In considering the vast importance of water to plant growth, it must be remembered that seeds must absorb a very large quantity of water before germination can begin; that the growth of the young plant, while still dependent upon the seed, involves the employment of a constant supply of water in order that the transference of nutrients from the stores in the seed to the newly developed parts may proceed without interruption; that soils which do not contain more than 5 to 9 per cent, of moisture will yield none of it to the plant, and that when such low percentages of moisture are approached there is a constant struggle—often fatal to the plant—between the soil and the plant for water; that during the period of the plant’s active growth, the absorption of all mineral matter and all nitrogen compounds from the soil takes place through the medium of an exceedingly weak aqueous solution of these substances, which solution is indeed absorbed in such quantities that a single plant of barley needs the passage through it during the five months in which it occupies the ground of more than an imperial gallon of water. It should be also remembered that all vegetable produce when in a growing state contains an immense proportion of water, often 70 to 80 per cent., and sometimes as much as 92 to 96 per cent., the latter figures representing the percentage of water in turnips and watercress respectively.

From all this it will be readily understood that artificial supplies of water are needed for vegetation in many dry countries. An illustration of this need presents itself in the district which comprises parts of the south of Spain, Portugal, and Italy, including Sicily and Greece. Along this zone, which includes the Mediterranean coasts north of the rainless region of Africa, with its currents of hot dry air from the Sahara, the annual rainfall may be as high as 30 inches, but the amount during the summer quarter is but 4 per cent, of the whole. All the district suffers from droughts, which are often most severe. Again, in many parts of central and eastern Europe there are table-lands, as in Moravia, Poland, and parts of Russia, where the yearly rainfall is insufficient—from 10 to 15 inches only. There are about twice as many rainy days in western as in eastern Europe. In very many of these rainless or arid countries and districts there are remains (mostly in ruins) of important ancient irrigation works; Spain, Sicily, and Syria furnish abundant examples of aqueducts and canals for agricultural irrigation. In Egypt, and in some parts of Persia, India, and China, artificial watering is employed for the reasons given above; while in Peru anti many other parts of America the same scarcity or irregular distribution of rain occurs. Special reference will be made further on to the very important irrigation works of India.

The next point to which reference has been made is the peculiar aquatic or semi-aquatic nature of some of the plants which are grown by means of artificial watering. Rice is the chief example of a plant of this kind; a rice swamp is proverbial, and wherever rice is grown in China, in India, in Japan, in Egypt, or in Italy, the land is underwater till the crop is just ready for harvesting.

The third reason for irrigating mentioned above is the determining cause of nearly all the artificial watering of land in temperate climates. It is not performed because the soil is dry and hot, for it is carried out mainly in the wettest and coldest months of the year. It is not performed because the crop to be raised is of an essentially aquatic nature, for ordinary grasses and meadow herbage only are watered. But it is performed that growth may be stimulated and fed, through certain agencies which the water brings to bear upon the vegetation in question. The watermeadows of England afford examples of this kind of irrigation. These are, in some instances, of immemorial origin, and may, like those of the Avon in Wiltshire and the Churn in Gloucestershire, be traceable back to Roman times. In the early part of the present century the system received further developments, but at present there is some tendency to depreciate the value of this kind of irrigation.

A fourth reason for irrigation is found where the solid matter suspended in the water is valuable and valued for its richness as manure, and for the actual increase which its deposition on the land makes to the height or level of the country. In England this kind of irrigation is practised mainly in the estuary of the Humber. But wherever a decided deposition of fertilizing silt, clay, or mud from water allowed to rest on the land takes place, there “warping,” the name given to this kind of irrigation, may be said to be practised. The waters of the overflowing Nile in Egypt act, partly at least, in this manner, for their dissolved constituents (about 10 grains per gallon) are perfectly insignificant when compared with those which are suspended.

In addition to these various kinds of irrigation with ordinary -water, there are several systems in which town sewage is employed. These involve the introduction of many new and complex conditions, and may be more conveniently considered under the heading Sewage.

It is the irrigation determined by the third of the foregoing reasons—water-meadow irrigation—that calls for more particular notice here. The subject may be conveniently treated in the following order :—quantity of water; quality of water; influence of mining refuse on water - meadows; grasses suitable for water - meadows; changes in irrigated herbage; methods of irrigation, including (1) bedwork irrigation, (2) catch work irrigation, (3) upward irrigation, and (4) warping; management and advantages of water-meadows; theory of irrigation of water-meadows. The article will close with some account of irrigation in India, and in Italy, France, and Belgium, and of the history of irrigation.

Before the systematic conversion of a tract into watermeadows can be safely determined on, care must be taken to have good drainage, natural or artificial, a sufficient supply of water, and water of good quality. It might indeed have been thought that thorough drainage would be unnecessary, but it must be noted that porous subsoils or efficient drains do not act merely by carrying away stagnant water which would otherwise cool the earth, incrust the surface, and retard plant growth. They cause the soil to perform the office of a filter. Thus the earth and the roots of grasses absorb the useful matters not only from the water that passes over it, but from that which passes through it. These fertilizing materials are-found stored up in the soil ready for the use of the roots of the plants. Stagnation of water is inimical to the action of the roots, and does away with the advantageous processes of flowing and percolating currents. Some of the best water-meadows in England have but a thin soil resting on gravel and flints, this constituting a most effectual system of natural drainage. The fall of. the water supply must suffice for a fairly rapid current, say 10 inches or 1 foot in from 100 to 200 yards. If possible the water should be taken so far above the meadows as to have sufficient fall without damming up the river. If a dam be absolutely necessary, care must be taken so to build it as to secure the fields on both sides from possible inundation; and it should be constructed substantially, for the cost of repairing accidents to a weak dam is very serious.

Quantity of Water.— Even were the objects of irrigation always identical, the conditions under which it is carried on are so variable as to preclude calculations of quantity. Mere making up of necessary water in droughty seasons is one thing, protection against frost is another, while the addition of soil material is a third. Amongst causes of variation in the quantity of water needed will be its quality and temperature and rate of flow, the climate, the season, the soil, the subsoil, the artificial drainage, the slope, the aspect, and the crop. In actual practice the amount of water varies from 300 gallons per acre in the hour to no less than 28,000 gallons. Where water is used, as in dry and hot countries, simply as water, less is generally needed than in cold, damp, and northerly climates, where the higher temperature and the action of the water as manure are of more consequence. But it is necessary to be thoroughly assured of a good supply of water before laying out a water-meadow. Except in a few places where unusual dryness of soil and climate indicate the employment of water, even in small quantity, merely to avoid the consequences of drought, irrigation works are not to be commenced upon a large area, if only a part can ever be efficiently watered. The engineer must not decide upon the plan till he has gauged at different seasons the stream which has to supply the water, and has ascertained the raincollecting area available, and the rainfall of the district, as well as the proportion of storable to percolating and evaporating water. Reservoirs for storage, or for equalizing the flow, are rarely resorted to in England; but they are of absolute necessity in those countries in which it is just when there is least water that it is most wanted. It is by no means an injudicious plan before laying out a system of water-meadows, which is intended to be at all extensive, to prepare a small trial plot, to aid in determining a number of questions relating to the nature and quantity of the water, the porosity of the soil, &c.

Quality of Water.— The quality of the water employed for any of the purposes of irrigation is of much importance. Its dissolved and its suspended matters must both be taken into account. Clear water is usually preferable for grass land, thick for arable land. If it is to be used for warping, or in any way for adding to the solid material of the irrigated land, then the nature and amount of the suspended material are necessarily of more importance than the character of the dissolved substances, provided the latter are not positively injurious. For use on ordinary watermeadows or on rice-fields, however, not only is very clear water often found to be perfectly efficient, but water having no more than a few grains of dissolved matter per gallon answers the purposes in view satisfactorily. Water from moors and peat-bogs or from gravel or ferruginous sandstone is generally of small utility so far as plant food is concerned. River water, especially that which has received town sewage, or the drainage of highly manured land, would naturally be considered most suitable for irrigation, but excellent results are obtained also with waters which are uncontaminated with manurial matters, and which contain but 8 or· 10 grains per gallon of the usual dissolved constituents of spring water. Experienced English irrigators generally commend as suitable for watermeadows those streams in which fish and waterweeds abound. But the particular plants present in or near the water-supply afford further indications of quality. Watercress, sweetflag, flowering rush, several potamogetons, water milfoil, water ranunculus, and the reedy sweet watergrass {Glyceria acpιatica) rank amongst the criteria of excellence. Less favourable signs are furnished by such plants as Arundo Donax (in Germany), Cicuta virosa, and Typha latifolia, which are found in stagnant and torpid waters. Water when it has been used for irrigation generally becomes of less value for the same purpose. This occurs with clear water as well as with turbid, and obviously arises mainly from the loss of plant food which occurs when water filters through or trickles over poor soil. By passing over or through rich soil the water may, however, actually be enriched, just as clear water passed through a charcoal filter which has been long used becomes impure. It has been contended that irrigation water suffers no change in composition by use, since by evaporation of a part of the pure water the dissolved matters in the remainder would be so increased as to make up for any matters removed. But it is forgotten that both the plant and the soil enjoy special powers of selective absorption, which remove and fix the better constituents of the water, and leave the less valuable.

The Influence of Mining and other Defuse.— In some of the districts of Devonshire and of Wales, in which the sloping sides of narrow valleys have been converted into small catch work irrigated meadows, the injurious effects of water from mines have been most marked. A stranger visiting the district in early spring would notice, along the sides of a valley, a number of small irrigated fields. Some of these, watered directly from little streams behind and above them, would show grass of great luxuriance, especially close to the main and secondary carriers. But where the river-water, contaminated by mining refuse, had been used, the grass bordering the water-courses would show a sickly yellow tint, and be generally less developed than the herbage of the rest of the field. This difference between the fields irrigated by small local brooks and those watered by the river cannot be explained by any inferiority in the river water as river water; for above the entrance of the refuse from the first mine it was everything that could be wished. But just below the place of entrance of the mine water the grass on the banks looked as if it had been burnt up with vitriol, while in the stream itself not a vestige of a living waterweed could be detected.

The injurious effects thus caused by the mine water have led to its partial disuse for the purposes of irrigation. Some of the most profitable water-meadows are no longer irrigated: the herbage in these is now of inferior character, and mosses and weeds, suppressed by total immersion, have reappeared, to the detriment of the more valuable grasses. Besides, there is now no early feed. Manure, not before wanted, has now to be applied, and the yield of grass is reduced in annual value by 30s. to 60s. per acre. To get a fair growth of grass the plant-food which the water formerly brought at little expense has to be furnished by costly farmyard manure; and even with this the crop is late and light.

It is clear in the particular instance to which reference has been made that water pumped from copper mines, or used in dressing the ores is the origin of the mischief. Several changes in the composition of the water have been found to occur in its passage through the mine. The original water lost much of its free carbonic acid gas; its carbonates were converted into sulphates; and it contained now the metals iron, manganese, cobalt, and aluminium, all as sulphates,—hardly a trace of any of these metals being present in the uninjured water. And matters in suspension were found to be both more abundant and more injurious than matters in solution. They consisted chiefly of copper pyrites and iron pyrites, in a very fine state of division. According to their degree of fineness the pyritic particles which escape from the settling pits at the mines travel varying distances down the stream, and may even be detected several miles below, both in the mud of the bed of the stream and on the leaves of grass and other occasionally immersed plants. Oxidation of the pyrites into the sulphates of copper and iron was proved to occur all through the course of the stream,—these salts, with their concomitant free sulphuric acid, producing a most injurious effect on vegetable growth. The presence of this acid in the free state has been detected in the waste water from a Welsh lead mine, in sufficient abundance to kill instantly, on several occasions, many salmon in the river into which it was discharged. The evil done by some Of the most deleterious materials in mine-waters can be arrested by the interposition of conduits filled with chalk or limestone, which act as chemical filters. The carbonate of lime neutralizes the free sulphuric acid and stops the heavy metals by converting them from soluble sulphates into insoluble carbonates.

Among the most injurious sorts of refuse which can find •their way into streams used for irrigating meadows are the chemical wastes from mills and factories in which the processes of dyeing, paper making, metal working, &c , are carried on. In the majority of such cases the fatal effects on vegetation are obvious, and the rivers polluted in this way, even if their volume of pure water be very large, cannot be used at all for irrigation.

The Seeds for Water-Meadows.— Of the few leguminous plants which are in any degree suitable for water-meadows, Lotus corniculatus major, Trifolium hybridum, and T. pratense are those which generally flourish best: T. repens .is less successful. Amongst grasses the highest place must be assigned to ryegrass, especially to the Italian variety, commonly called Lolium italicum. The mixture of seeds for sowing a water-meadow demands much consideration, and must be modified according to local circumstances of soil, aspect, climate, and drainage. From the peculiar use which is made of the produce of an irrigated meadow, and from the conditions to which it is subjected, it is necessary to include in our mixture of seeds some that produce an early crop, some that give an abundant growth, and some that impart sweetness and good flavour, while all the kinds sown must be capable of flourishing on .irrigated soil.

The following mixtures of seeds (stated in pounds per acre) have been recommended for sowing on water-meadows, Messrs Sutton of Reading, after considerable experience, (regarding No. I. as the more suitable :— [table]

Changes in Irrigated Herbage.— In irrigated meadows, though in a less degree than on sewaged land, the reduction of the amount or even the actual suppression of certain species of plants is occasionally well-marked. Sometimes this action is exerted upon the finer grasses, but happily also upon some of the less profitable constituents of the miscellaneous herbage. Thus Ranunculus bulbosus has been observed to become quite rare after a few years watering of a meadow in which it had been most abundant, R. acris rather increasing by the same treatment; Plantago media was extinguished and P. lanceolata reduced 70 per cent. Amongst the grasses which may be spared, Aira caespitosa, Bmza media, and Cynosurus cristatus are generally much reduced by irrigation. Useful grasses which are increased are Lolium perenne and Alopecumιs pratensis, and among those of less value Avena fαvesceηs, Dactylis glomerata, and Poa pratensis. Methods of Irrigation.— There are four ways of irrigating land with water practised in England :—(1) bed work irrigation, which is the most efficient although it is also the most costly method by which currents of water can be applied to level land; (2) catchwork irrigation, in which the same water is caught and used repeatedly ; (3) subterraneous or rather upward irrigation, in which the water in the drains is sent upwards through the soil towards the surface; and (4) warping, in which the water is allowed to stand over a level field until it has deposited the mud suspended in it.

There are two things to be attended to most carefully in the construction of a water-meadow on the first or second of these plans. First, no portion of them whatever should be on a dead level, but every part should belong to one or other of a series of true inclined planes. The second point of primary importance is the size and slope of the main conductor, which brings the water from the river to the meadow. The size of this depends...