Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/269

Rh NAVIGATION 257 these were published by Dominic Cassini at Bologna in 1688, and were repeated in a more correct form at Paris in 1693 by his son, who was followed by Pound, Bradley, Wargentin, and many other astronomers. But this method, though useful on land, is not suited to mariners ; when Whiston, for example, in 1737 recommended that the satellites should be observed by a reflecting telescope, he did not sufficiently consider the difficulty of using a telescope at sea, or the infrequency of the occultations, and it is the lunar problem which will chiefly concern us in what follows. The study of this problem was stimulated by the reward of 1000 crowns offered by Philip III. of Spain in 1598 ; the states-general followed with an offer of 10,000 florins. But for a long time nothing practical came of this ; a pro posal by J. Morin, submitted to Richelieu in 1633, was pronounced by commissioners appointed to judge of it to be incomplete through the imperfection of the lunar tables, and in like manner when the question was raised in England in 1674 by a proposal of St Piere to find the longitude by using the altitudes of the moon and two stars to find the time each was from the meridian, and when the king was pressed by St Piere, Sir J. Moore, and Sir C. Wren to establish an observatory for the benefit of naviga tion, and especially that the moon s exact position might be calculated a year in advance, Flamsteed gave his judg ment that the lunar tables then in use were quite useless, and the positions of the stars erroneous. The result was that the king decided upon establishing an observatory in Greenwich Park, and Flamsteed was appointed astro nomical observer on March 4, 1675, upon a salary of 100 a year, for which also he was to instruct two boys from Christ s Hospital. While the small building in the Park was in course of erection he resided in the Queen s House (now the central part of the Greenwich Hospital school), and removed to the house on the hill, July 10, 1676, which came to be known as &quot;Flamsteed House.&quot; The institution was placed under the surveyor-general of ordnance, perhaps because that office was then held by Sir Jonas Moore, himself an eminent mathematician. Though this was not the first observatory in Europe, it was destined to become the most useful, and has fulfilled the important duties for which it was established. It was established to meet the exigencies of navigation, as was. clearly stated on the appointment of Flamsteed, and on several subsequent occasions ; and we see now what an ex cellent foster-mother it has been to the higher branches of that science. This has been accomplished by much labour and patience ; for, though the most suitable man in the kingdom was placed in charge, it was so starved and neglected that it was almost useless during many years. The Government did not provide a. single instrument. Flamsteed entered upon his important duties with an iron sextant of 7 feet radius, a quadrant of 3 feet radius, two telescopes, and two clocks, the last given by Sir Jonas Moore. Tycho Brahe s catalogue of about a thousand stars was his only guide. In 1681 he fitted a mural arc which proved a failure. Seven years after another mural arc was erected at a cost of 120, with which he set to work in earnest to verify the latitude, and to determine the equinoctial point, the obliquity of the ecliptic, the right ascension and declination of the stars, till he numbered two or three thousand which appeared in the &quot; British catalogue.&quot; See FLAMSTEED and ASTRONOMY. Flamsteed died in 1720, and was succeeded by Halley, who paid particular attention to the motions of the moon with a view to the longitude problem. A paper which he published, in the Phil. Trans., 1731, No. 421, shows what had been accomplished up to that date, and proves that it was still impossible to find the longitude correctly by the moon. 1 He repeats what he had published twenty years before in an appendix to Street s Caroline tables, which contained observations made by him (Halley) in 1683-84 for ascertaining the moon s motion, which he thought to be the only practical method of &quot; attaining &quot; the longitude at sea. He found that it only needed a little patience to be able to manage a 5 or 6 foot telescope, capable of showing the occultations of the fixed stars by the moon, on shipboard in moderately fine weather (ships must have been very steady in his days). The Caroline tables of Street, though better than those before him, as well as those of Tycho, Kepler, Bullialdus, and Horrox, were uncertain ; sometimes the equations would compensate one another ; at other times when they fell the same way the result might be 100 leagues in error. He hopes that the tables will be so amended that an error may scarce ever exceed 3 minutes of motion (equal to 1^ of longitude). Sir Isaac Newton s tables, corrected by himself (Halley) and others up to 1713, would admit of errors of 5 minutes, when the moon was in the third and fourth quarters. He blames Flamsteed for neglecting that portion of the work, as he was at the observatory more than two periods of eighteen years.- He himself had at this time seen the whole period of the moon s apogee less than nine years during which period he observed the right ascension of the moon at her transit, with great exactness, almost fifteen hundred times, or as often as Tycho Brahe, Hevelius, and Flamsteed together. He hoped during 1731 and a few future years to com pute the moon s position within 2 minutes of motion with certainty, which will be equal to 20 leagues at the equator and 1 5 in the Channel ; and Hadley s quadrant might be applied to taking distances at sea with the desired accuracy. 2 The last remark calls us to consider this great improve-- ment in the instrumental means for taking observations at sea. Up to this time all instruments either depended on a plumb line or required the observer to look in two directions at once. Their imperfections are clearly pointed out in a paper by Pierre Bouguer (1729) which o received the prize of E / the Paris Academy of Sciences for the best method of taking the altitude of stars at sea. Bouguer himself pro poses a modification of what he calls the English quadrant, pro bably the one proposed by Wright and im proved by Davis. Fig. u r n 11 represents the in strument as proposed, capable of measuring fully 90 from E to N. A fixed pinule was recommended to be placed at E, through which a ray from the sun would pass to the sight C. The sight F was movable. The observer, standing with his back to the sun, 1 This did not prevent many attempts being made. Thus John Herne s Longitude Unvailed, 1678, proposes to find the moon s meridian passage at sea by equal altitudes with the cross-staff, and then compare apparent time at ship with London time. In 1717 Henry Kindon proposed both the magnet and meridional lunar distances ; Kobert Browne attempted to calculate lunar tables, which he submitted to the board of longitude; while Francis Bodkin proposed the use of telescopes upon the planets and their satellites. The proposals submitted to the board of longitude to find the longitude by the moon s meridian passage are too numerous to be mentioned. 2 Halley s observations were published posthumously in 1742, and in 1765 the commissioners of longitude paid his daughter 100 for MS. supposed to be useful to navigation. XVII. - 33