite, as the heat equivalent of the hydrogen in excess of that required to form water with the oxygen of the coal is calculated as carbon, while it is really about four times as great. Sulphur in iron pyrites also acts as a reducing agent upon litharge, and increases the apparent effect in a similar manner.

The theoretical evaporative power of a coal found by either of the above methods is always considerably above that obtained by actual combustion under a steam boiler, as in the latter case numerous sources of loss, such as imperfect combustion of gases, loss of unburnt coal in cinders, <kc., come into play, which cannot be allowed for in theoretical experiments. It is usual, therefore, to determine the value of a coal by the combustion of a weighed quantity in the furnace of a standard boiler, and measuring the amount of water evaporated by the heat developed. Various investigations of this kind have been made at different times, both in Europe and America, the most extensive being the following :—

Johnson, Report on American Coals, Washington, 1844; De la Beche and. Playfair, Three Reports on Coal suited to the Steam Navy, London, 1848-49-51; P. W. Brix, On the Heating Power of Fuel used in Prussia, Berlin, 1853; Hartig, Heating Power of Saxon Coal, Dresden, 1860.

The following table of the average results obtained from these investigations shows the number of pounds of water evaporated for every pound of the different kinds of coal burnt.

[table]

The literature relating to coal and coal mining, is very extensive, but the following list includes the titles of the more important works upon these subjects.

England and America.— The Report of the Royal Coal Commission (3 vols., fol., with Atlas, London, 1870). This is the most comprehensive work upon the subject. Hull, Coal Fields of Great Britain (3d ed. London, 1873). Reports and Maps of the Geological Surveys of the United Kingdom. Descriptive memoirs of each coal field published as completed. Percy, Metallurgy, vol. i., on Fuel (2α ed. London, 1875), containing full details of the chemistry of coal. Greenwell, Practical Treatise on Mine Engineering (2d ed. London, 1869). André, Practical Treatise on Coal Mining (London, 1876). Smyth, Coal and Coal Mining (2d ed. London 1872). Jevons, The Coal Question (2d ed. London, 1866). Rogers, Geology of Pennsylvania (2 vols., Edinburgh, 1850). Proceedings of the South Wales Institute of Engineering (8 vols., Merthyr, 1858-73). Transactions of the North of England Institute of Mining Engineers (23 vols., Newcastle, 1852-74). Various Geological Reports of the State a⅛d General Governments of the United States; including Newberry’s Ohio Reports, Cox’s Indiana Reports, and Hayden’s Reports of Geological Survey of the Territories. France and Belgium. —Burat, Géologie de la France (8vo. Paris, 1864). Cours d’Exploitation de Mines (1871). Matériel des Houïllieres en France, &c. (1861-68). Bulletin de la Société de l'industrie Minérale, S. Etienne (20 vols, since 1855). Ponson, Traité de T Exploitation des Mines de Houille (2d ed. Liege, 1868-71). Supplement to the above (1867-72). De Kuyper, Revue Universelle des Mines, <fcc. (Liège, since 1854).

Germany. —Geinitz, Die Steinkohlen Deutschlands, ékc. (3 vols. 4to, Munich, 1865). This is the most complete book on the subject. Zincken, Die Braunkohle (2 vols., Hanover, 1865-71) Zeitschrift fur Berg Hütten und Salinenwesen, dkc. (4to. Berlin, 22 vols, since 1854).