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had we been furnished with such an account of its situation, as we could fully have depended on."
The latitude of a place the sailor can easily discover; but the longitude is a subject of the utmost difficulty, for the discovery of which so many methods have been devised. It is, indeed, of so great consequence, that the Parliament of Great Britain proposed a reward of 10,000/. if it extended only to one degree of a great circle, or 60 geographical miles; 1o^OOO/. if found to 40 such miles; and 20,000/. to the person that can find it within 3O minutes, of a great circle, or 30 geographical miles.
As I cannot enter fully into this subject in these Essays, it will, I hope, be deemed sufficient, if I give such an account as will enable the reader to form a general idea of the solution of this important problem.
From what has been seen in the preceding pages, it is evident that 15 degrees in longitude answer to one hour in time; and, consequently, that the longitude of any place would be known, if we knew their difference in time; or, in other w'ords, how much sooner the sun, &e. arrives at the meridian of one place, than that of another. The hours and degrees being, in this respect, commensurate, it is as proper to express the distance of any place in time as in degrees.
Now it is clear, that this difference in time would be easily ascertained by the observation of any instantaneous appearance in the heavens, at two distant places; for, the difference in time, at.which the same phenomenon is observed, will be the distance of the two places from each other in longitude. On this principle, most of the methods in general use are founded.
Thus, if a clock, or watch, was so contrived, as to go uniformly in all seasons, and in all places; such a watch being regulated to London time, would always shew the time of the day at London; then, the time of the day under any other meridian being found, the difference between that time, and the corresponding London time, would give the difference in longitude.
For, suppose any person, possessed of one of these time-pieces, to set out on a journey from London, if his time-piece be accurately adjusted, wherever he is, he will always know the hour at London exactly; and when he has proceeded so far either eastward or westward, that a differ-ence is perceived betwixt the hour shewn by his time-piece, and those of the clocks and watches at the places to which he goes, the distance of those places from London in longitude will be known. But to whatever degree of perfection such movements may be made, yet, as every mechanical instrument is liable to be injured by various accidents, other methods are obliged to be used, as the eclipses of the surt and moon, or of Jupiter's satellites. Thus, supposing the moment of the beginning of an eclipse was at ten o'clock at night at London, and by account from tvi-o observers in two other places, it appears that it began with one of them at nine o'clock, and with the other at midnight; it is plain, that the place where it began at nine is one hour, or 15 degrees, cast in longitude from London; the other place where it began at midnight, is 3O degrees distant hi west longitude from London. Eclipses of the sun and moon do not, however, happen often enough to answer the purposes of navigation; and the motion of a ship at sea prevents the observations of those of Jupiter's satellites.
If the place of any celestial body be computed; for example, as in an almanack, for every day, or to pails of days, to any given meridian, and the place of this celestial body can be found by observation at sea, the difference of time between the time of observation, and the computed time, will be the difference of longitude in time. The moon is found to be the most proper celestial object, and the observation of her appulses, to any fixed star, is reckoned one of the best methods for resolving this difficult problem.
LENGTH OF THE DEGREES OF LONGITUDE.
Supposing the earth to be a perfect globe; the length of a degree upon the meridian has been estir mated to be 69,1 miles; but as the earth is an oblate spheroid, the length of a degree on the equator will be somewhat greater.
Whether the earth be considered as a spheroid or a globe, all the meridians intersect one another at the poles. Therefore, the number of miles in a degree must always decrease as you go north or south from the equator. This is evident by inspection of a globe, where the parallels of latitude are found to be smaller in proportion, as they are nearer the pole Hence it is, that a degree of longitude is no wherethe same, but upon the same parallel; and that a degree of longitude is equal to a degree of latitude only upon the equator.
The following table shews how many geographical, miles, and decimal parts of a mile, would be contained in a degree of longitude, at each degree of latitude from the equator to the poles, if the earth was a perfect sphere, and the circumference of its equinoctial line 360 degrees, and each degree 60 geographical miles.
This table enables us to determine the velocity with which places upon the globe revolve eastward; for the velocity is different, according to the distance of the places from the equator, being swiftest as passing through a greater space, and so, by degrees, slower towards the pole, as passing through a less space in the same time. Now, as every part of the earth is moved through the space of its circumference, or 36o degrees, in 24 hours; the space described in one hour, is found by dividing 36%O by 24, which gives in the quotient 15 degrees; and so many degrees does every place on the earth move in an hour. The number of miles contained in so many degrees, in any latitude, is readily found from the table.