and bring the artificial horizon to that degree, and the intersection of its edge with the meridian is the situation required,

may

be seen

By this problem, any place not represented on the globe may be laid down thereon, and it where a ship is, when its latitude and longitude are known.

Example. The latitude of Smyrna, in Asia, is 38 degrees 28 minutes north; its longitude 27 dedegrees 30 minutes east of London; therefore, bring 27 degrees 30 minutes counted eastward on the equator, to the moveable meridian, and slide the diameter of the artificial horizon to 38 degrees 28 minutes north-latitude, and its centre will be correctly placed over Smyrna.

It may be proper, in this place, just to shew the pupil, that the latitude of any place is always equal to the elevation of the pole of the same place above the horizon. The reason of this is, that from the equator to the pole are 90 degrees, from the zenith to the horizon are also 90 degrees; the distance of the zenith to the pole is common to both; and, therefore, if taken away from both, must leave equal remains; that is, the distance from the equator to the zenith, which is the latitude, is equal to the elevation of the pole.

OF FINDING THE LONGITUDE.

As the finding the longitude of places forms one of the most important problems in geography and

astronomy, some further account of it, it is presumed, will prove entertaining and useful to the reader.

"For what can be more interesting to a person 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 designs to visit? These important particulars are all determined by knowing the latitude and longitude of the place under consideration. 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 absolute necessity. Floating in a frail vessel, upon an uncertain abyss, he has consigned himself to the mercy of the winds and waves, and knows not where he is*."

The following instance will prove of what use it is to know the longitude of places at sea. The editor of Lord Anson's voyage, speaking of the island of Juan Fernandez, adds, "The uncertainty we were in of its position, and our standing in for the main on the 28th of May, in order to secure a sufficient easting, when we were, indeed, extremely near it, cost us the lives of between 70 and 80 of our men, by our longer continuance at sea; from which fatal accident we might have been exempted,

*Bonnycastle's Astronomy.

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 10,000l. if it extended only to one degree of a great circle, or 60 geographical miles; 15,000, if found to 40 such miles; and 20,000l. to the person that can find it within 30 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 words, how much sooner the sun, &c. 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 difference 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 sun 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 two 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, east in longitude from London; the other place where it began at midnight, is 30 degrees distant in 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 parts 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 estimated 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