| XXX | (449) | XXX |
44P ASTRO N O M Y. times ftationary, and thrice retrograde. Let -us excepted. And if we were to trace out all their appa- isnowfixtrace his motion for one year in the figure. rent paths, and put the figures of them together in one Plate XL. fig. 2. fupppfe Mercury to be fetting diagram, they would appear fo anomalous and confufed, outInfrom A towards B, (between the earth and left hand that no man in his fenfes could believe them to be reprecorner of the Plate), and as feen from the earth, his motion fentations of their real paths; but would immediately will then be direft, or according to the order of the figns. conclude, that fuch apparent irregularities muft be owing when he comes to J?, he appears to ftand ftill in the to fome optic illufions : And after a good deal of in- But degree of at F, as ftiewn by the line BF. Whilft quiry, he might perhaps be at a lofs to find out the true he23dgoes to C, the line BF, fuppofed ,to move eaufe of thefe inequalities; efpecially if he were one of with him,fromgoesB backward contrary to thofe who would rather, with the greateft juftice, charge the order of figns ; and whenfromhe isF atto C,E, heor appears ftafrail man with ignorance, than the Almighty with being tionary at E, having gone back ii-‘- degrees. Now, the author of fuch cofifufion. ' Dr Long, in his firft volume of Aftronbmy, has given fuppofe him ftationary on the firft of January at C, on the 10th thereof he will appear in the heavens as at 20, us figures of the apparent paths of all the planets fepa- near F; on the 20th, he will be feen as at C; on the rately from Caflini; from them Mr Fergufon firft thought FT; on the 10th of February, at 7; on the 20th, of attempting to trace fome of them by an orrery, that at3 ift,K;atand on the 28th, at L; as the dotted lines fhew, fliews the motions of the fun, Mercury,'Venus, the earth, which are drawn through every tenth day’s motion in Lis and moon, according to the Copernican fyftem. Having path, and continued to the ecliptic. On rthe 10th taken off" the fun, Mercury, and Venus, he put.black looped lead pencils in their places, with the points turned up- of March, he appears at 717; on the 20th, at A -, and on On the 10th of April, he appears ftationary ward, and fixed a circular fheet of pafteboard fo that the 3 ;ift,onatthe0. 2dth, he feems to have gone back again to the earth kept conftantly under its centre in going round at0;Pand the 30th, he appears ftationary. at §>_, having the fun, and the pafteboard kept its parallelifm. Then, gone backon114 degrees. Thus Mercury feems to go forpreffing gently with one hand upon the pafteboard to degrees, or 131 degrees, and to go back make it touch the three pencils, with the other hand he ward 4 figns1211degrees, at a mean rate. From the 30th turned the winch that moves the whole machinery : and only 11 oriq the 10th of May, he feems to move from as the earth together with the pencils in the places of ofto April Mercury and Venus had their proper motions round R ; and on the 20th, he is feen at S, going forthe fun’s pencils, which kept at reft in the centre of the ward in the fame manner again, according to the order letters ; and backward when they go back ; which it machine, all the three pencils defcribed a diagram, ofis needlefs to explain any farther, as the reader can trace from which fig. 2. of Plate XL. is truly copied in a Imaller fize. As the earth moved round the fun, the him out fo eafily through the reft of the year. The fun’s pencil defcribed the dotted circle of months, whilft fame appearances happen in Venus’s motion ; but as fhe flower than Mercury, there are longer intervals Mercury’s pencil drew the curve with the greateft num- moves ber of loops, and Venus’s that with the feweft. In their of time between them. X inferior conjunftions they come as much nearer the earth, or within the circle of the fun’s apparent motion round the heavens, as they go beyond it in their fuperior con- Chap. V. The phyfical Ccntfes . of the junftions. On each fide of the loops they appear fta- Motions of the Planets. The Excentricities tionary; in that part of each loop next the earth retro- of their Orbits The Times in is. bid: the grade ; and in all the reft of their paths direft. If Caflini’s figures of the paths of the fun, Mercury, Action of Gravity alone would bring them to and Venus, were put together, the figure as above traced the Sun. put would be exaftly like them. It reprefents the fun’s apparent motion found the ecliptic, which is the fame e- From the uniform projeftile motion of bodies in ftraight very year; Mercury’s motion for feven years, and Ve- lines,'and the univerfal power of attraftion which draws nus’s for eight; in which time Mercury’s path makes 23 them off from thefe lines, the curvilineal motions of all loops,. crofling itfelf fo many times, and Venus’s only the planets arife. InPLteXL. fig. 3. if the body A~b& five. In eight years, Venus falls fo nearly into the fame projefted along the right line ABX, in open fpace, where apparent path again, as to deviate very little from it in it meets with no refiftance, and is not drawn afide by any fome ages; but in what rfumber of years Mercury and other power, it will for ever go on with the fame velocity, the reft of the planets would defcribe the fame vifible and in the fame direftion. For the force which moves it paths over again, it is hard to determine. Having fi- from y7 to 7? in any given time, will carry it from 2? to X nifhed the above figure of the paths of Mercury and Ve- in as much more time, and fo on, there being nothing to nus, he put the ecliptic round them as in the Doftor’s obftruft orialter its motion. But if when this projeftile book, and added the dotted lines from the earth to the force has carried it, fuppofe to F, the body S begins to ecliptic for ihewing Mercury’s apparent or geocentric attraft it, with a power duly adjufted, and perpendicular motion therein for one year; in which time his path to its motion at B, it will then be drawn from the ftraight makes three loops, and goes on a little farther; which line ABX, and forced to revolve about S in the circle fhews that he has three inferior, and as many fuperior BYTU. When the body A comes to U, or any other' conjunftions with the fun in that time ; and alfo that he part of its orbit, if the fmall body k, within the fphere 5 'X of Von, I. No. 19. 3
