designed. It was established to meet the exigencies of navigation, as was clearly stated on the appointment of Flamsteed, and on several subsequent occasions; we see now what an excellent foster-mother it has been to the higher branches of that science. This has been accomplished by much labour and patience; for, though originally the most suitable man in the kingdom was placed in charge, it was so starved and neglected as to be 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 ft. radius, a quadrant of 3 ft. radius, two telescopes and two clocks, the last given by Sir Jonas Moore. Tycho Brahe’s catalogue of 777 stars, formed in about 1590, 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 position of the equinoctial point, the obliquity of the ecliptic and the right ascensions and declinations of the stars; he obtained the positions of 2884 which appeared in the “British catalogue” in 1723 (see Flamsteed, and Astronomy).
Flamsteed died in 1719, 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) shows what had been accomplished up to that date, and proves that it was still impossible to find the longitude correctly by any observation depending upon the predicted position of the moon. He repeats what he had published twenty years before in an appendix to Thomas Street’s Caroline tables, which contained observations made by him (Halley) in 1683–1684 for ascertaining the moon’s motion, which he thought to be the only practical method of “attaining” the longitude at sea. The Caroline tables of Street, though better than those before his time as well as those of Tycho, Kepler, Bullialdus and Horrox, were uncertain; sometimes the errors would compensate one another; at others when they fell the same way the result might lead to a position being 100 leagues in error. He hopes that the tables will be so amended that an error may scarce ever exceed 3 minutes of arc (equal to 112° 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 astronomical 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 he observed the right ascensions at her transit, with great exactness, almost fifteen hundred times, or as often as Tycho Brahe, Hevelius and Flamsteed together. He hoped to be able to compute the moon’s position within 2 minutes of arc with certainty, which would reduce errors of position to 20 leagues at the equator and 15 in the Channel; he thought Hadley’s quadrant might be applied to measure lunar distances at sea with the desired accuracy.[1]
The rise of modern navigation may be fairly dated from the invention of the sextant in 1731 and of the chronometer in 1735; the former a complete nautical observatory in itself, and the latter an instrument which in its modern development has become an almost perfect time-keeper. It was a curious coincidence that these two invaluable instruments were invented at so nearly the same time. Until 1731 all instruments in use at sea for measuring angles 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 received the prize of the Paris Academy of Sciences for the best method of taking the altitude of stars at sea. Bouguer himself proposes a modification of what he calls the English quadrant, probably the one suggested by Wright and improved by Davis. Fig. 6 represents the instrument 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 would look through F and C at the horizon, shifting the sight F up or down till the ray from the sun coincided with the horizon. The space from E to F would represent the altitude, and the remaining part F to N the zenith distance. The English quadrant which this was to supersede differed in having about half the arc from E towards N, and, instead of the pinule being fixed at E, it was on a smaller arc represented by the dotted line eB, and movable. It was placed on an even number of degrees, considerably less than the altitude; the remainder was measured on the larger arc, as described.
Hadley’s instrument, on the other hand, described to the
Royal Society in May 1731 (Phil. Trans.), embodies Newton’s
idea of bringing the reflection of one object to coincide with the
direct image of the other. He calls it an octant, as the arc is
actually 45°, or the eighth part of a circle; but, in consequence
of the angles of incidence and
reflection both being changed
by a movement of the index.
it measures an angle of 90°, and
is graduated accordingly; the
same instrument has therefore
been called a quadrant. It was
very slowly adopted, and no
doubt there were numerous
mechanical difficulties of centring,
graduating, &c., to be
overcome before it reached perfection.
In August 1732, in
pursuance of an order from the
Admiralty, observations were
made with Hadley’s quadrant
on board the “Chatham”
yacht of 60 tons, below Sheerness,
in rough weather, by persons—except the master attendant—unaccustomed
to the motion; still the results were very satisfactory.
A year later Hadley published (Phil. Trans., 1733) the
description of an instrument for taking altitudes when the
horizon is not visible. The sketch represents a curved tube or
spirit-level, attached to the radius of the quadrant, since which
time many attempts have been unsuccessfully made to construct
some form of artificial horizon adapted to use at sea on board
ship, a discovery which would greatly facilitate observations at
night and at the many times when the natural or sea horizon is
imperfectly visible.
From the year 1714 the history of navigation in England is closely associated with that of the “Commissioners for the discovery of longitude at sea,” a body constituted in that year with power to grant annually sums not exceeding £2000 to assist experiments and reward minor discoveries. and also to judge on applications for much greater rewards which were from time to time offered to open competition. For a method of determining the longitude within 60 geographical miles, to be tested by a voyage to the West Indies and back, the sum of £10,000 was offered; within 40 m., £15,000; within 30 m., £20,000. £10,000 was also to be given for a method that would determine longitude within 80 m. near the shores of greatest danger. No action seems to have been taken before 1737; the first grant made was in that year, and the last in 1815, but the board continued to exist till 1828, having disbursed in the course of its existence £101,000 in all.[2] In the interval a number of other acts had been passed either dealing with the powers, constitution and funds of the commissioners or encouraging nautical discovery; thus the act 18 George II. (1745) offered £20,000 for the discovery by a British ship of the North-West Passage, and the act 16 George III. (1776) offered the same reward for a passage to the Pacific either north-west or north-east, and £5000 to any one who should approach by sea within one degree of the North Pole. All these acts were swept away in 1828, when the longitude problem had ceased to attract competitors, and voyages of discovery were nearly over.
The suggestions and applications sent in to the commissioners were naturally very numerous and often very trifling; but they sometimes furnish useful illustrations of the state of navigation. Thus, in a memorial by Captain H. Lanoue (1736), he records a number of recent casualties, which shows how carelessly the largest ships were then navigated. Several men-of-war off Plymouth in 1691 were
- ↑ Halley’s observations were published posthumously in 1742 and in 1765 the commissioners of longitude paid his daughter £100 for MSS. supposed to be useful to navigation. As the moon passes the stars lying in her course through the heavens at the mean rate of 33″ in one minute of time, it is obvious that an error to that amount in measuring the distance from a star would produce an error of 15 m. in longitude. As the moon’s motion with regard to the sun is nearly one degree a day less, a similar error in the distance would produce still more effect.
- ↑ This total comprises the large sums awarded to Harrison and to the widow of Mayer, the cost of surveys and expeditions in various parts of the globe, large outlays on the Nautical Almanac and on subsidiary calculations and tables, rewards for new methods and solutions of problems, and many minor grants to watchmakers or for improvements in instruments. Thus Jesse Ramsden received in 1775 and later about £1600 for his improvements in graduation (q.v.), and E. Massey in 1804 got £200 for his log (see Log).
