NAVIGATION is the art of conducting a ship across the ocean. It is here treated to the exclusion of seamanship, which forms a distinct subject. The present article will give, first, a view of the history of the art from the time of the epoch-making voyages of Columbus and the Portuguese, with special reference to advances made in England, and then a sketch of practical navigation as the art now stands.
The backward state of navigation at this time is best understood from a sketch of the few rude appliances which the mariner had. He had a compass, a cross-staff or astrolabe, a moderately good table of the sun’s declination, a correction for the altitude of the pole star, and occasionally a very incorrect chart. The first map or sea chart seen in England was brought by Bartholomew Columbus in 1489, and the first map of England was made in 1520. Decimal arithmetic was invented by Simon Stevin about the end of the 16th century. Watches were unknown till 1530, and immediately Gemma Frizon or Frisius seized the idea for the purpose of ascertaining the difference of longitude between two places. They were too rough to be of use, and their advocate proposed to correct them by water-clocks or sand-clocks. Almanacs were first published in Poland in 1470, and in London three years later. These contained tables of the sun’s declination and that of many of the stars, and tables for finding the latitude by the pole star and the “pointers.” There was not till 1607 any means whatever of measuring a ship’s progress through the water, and none in general use till twenty or thirty years later (see Log).
The “cross-staff” appears to have been used by astronomers at a very early period for measuring heights and distances, more recently by seamen for measuring altitudes. It was one of the few instruments possessed by Columbus and Vasco da Gama. The old cross-staff, called by the Spaniards “ballestilla,” consisted of two light battens. The part we may call the staff was about an inch and a half square and 36 inches long. The cross was made to fit closely and to slide upon the staff at right angles; its length was a little over 26 inches, so as to allow the “pinnies” or sights to be placed exactly 26 inches apart. A sight was also fixed on the end of the staff for the eye to peep through at the other two sights and objects to be measured. It was made by describing the angles on a table, and laying the staff upon it (fig. 1). The scale of degrees was marked on. the upper face. Afterwards shorter crosses were introduced, so that smaller angles could be taken by the same instrument. These angles were marked on the sides of the staff.
Another primitive instrument in common use at the beginning of the 16th century was the astrolabe, which was more convenient than the cross-staff for taking altitudes, but was incapable of measuring distances. Fig. 2 represents an astrolabe as described by Martin Cortes. It was made of copper or tin, about one-fourth of an inch in thickness and 6 or 7 inches in diameter, and was perfectly circular except at one place, where a projection was provided for a hole by which it was suspended. Weight was considered desirable in order to keep it steady when in use. The face of the metal having been well polished, a plumb line from the point of suspension marked the vertical line, which when carefully subdivided gave the horizontal line and centre. The upper left quadrant was divided into degrees. The second part was a pointer pt of the same metal and same thickness as the circular plate, about 1∣ inches wide, and in length equal to the diameter of the circle. The centre was bored, and a line was drawn across it the full length, which was called the line of confidence. On the ends of that line were fixed plates s, s, having each a larger and a smaller hole, both exactly over the line of confidence, as sights for the sun or stars. The pointer moved upon a centre the size of a goose quill. When the instrument was suspended the pointer was directed by hand to the object, and the angle read on the one quadrant only. Some years later the other quadrant was also graduated, to give the benefit of a second reading.
Among the earliest writers who touched upon navigation was John Werner of Nuremberg, who in 1514, in his notes upon Ptolemy’s geography, describes the cross-staff as a very ancient instrument, but says that it was only then beginning to be introduced among seamen. He recommends measuring the distance between the moon and a star as a means of ascertaining the longitude.
Thirty-eight years after the discovery of America, when long voyages had become comparatively common, Gemma Frisius wrote upon astronomy and cosmogony, with the use of the globes. His book comprehended much valuable information to mariners of that day, and was translated into French fifty years later (1582) by Claude de Bossière. The system adopted is that of Ptolemy. The following are some of the points of interest for the subject before us. There is a good description of the sphere and its circles; the obliquity of the ecliptic is given as 23° 30'. The distance between the meridians is to be measured on the equator, allowing 15° to an hour of time; longitude is to be found by eclipses of the moon and conjunctions, and reckoned from the Fortunate Islands (Azores). Latitude should be measured from the equator, not from the ecliptic, “as Clarean says.” The use of globes is very thoroughly and correctly explained. The scale for measuring distances was placed on the equator, and 15 German leagues, or 60 Italian leagues, were to be considered equal to one degree. The Italian league was 8 stadia, or 1000 paces, therefore the degree is taken much too small. We are told that, on plane charts, mariners drew lines from various centres (i.e., compass courses), which were very useful since the virtue of the loadstone had become known; it must be remembered that parallel rulers were unknown. Such a confusion of lines has been continued upon sea charts till very recently. Frisius gives rules for finding the course and distance correctly, except that he treats difference of longitude as departure. For instance, if the difference of latitude and difference of longitude are equal, the course prescribed is between the two principal winds,—that is, 45°. He points out that the courses thus followed are not straight lines, but curved, because they do not follow the great circle, and that distances could be more correctly measured on the globe. The tide is said to rise with the moon, high water being when it is on the meridian and nadir. From a table of latitudes and longitudes a few examples are here selected, by which it appears that even the latitude was much in error. The figures in brackets represent the positions according to modern tables, counting the longitude from the western extremity of St Michael. Flores is 5° 8' farther west.
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In 1534 Gemma produced an “astronomical ring, which he dedicated to the secretary of the king of Hungary. He admitted that it was not entirely his own invention, but asserted that it could accomplish all that has been said of quadrants, cylinders, and astrolabes,—also that it was a pretty ornament, worthy of a prince. As it displayed great ingenuity, and was followed by many similar contrivances during two centuries, a sketch is here given (fig. 3). The description must necessarily be brief.
The outer and principal sustaining circle EPQπ represents the meridian, and is about 6 inches in diameter; P, π are the poles. The upper quadrant is divided into degrees. It is suspended by a fine cord or wire placed at the supposed latitude. The second circle EQ is fixed at right angles to the first, and represents the equinoctial line. The upper side is divided into twenty-four parts, representing the hours from noon or midnight. On the inner side of that circle are marked the months and weeks. The third ring CO is attached to the first at the poles, and revolves freely within it. On the interior are marked the months, and on another side the corresponding signs of the zodiac; another is graduated in degrees. It is fitted with a groove which carries two movable sights. On the fourth side are twenty-four unequal divisions (tangents) for measuring heights and distances. Its use is illustrated by twenty problems; it can do roughly all that any instrument for taking angles can. Thus, to find the latitude, set the sights C, C to the place of the sun in the zodiac, and shut the circle till it corresponds with 12 o’clock. Peep through the sights and alter the point of suspension till the greatest elevation is attained; that time will be noon, and the point of suspension will be the latitude. The figure is slung at lat. 40°, either north or south. To find the hour of the day, the latitude and declination being known: the sights C, C being set to the declination as before, and the suspension on the latitude, turn the ring CO freely till it points to the sun, when the index opposite the equinoctial circle will indicate the time, while the meridional circle will coincide with the meridian of the place. In this we may see the germ of the “equatorial” now used in the principal observatories.
There is at present in the museum attached to the Royal Naval College at Greenwich an instrument, ticketed as Sir Francis Drake’s astrolabe, prior to 1570. It is not an astrolabe, but may be a combination of astronomical rings as invented by Gemma with other things, probably of a later date. It has the appearance of a large gold watch, about 21 inches in diameter, and contains several parts which fall back on hinges. One part is a sun-dial, the gnomon being in connexion with a graduated quadrant, by which it could be set to the latitude of the place. There are a small compass and an hour circle. It is very neat, but too small for actual use; it may be simply an ornament representing a larger instrument. There is a table of latitudes engraved inside one lid; that given for London is 51° 34', about three miles too much.
In 1537 Pedro Nunez (Nonius), cosmographer to the king of Portugal, published a work on astronomy, charts, and some points of navigation. He recognized the errors in plane charts, and tried to rectify them. Among many astronomical problems is one for finding the latitude of a place by knowing the sun’s declination and the altitude when on two bearings, not less than 40° asunder. Gemma did a similar thing with two stars; therefore the problem now known as a “double altitude” is a very old one. They could do it by the globe within a degree. To Nuiïez has been erroneously attributed the present mode of reading the exact angle on a sextant, the scale of a barometer, and various other things, the credit of which is due to Vernier nearly a hundred years later. The mode of dividing the scale which Nunez published in 1542 was the following. The arc of a large quadrant was furnished with forty-five concentric segments, or scales, the outer one graduated to 90°, the others to 89, 88, 87, <fcc., divisions. As the fine edge of the pointer attached to the sights passed among those numerous divisions it touched one of them, suppose the fifteenth division on the sixth scale, then the angle was ⅜i of 90° = 15° 52' 56". This was a laborious method; Tycho Brahe tried it, but abandoned it in favour of the diagonal lines then in common use, and still found on all scales of equal parts.
In 1545 Dr Pedro de Medina published The Art of Navigation at Valladolid, dedicated to Don Philippo, prince of Spain. This appears to be the first book ever published professedly on navigation. It was soon translated into French and Italian, and many years after into English by John Frampton. Though this pretentious work came out two years after the death of Copernicus, the astronomy is still that of Ptolemy. The general appearance of the chart of the Mediterranean, Atlantic, and part of the Pacific is in its favour, but examination shows it to be very incorrect. A scale of equal parts, near the centre of the chart, extends from the equator to what is intended to represent 75° of latitude; by this scale London would be in 55° instead of 51∣°, Lisbon in 37∣ o instead of 38° 42'. The equator is made to pass along the coast of Guinea, instead of being over four degrees farther south. The Gulf of Guinea extends 14 degrees too far east, and Mexico is much too far west. Though there are many vertical lines on the chart at unequal distances they do not represent meridians; there is no indication of longitude. A scale of 600 leagues is given (German leagues, fifteen to a degree). By this scale the distance between Lisbon and the city of Mexico is 1740 leagues, or 6960 miles; by the vertical scale of degrees it would be about the same; whereas the actual distance is 4820 miles. Here two great wants become apparent,—a knowledge of the actual length of any arc, and the means of representing the surface of the globe on flat paper. There is a table of the sun’s declination to minutes; on June 12th and December 11th (o.s.) it was 23° 33'. The directions for finding the latitude by the pole star and pointers appear good. For general astronomical information the book is inferior to that of Gemma.
In 1556 Martin Cortes published at Seville The Art of Navigation. He gives a good drawing of the cross-staff and astrolabe, also a table of the sun’s declination for four years (the greatest being 23° 33'), and a calendar of saints’ days. The motions of the heavens are described according to the notions then prevalent, the earth being viewed as fixed. He recommends the height of the pole being found frequently, as the estimated distance run was imperfect. He devised an instrument whereby to tell the hour, the direction of the ship’s head, and where the sun would set. A very correct table is given of the distances between the meridians at every degree of latitude, whereby a seaman could easily reduce the difference of longitude to departure. In the rules for finding the latitude by the pole star, the star is supposed to be then 3° from the pole; it is now (1883) 1° 18' 54". Martin Cortes attributes the tides entirely to the influence of the moon, and gives instructions for finding the time of high water at Cadiz, when by means of a card with the moon’s age on it, revolving within a circle showing the hours and minutes, the time of high water at the place for which it was set would be indicated. He deplores the loss of the earl of Niebla and other valiant captains of Spain, before Gibraltar, in 1436, because the mariners kept no account of the tides. In this instance it was more probably the effect of current. There is a chapter upon the signs which prognosticate fair or foul weather, from Pliny and Aristotle; another upon “shining exhalations,” the “fire of St Elmo,” and other old superstitions. Directions are given for making a compass similar to those then in common use, also for ascertaining and allowing for the variation. The east is here spoken of as the principal point, and marked by a cross, after that the true north.
There is a table of difference of latitude and departure in proportion to the tangent of the course.
The third part of Martin Cortes’s work is upon charts; he laments that wise men do not produce some that are correct, and that pilots and mariners will use plane charts which are not true. In the Mediterranean and Channel of Flanders the want of good charts is (he says) less inconvenient, as there they do not navigate by the altitude of the pole.
As some subsequent writers have attributed to Cortes the credit of first thinking of the enlargement of the degrees of latitude on Mercator’s principle, his precise words may be cited. In making a chart, it is recommended to choose a well-known place near the centre of the intended chart, such as Cape St Vincent, which call 37°, “and from thence towards the Arctic pole the degrees increase; and from thence to the equinoctial line they go on decreasing, and from the line to the Antarctic pole increasing.” It would appear at first sight that the degrees increased in size as well as being called by a higher number, but a specimen chart in the book does not justify that conclusion. It is from 34° to 40°, and the divisions are unequal, but evidently by accident, as the highest and lowest are the largest. He states that the Spanish scale was formed by counting the Great Berling as 3° from Cape St Vincent (it is under 2∣ o ). Twenty English leagues are equal to 17⅜ Spanish or 25 French, and to 1° of latitude. Cortes was evidently at a loss to know the size of a degree, and consequently the circumference of the globe. The degrees of longitude are not laid down, but for a first meridian we are told to draw a vertical line “through the Azores, or nearer Spain, where the chart is less occupied.” It is impossible under such circumstances to understand or check the longitudes assigned to places at that period. Martin Cortes’s work was held in high estimation in England for many years, and appeared in several translations. One by Richard Eden in 1609 gives an improved table of the sun’s declination from 1609 to 1625—the greatest declination being 23° 30'. The declinations of the principal stars and the times of their passing the meridian, and other improved tables, are given, with a very poor traverse table for eight points. The cross-staff, he said, was in most common use; but he recommends Wright’s sea quadrant.
William Cuningham published in 1559 a book called his Astronomical Glass, in which he teaches the making of charts by a central meridional line of latitude in equal parts, with other meridians on each side, distant at top and bottom in proportion to the departure at the highest and lowest latitude, for which purpose a table of departures is given very correctly to the third place of sexagesimals. The chart would be excellent were it not that the parallels are drawn straight instead of being curved. In another example, which is one-fourth of the sphere, the meridians and parallels are all curved; it would be good were it not that the former are too long. The hemisphere is also shown upon a projection approaching the stereographic; but the eighteen meridians cut the equator at equal distances, instead of being smaller towards the primitive. He gives the drawing of an instrument like an astrolabe placed horizontally, divided into 32 points and 360 degrees, and carrying a small magnetic needle to be used as a prismatic compass, or even as a theodolite (fig. 4). A sketch is given of Ptolemy observing the sun with a primitive instrument, likely from its great size to give good results...
