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[[start page]] [[page number]] [[left justified]] 131) [[/left justified]] [[left margin]] An Essay for finding the Longitude at Sea, by [[underline]] Michael Woods [[/underline]], Mathematician in [[underline]] Liverpool. [[/underline]] Gents Mag. for Septr. 1767 p. 449. [[/left margin]] Having observed several essays for finding the longitude, I have sent you one which differs from them all. The notion that some machine must be contrived to measure exactly the space of a solar day, commonly supposed to contain 24 hours, hath hitherto, in my opinion, defeated every attempt to discover the longitude by a time-keeper, a thing neither necessary nor practicable, with any certainty, by reason of the inequality of the solar days: for the time between one meridian shadow, on a sundial, and the next, is not equal, and that inequality is ever more or less, according to the sun's position in the ecliptic, etc. The only probable machine that has been made, is by one [[underline]] John Harrison [[/underline]], finished about [[underline]] Christmass [[/underline]] 1765, which machine I went to see at [[underline]] Greenwich [[/underline]]. But as I apprehend, the only portion of time necessary to be measured, is that of the earth's diurnal motion on its own axis, which, by the following directions, may be determined with great exactness. As the earth's revolutions upon its axis, from west to east, are ever equal in time one to another, so all fixed stars, whether they rise or set, or are always above the horizon, if observed from any particular place, must appear to revolve in the same equal time. Provide yourself, therefore, with a sand-glass, large enough to contain such a quantity of sand as shall take up that whole period in running out, so that the glass need [[strikethrough]] [illegible?] [[/strikethrough]] be turned but once to each revolution. Being provided with such a glass, take a small tube, whose diameter must not exceed the apparent diameter of a star; turn it in the night to any fixed star that may suit your purpose, and the moment you have the center of the star against the center of the tube, fix your tube fast, causing the glass to be turned at the same instant. The next night note if the sand in your glass is all run out, or not, at the time the same star comes again opposite to the center of your tube, which must remain all the time as at first fixed. Repeat your observation in the same manner every night, till you bring it exactly to the time required. Your glass being thus regulated, for the method of using it at sea, in order to know your longitude observe this [[strikethrough]] most [[/strikethrough]] general and most useful rule. Observe any two fixed stars near the elevated pole, whose right ascension is the same; or any two whose difference of right ascension is 12 hours; the first always comes on the meridian at the same time, either above or below the pole; the latter likewise comes on the meridian, but have always the pole between them; either will do. The star in [[underline]] CassiopÓ•ia's [[/underline]] side, the polar star, and the last but two in the [[underline]] Great Bear's [[/underline]] Tail, come on the meridian near the same time. [[left margin]] I have tried thus to suspend a plumet & line on board, but find it utterly impracticable. [[cursive]] MS [[/cursive]]. [[/left margin]] When you find two stars you intend to Observe are near the meridian, hold up a thread and plummet; note, when they cut the thread, and at that instant cause your glass to be turned; then, if you continue on the same meridian, you will find that your glass will be always out when those stars come perpendicular to the thread and plummet, or on the meridian; but if you move to the eastward or westward, the difference will be equal to your difference of longitude east or west. As the polar star is never above two and a half degrees from [[end page]]
