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As the finding the longitude of places forms one of the most important problems in geography and aftronomy, fome further account of it, it is prefumed, will prove entertaining and useful to the reader.

"For what can be more interefting to a perfon in a long voyage, than to be able to tell upon what part of the globe he is, to know how far he has travelled, what distance he has to go, and how he must direct his course to arrive at the place he defigns to vifit; thefe important particulars are all determined by knowing the latitude and longitude of the place under confideration. When the discovery of the compass invited the voyager to quit his native shore, and venture himself upon an unknown ocean, that knowledge which before he deemed of no importance, now became a matter of abfolute neceffity. Floating in a frail veffel, upon an uncertain abyss, he has configned himself to the mercy of the winds and waves, and knows not where he is."*

The following inftance will prove of what use it is to know the longitude of places at fea. The editor of Lord Anfon's voyage, fpeaking of the ifland

* Bonnycaftle's Aftronomy.

ifland of Juan Fernandez, adds, "The uncertainty we were in of it's pofition, and our ftanding in for the main on the 28th of May, in order to secure a fufficient eafting, when we were indeed extremely near it, coft us the lives of between 70 and 80 of our men, by our longer continuance at fea; from which fatal accident, we might have been exempted, had we been furnished with such an account of it's fituation, as we could fully have depended on."

The latitude of a place the failor can eafily discover; but the longitude is a subject of the utmoft difficulty, for the discovery of which many methods have been devifed. It is indeed of fo great confequence, that the parliament of Great Britain proposed a reward of 10,000/. if it extended only to 1 degree of a great circle, or 60 geographical miles; 15,000l. if found to 40 fuch miles; and 20,000l. to the person that can find it within 30 minutes of a great circle, or 30 geogra phical miles.

We cannot enter fully into this subject in these effays; it will, I hope, be deemed fufficient, if we give fuch an account as will enable the reader to form a general idea of the folution of this important problem.

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From what has been feen in the preceding pages, it is evident that 15 degrees in longitude. answer to one hour in time, and confequently that the longitude of any place would be known, if we knew their difference in time; or in other words, how much fooner the fun, &c. arrives at the meridian of one place, than that of another. The hours and degrees being in this respect commenfurate, it is as proper to exprefs 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 inftantaneous appearance in the heavens, at two diftant places; for the difference in time at which the fame phænomenon is obferved, will be the distance of the two places from each other in longitude; on this principle, most of the methods in general ufe are founded.

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Thus if a clock, or watch, was fo contrived, as go uniformly in all feafons, and in all places; fuch a watch being regulated to London time, would always fhew 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 correfponding London time, would give the difference in longitude.

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For fuppofing any perfon poffeffed of one of these time-pieces, to fet 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 fo far either eastward or weftward, that a difference is perceived betwixt the hour fhewn by his timepiece, 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 fuch movements may be made, yet as every mechanical inftrument is liable to be injured by various accidents, other methods are obliged to be used, as the eclipfes of the fun and moon, or of Jupiter's fatellites. Thus fuppofing the moment of the beginning of an eclipfe was at ten o'clock at night at London, and by accounts from two ob-. fervers 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 30 degrees diftant in west longitude from London. Eclipfes of the fun and moon do not, however, happen often enough to answer the purposes of navigation; T 3

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and the motion of a fhip at fea prevents the observations of those of Jupiter's fatellites.

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If the place of any celeftial body be computed, for example, as in an almanack, for every day, or to parts of days, to any given meridian and the place of this celeftial body can be found by obfervation at fea, the difference of time between the time of obfervation and the computed time, will be the difference of longitude in time. The moon is found to be the most proper celeftial object, and the obfervations of her appulfes to any fixed ftar is reckoned one of the beft methods for refolving this difficult problem.

LENGTH OF THE DEGREES OF LONGITUDE.

Suppofing the earth to be a perfect globe, the length of a degree upon the meridian has been eftimated to be 69,1 miles; but as the earth is an oblate fpheroid, the length of a degree on the equator will be fomewhat greater.

Whether the earth be confidered as a spheroid or a globe, all the meridians interfect one another at the poles. Therefore, the number of miles in a degree must always decreafe as you go north or fouth from the equator. This is evident by infpection

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