The determination of the longitude requires simply accurate instru ments for the measurement of the positions of the heavenly bodies, and one or other of the two following—either perfectly correct watches, or perfectly accurate tables of the lunar motions. The legislature of Queen Anne, which passed an Act offering a reward for the discovery of the longitude, the problem being then very inaccurately solved, for want of one or the other, good watches or lunar tables, never contem pleted the invention of a method, hut only the means of making existing methods sufficiently accurate. And the legislature of George which repealed the former Act and substituted another, specifically limited the reward to those who should either proceed by improve ment of chronometers or of lunar table. The rewards which were given were to Harrison for the former, and to 3layer's executors for the latter. The latter Act is now repealed, and there does not exist any parliamentary offer of a sum of money for further fmprovementa.
Many persons, imagining that, as in the case of the quadrature of the circle, /tc., a theoretical difficulty existed, have employed themselves in endeavouring to invent a method, imagining that they should obtain the prize held out by the legislature. Some persons still-occupy them selves in this manner ; and it is impossible to persuade them either of the repeal of the Acts of Parliament, or of their having mistaken the nature of the difficulty, which is now, for all practical purposes, entirely conquered. It is impossible to find the latitude of a place without knowing the position of the equator in the heavens, or the longitude without knowing the meridian of Greenwich. The equator
has a real existence in the heavens, since its pole is the immovable point of the heavens, which can be detected (though it is not absolutely occupied by a star) from observation of the motion of the stars, which always preserve their distance from the pole. But the meridian of Greenwich, a purely arbitrary circle of the earth, determined merely by the will of Charles IL, that an observatory should be built on a certain hill near London, has no representative in the heavens. The only method, then, of finding longitude of the heavenly bodies is by finding the hour of the day which it is at Greenwich, at a particular hour on the spot whose longitude is required. It is then known how much of 360 degrees is revolved -through by the earth in.the period which brings a star from the meridian of the place upon the meridian of Greenwich, or vice verse' ; and this angle is the longitude. A watch which goes correctly and is set at Greenwich will carry the time at that place all over the world ; or a celestial phenomenon, of which the Greenwich time may be predicted, will, if the moment of its happening be observed at any other place, give the 'difference of times at the moment of observation. Any proposal for finding the longitude astro nomically, which does not depend on one or the other of these prin ciples, is useless, unless it be that of actually measuring the distance between the given place and Greenwich, the latitudes of both being known. Whether it be possible to use any other than astronomical means for the purpose, it would be presumptuous to decide ; but there certainly is no other method which offers the most distant prospect of success.