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before those stars come to the ^[[same]] position by your line and plummet, as when first observed at your departure; hence you may conclude your difference of longitude to be 41 degrees 15 minutes westerly, two hours 45 minutes reduced to degrees and minutes, being equal to 41 degrees 15 minutes. Note, if your glass be out before you have your observation, the difference of longitude is westerly; and, on the contrary, if you have your observation first, the difference is easterly. Thus you may settle your longitude every clear night, and if due care be taken in turning the glass, you will not have an error therein of 15 minutes in the longest voyage, for the observation may be always made in less than half a minute of time.
    By decreasing your latitude, the star you took your first observation by, at departure, may be depressed below the horizon; or, by length of time, may come on the meridian before night; in either of which cases, it is but taking your observation in time by some other star, more convenient to your purpose, and turning another glass, as at first, and your journal may be continued with the same exactness as if you still made your observation by the same star. It may possibly happen that the sand, continually running for a long space of time, may wear the orifice thro' which it passes; or the grains, by rubbing against each other, may be so polished as to run something faster than at first; but this may be easily tried and remedied by spare glasses, of which it will be necessary for every captain to have two or three at least, as well on this account as on account of those above mentioned; which, as the expence is small, can be no great inconvenience. The only objection that occurs to me, against this method, is, its being impracticable beyond the polar circles, by reason of the sun's continuance there above the horizon, the only time when those seas are navigable; but, as they only include a small part of the world, and few ships frequenting those parts, I think it can be of no great weight. Thus the main point of navigation my be determined, without any regard to the solar time.

[[left margin]]Friday 11th of 7ber. Gents Mag.p.475.for Sept.r 1767. New accurate instrument for celestial observ.ns [[/left margin]]

The [[underline]] Abbe Rochon [[/underline]], who sailed from [[underline]] Brest [[/underline]] in April last, in order to make trial of some instruments of his own invention, for taking Altitudes at Sea, returned from his Voyage, in course of which he had observed several eclipses of [[underline]] Jupiter's [[/underline]] satellites: and it is asserted, that by his instrument the observer can never be above four seconds without recovering the star, let the motion of the ship be ever so violent, which must be of infinite advantage in making observations for discovering the longitude at Sea.