Page:EB1911 - Volume 19.djvu/306

 wrecked through mistaking the Deadman for Berry Head. Admiral Wheeler’s squadron in 1694, leaving the Mediterranean, ran on Gibraltar when they thought they had passed the Strait. Sir Cloudesley Shovel’s squadron, in 1707, was lost on the rocks off Scilly, by erring in their latitude. Several transports, in 1711, were lost near the river St Lawrence, having erred 15 leagues in the reckoning during twenty-four-hours. Lord Belhaven was lost on the Lizard on the 17th of November 1721, the same day on which he sailed from Plymouth.

Many rewards were paid by the commissioners for methods by which the tedious calculations involved in “clearing the lunar distance” could be abbreviated; thus Israel Lyons (1739–1775) received £10 for his solution of this problem from the commissioners in 1769; and in 1772 he and Richard Dunthorne (1711–1775) each obtained £50. George Whichell, master of the Royal Naval Academy, Portsmouth, conceived a plan whereby the correction could be taken from a table by inspection. In October 1765 the commissioners of longitude awarded him £100 to enable him to complete and print 1000 copies of his table. On the following April they gave him £200 more. The work was continued on the same plan by Antony Shepherd, the Plumian professor of astronomy, Cambridge, with some additions by the astronomer-royal. The total cost of the ponderous 4to volume up to the time of publication in June 1772 was £3100, after which £200 more was paid to the Rev. Thomas Parkinson and Israel Lyons for examining the errata. It was a very large and expensive volume—ill-adapted for ship’s use. Considerable sums were paid by the commissioners from time to time for other tables to facilitate navigation—not always very judiciously. It is sufficient to mention here the tables of Michael Taylor and those of Mendoza, published in 1815. The proposals submitted to the board to find the longitude by the time of the moon’s meridian passage are very numerous.

One of the first points to which the attention of the commissioners was directed was the survey of the coasts of Great Britain, which was pressed on them by Whiston in 1737. He was appointed surveyor of coasts and headlands, and in 1741 received a grant for instruments. An act passed in 1740 enabled the commissioners to spend money on the survey of the coasts of Great Britain and the “plantations.” At a later date they bore part of the expenses of Cook’s scientific voyages, and of the publication of their results. Indeed it is to them that we owe all that was done by England for surveys of coasts, both at home and abroad, prior to the establishment of the hydrographic department of the Admiralty in 1795. But their chief work lay in the encouragement they gave on the one hand to the improvement of timepieces, and on the other to the perfecting of astronomical tables and methods, the latter being published from time to time in the Nautical Almanac. Before we pass on to these two important topics we may with advantage take a view of the state of practical navigation in the middle of the 18th century as shown in two of the principal treatises then current.

John Robertson’s Elements of Navigation passed through six editions between 1755 and 1796. It contains good teaching on arithmetic, geometry, spherical trigonometry, astronomy, geography, winds and tides, also a small useful table for correcting the middle time between the equal altitudes of the sun—all good, as is also the remark that “the greater the moon’s meridian altitude the greater generally the tides will be.” He states that Lacaille recommends equal altitudes being observed and worked separately, in order to find the time from noon, and the mean of the results taken as the truth. There is a sound article on chronology, the ancient and modern modes of reckoning time. A long list of latitudes, longitudes and times of high water finishes vol. i. The second volume is said by the author to treat of navigation mechanical and theoretical; by the former he means seamanship. He gives instructions for all kinds of sailings, for marine surveying and making Mercator’s chart. There are two good traverse tables, one to quarter points, the other to every 15 minutes of arc; the distance to each is 120 m. There is a table of meridional parts to minutes, which is more minute than customary. Book ix., upon what is now called “the day’s work,” or dead-reckoning, appears to embrace all that is necessary. A great many methods, we are told, were then used for measuring a ship’s rate of sailing, but among the English the log and line with a half-minute glass were generally used. Bouguer and Lacaille proposed a log with a diver to avoid the drift motion (1753 and 1760). Robertson’s rule of computing the equation of equal altitudes is as good as any used at the present day. He gives also a description of an equal-altitude instrument, having three horizontal wires, probably such as was used at Portsmouth for testing Harrison’s timekeeper. The mechanical difficulties must have been great in preserving a perpendicular stem and a truly horizontal sweep for the telescope. It gave place to the improved sextant and artificial horizon. The second edition of Robertson’s work in 1764 contains an excellent dissertation on the rise and progress of modern navigation by Dr James Wilson, which has been greatly used by all subsequent writers.

Don Jorge Juan’s Compendio de Navegacion, for the use of midshipmen, was published at Cadiz in 1757. Chapter i. explains what pilotage is, practical and theoretical. He speaks of the change of variation, “which sailors have not believed and do not believe now.” He describes the lead, log and sand-glass, the latter corrected by a pendulum, charts plane and spherical. Supposing his readers to be versed in trigonometry, he explains what latitude and longitude are, and shows a method for finding the latter different from what has been taught. He explains the error of middle latitude sailing, and shows that the longitude found by it is always less than the truth. (It is strange that while reckoning was so rough and imperfect in many respects such a trifle as that is in low latitudes should be noticed.) After speaking of meridional parts, he offers to explain the English method, which was discovered by Edmund Halley, but omits the principles upon which Halley founded his theory, as it was “too embarrassing.” He gives instructions for allowing for currents and leeway, tables of declination, positions of a few stars, meridional parts, &c. It is worthy of remark that, after giving a form for a log-book, he adds that this had not been previously kept by any one, but he thought it should not be trusted to memory. He only requires the knots, fathoms, course, wind and leeway to be marked every two hours. He gives a sketch of Halley’s quadrant, but without a clamping screw or tangent screw.

To ascertain local time at sea by astronomical observations by the altitude of suitably-situated heavenly bodies was an old, well-known and frequently practised operation, so that a comparison could thus be easily made between such local time and the Greenwich time if known at the same instant. The introduction of timekeepers by which Greenwich time can be carried to any part of the world, and the longitude found with ease, simplicity and certainty is due to the invention of John Harrison.

The idea of keeping time at sea by watches was no novelty, but the practical difficulty arose from their very irregular rates owing to changes of temperature and the motion of the ship. Huygens had applied pendulums to the regulation of clocks on shore in 1656, and in 1675 his application of spiral springs as regulators of watches made them available for use at sea. William Derham published a scientific description of various kinds of timekeepers in The Artificial Clock-Maker, in 1700, with a table of equations from Flamsteed to facilitate comparison of mean time with that shown by the sun-dial or apparent time. In 1714 Henry Sully, an Englishman, published a treatise at Vienna, on finding time artificially. He went to France, and spent the rest of his life in trying to make a timekeeper for the discovery of the longitude at sea. In 1716 he presented a watch of his own make to the Academy of Sciences, which was approved; and ten years later he went to Bordeaux to try his marine watches, but died before embarking. Julien le Roy was his scholar, and perfected many of his inventions in watchmaking.

Harrison’s great invention was the principle of compensation through the unequal contraction of two metals, which he first applied in the invention in 1726 of the compensation (gridiron) pendulum, still in use, and then modified so as to fit it to a watch, devising at the same time a means by which the watch retains its motion while being wound up. With regard to the success of the trial journey (see ) to Jamaica in 1761–1762, it may be noted that by the journal of the House of Commons we find that the error of the watch was ascertained by equal altitudes at Portsmouth and Barbados, the calculations being made by Short; these errors came greatly within the limits of the act. At Jamaica the watch was only in error five seconds (assuming that the longitude previously found by the transit of Mercury could be closely depended) on, which as we now know, was not the case, the observations being too few in number, and taken with an untrustworthy instrument). Short at Portsmouth found the whole unallowed-for error from November 6th, 1761, till April 2nd, 1762, to be 1m54s.5＝18 geographical miles in the latitude of Portsmouth. During the passage home in the “Merlin” sloop-of-war the timekeeper was placed in the after part of the ship, because it was the dryest place, and there it received violent shocks which retarded its motion. It lost on the voyage home 1m 49s＝16 geographical miles.

One might have supposed that Harrison had now secured the prize; but there were powerful competitors who hoped to gain it by lunars, and a bill was passed through the House in 1763 which left an open chance for a lunarian during four years. A second West Indies trial of the watch took place between November 1763 and March 1764, in a voyage to Barbados, which occupied four months; during which time it is said, in the preamble to act 5 Geo. III. 1765, not to have erred 10 geographical miles in longitude. We only find in the public records the equal altitudes taken at Portsmouth and at Bridgetown, Barbados. William Harrison assumed an average rate of 1s a-day gaining, and he anticipated that it would go slower by 1s for every 10° increase in temperature. The longitude of Bridgetown was determined by N. Maskelyne and C. Green by nine emersions of Jupiter’s first satellite, against five of Bradley’s and