Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/271

NAVIGATION 259 shipmen, 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 described 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 will explain what latitude and longi tude are, and show a method for finding the latter different from what has been taught. He will show the error of middle latitude sailing, and show 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 they should strain at such a trifle as that is in low latitudes. ) He promised to find the difference of longitude without a departure (a similar rule to that of Bond). After speaking of meridional parts, he offered 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 was not the first.) He gives instructions for currents and lee way, tables of declination, a few stars, meridional parts, &c. It is worthy of remark that, after giving a form for a log-book, he added that this had not been previously kept by any one, but he thought it should not be trusted to memory. He only required the knots, fathoms, course, wind, and leeway to be marked every two hours. Every hour is quite long enough, and that is often divided now. He gave a sketch of Hadley s quadrant, in shape like those in use fifty years back, but without a clamping screw or tangent screw. Back glasses were much valued in those days, the force of habit, no doubt. The book is quite free from all extraneous rubbish. 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 was no novelty. HTJYGENS (q.v.) made pendulum watches for the purpose prior to 1665, at which date Major Holmes communicated to the Royal Society (Phil. Trans., i. 13) the fact of his having tried two of them on the coast of Guinea. He sailed from St Thomas, set his watches, sailed west 700 or 800 leagues, without changing course, then steered towards the coast of Africa N.N.E. 200 or 300 leagues. The masters of the other ships under his charge, fearing the want of water, wished to steer for Barbados. Holmes, on com paring the calculations, found them to differ from him from 80 to over 100 leagues. He considered that they were only 30 leagues from the Cape Verd Islands, where they arrived next afternoon. The vague manner of estimat ing distance is worth notice. 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 with the sun-dial. In 1714 Henry Sully, an Englishman, pub lished 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, and 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 com pensation through the unequal contraction of two metals, which he first applied in the invention of the compensation 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. To what has been said in the article HARBISON on his successive attempts, and the success of the trial journey to Jamaica in 1761-62, it may be added that by the journal of the House of Commons we find that the error of the watch (as if there were only one) was ascertained by equal altitudes at Portsmouth and Barbados, the calculations being made by Short. The watch came greatly within the limits of the Act. At Jamaica it was only in error five seconds (assuming that the longitude previously found j by the transit of Mercury could be so closely depended on, | which as we now know, was not the case; the observations were too few in number, and taken with an untrustworthy instrument). Short found the whole error from Novem ber 6, 1761, till April 2, 1762, to be l m 54 S 5 = 18 geographical miles in the latitude of Portsmouth. He considered that a position determined by a transit of Mercury was liable to an error of 30 s only, and by Jupiter s best satellite to 3 m 44 s . 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 l m 49 s = 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 1 s a-day gaining, as he anticipated that it would go slower by I 8 for every 10 increase in tempera ture. The longitude of Bridgetown was determined by Maskelyne and Green by nine emersions of Jupiter s first satellite, against five of Bradley s and two at Greenwich Observatory, to be 3 h 54 m 20 s west of Greenwich. In February 1765 the commissioners of longitude expressed an opinion that the trial was satisfactory, but required the principles to be disclosed and other watches made. Half the great reward was paid to Harrison under Act of Parliament in this year, and he and his son gave full de scriptions and drawings, upon oath, to seven persons appointed by the commissioners of longitude. 1 The other half of the great reward was promised to Harrison when he had made other timekeepers to the satisfaction of the commissioners, and provided he gave up everything to them within six months. The second half was not paid till 1773, after trials had been made with five watches. These trials were partly made at Greenwich by Maskelyne, who, as we shall see, was a great advocate of lunars, and was not ready to admit more than a subsidiary value to the watch. A bitter controversy arose, and Harrison in 1767 published a book in which he charges Maskelyne with exposing his watch to unfair treatment. The feud between the astronomer-royal and the watchmakers con tinued long after this date. Even after Harrison had received his 20,000, doubts were felt as to the certainty of his achievement, and fresh rewards were offered in 1774 both for timekeepers and for improved lunar tables or other methods. But the tests proposed for timekeepers were very discouraging, and the watchmakers complained that this was due to Maskelyne. A fierce attack on the astronomer s treatment of himself and other watchmakers was made by Mudge in 1792, in A Narrative of Facts, addressed to the first lord of the Admiralty, and Maskelyne s reply does not convey the conviction that full justice was done to timekeepers. Maskelyne at this date still says that he would prefer an eclipse of a bright star by the moon and a number of cor respondent observations by transits of the moon com pared with those of fixed stars, made by two astronomers at remote places, to any timekeeper. The details of these 1 The explanations and drawings are now at the British Museum; and two of his watches, one of which was used by Captain Cook in the "Resolution," are at Greenwich Observatory. In 1767 Harrison estimates that a watch could be made for 100, and ultimately for 70 or 80.