| XXX | (499) | XXX |
499' ASTRO N O M centre is parts of an inch from the earth’s the horizon to the Ltuude of your place on tl^e brafen inwhofe this machine, this being in juft proportion to the meridian, and the quadrant of altitude to the fouth point diliance from the earth. yS'vV is a bar of wood, of the horizon; which done, turn the globe with its fur- moon’s to be moved by hand round the axis g which is fixed niture till the quadrant of altitude comes right again!! wheel T. The circumference of this wheel is to thein the fun, vizr to his place in the ecliptic; and keeping it the of the Imall wheel L (below the other end there, fet the hour-index to the XII next the letter C; ofcircumference as 365^ days is to 294> or as a year is to a and the machine will be. redlified, not only for the fol- the bar)'The wheels are grooved round their edges, and lowing-problems, but for feveral others which the artift lunation. in the grooves is the cat-gut ftring GG crofting between may eafrly find out. the wheels at X. On the axis of the wheel L is the inF, in which is fixed the moon’s axis M for carrying Problem I. T'o find the amplitudes, meridian alti- dex round the earth E (fixed on the axis of the wheel tudes, and times of riftng, culminating, and felting, her L) in the time that the index goes round a circle of aqi of the fun, moon, and planets. parts, which are the days of the moon’s age. Turn the globe round eaftward, or according to the or- equalwheel P has the months and days of the year all der of figns ; and as the eaftern edge of the horizon comps The round its limb ; and in the bar A4 is fixed the index right again!! the fun, moon, or any planet, the hour- which points the days of the months anfwering to index will Ihew the time of its rifing ; and the inner edge the days of theoutmoon’s by the index F, in the of the ecliptic will cut its rifing amplitude in the horizon. circle of 29^ equal partsage,atftiewn end of the bar. Turn on, and as the quadrant of altitude comes right a- On the axis of the wheel L istheputother piece D, below gainft the fun, moon or planets, the ecliptic cuts their the cock C, in which this axis turnstheround in D meridian altitudes in the quadrant, and the hour-index are put the pencils e and m, diredtly under ;theandearth Ihews the times of their coming to the meridian. Con- and moon M-, fo that m is carried round e, as M Eis tinue turning, and as the wellern edge of the horizon round £► comes right again!! the fun, moon, or planets, their fet- Lay the machine on an even floor, prefling gently on ting amplitudes are cut in the horizon by the ecliptic; the wheel T, to caufe its fpiked feet (of which two apand the times of their fetting are {hewn by the index on pear at P and P, the third being fuppofed to be hid the hour-circle. from fight by the wheel) enter a little into the floor to the wheel from turning. Then lay a paper about Prob. II. To find the altitude and azimuth of the fecure feet long under the pencils e and m, crofs-wife to fnn, moon, and planets, at any time of their being four the bar; which done, move the bar flowly round the axis above the horizon. wheel T; and as the earth E goes round the fun Turn the globe till the index comes to the given time gS,ofthethe moon M will go round the earth with a duly iti the hour-circle, then keep the globe Iteady, and mo- proportioned
- and the fridtion-wheel /f'running ,
ving the quadrant of altitude to 'each planet refpedtively, on the floor, velocity will from bearing too heavily the edge of the ecliptic will cut the planet’s mean alti- on the pencils e andkeep,m,the-bar will delineate the paths tude on the quadrant, and the quadrant will cut the of the earth and moon.. Aswhich the index out the days planet’s azimuth, or point of hearing on the horizon. of the months,.the index ftiews the/points moon’s age on thefe in the circle of^i equal parts^ And as this laftProb. III. The fun's altitude being given at any time days, to the different days in its circle, the likeeither before or after noon, to find the hour of the index, points, figures may be fet to thofe parts of the curvesday, and variation of the conipafs, in any known la- numeral of the earth’s path and moon’s, where the pencils <? and titude. at thofe times refpedtively, to fhew the places of With one hand hold the edge of the quadrant right a.- mtheareearth and moon. the pencil e be pufhed a very gain!! the fun ; and, with the other hand, turn the globe little Off, as if from theIfpencil to about part of weftward, if it be in the forenoon, or eaftward if it be their diftance, and the pencil m m,puftied as much towards in the afternoon, until the fun’s place at the inner edge e, to bring them to the fame difMnces again, not ; of the ecliptic cuts the quadrant in the fun’s obferved. al- to the. fame points of fpace; then, as m goesthough e, . titude ; and then the hour-index will point out the time e will go as it were round the centre of gravity round between of the day, and the quadrant will cut the true azimuth, the earth e and moon m ; but this- motion will not fenor bearing of the fun for that time : The difference be- fibly alter the figure of the earth’s path or the moon’s. tween which, and the bearing {hewn by the azimuth if a pin, as/-, be put through the pencil w, with its" compafs, fhews the variation of the compafis in that place head towards that of the pin q in the pencil e, its head of the earth. vrill always keep thereto as m goes, round or as the The Ttajectorium Lunarte, Plate XTIX. fig. 2. fame fide of the moon is ftill'obverted to the earth. Bur This machine is for delineating the paths of the earth the which may be confidered as an equatorial diaand moon, Ihewing what fort of curves they make in the meterpin ofp, the moon, will turn quite round the point w, etherial regions. S is the fun, and E the earth, whofe ' making all poflible-angles the line of its progrefs, o centres are 81 inches dil!ant from each other; every line of the moon’s path. with This is an ocular proof of the inch anfwering to a million of miles. M is the moon, moon’s turning round her axis,. The
