See Fig.XI. and fo may be expected 1789, and 1918, and fo every 129 Years afterwards. The Fourth having laft appear'd 1680, 1681, and having its Period no lefs than 575 Years, cannot return till A. D. 2255. But then, as to the rest of the Comets, we cannot yet foretel the Periods of their Revolution, for want of Ancient exact Hiftories and Obfervations, but must leave their Determination to future Ages; which will, no doubt, if Aftronomical Learning continues in the World, be in Time fully discovered, and known by our Pofterity. N. B. The Obfervations fhew that the Comets are about the Bignefs of the Planets, and have vaft Amospheres about them; the Central Parts thereof being denfer than the Superficial; as alfo, that the Vapours and Clouds thereof are hurried about in great Disorder and Confufion, like Planets in a Chaotick State: It alfo appears from the like Obfervations when near the Sun, that their Atmospheres wind themselves round like a revolving Globe, and afcend towards the Regions oppofite to the Sun; as if the rareft Vapours whereof they are compos'd, were carried up by the Solar Rays; as also that thofe Tails are generally the longest which arife from the greatest Nearness to the Sun. N. B. The Motion of Comets being fome from North to South, others from South to North; fome from East to West, others from Weft to Eaft, in all Planes and Directions; and this from the very Sun downwards, to the Regions vaftly beyond Saturn upwards; and that Motion appearing moft exactly regular, without the leaft Retardation by any fenfible Resistance in their feveral Courfes, it is thence most certain,that there This is the Reprefentation of the lowest part of the Tail of a Comet near its Perihelion, with the purer part of its atmosphere winding itfelfinto if Tail, and the Cloudy part of the same placed round about the CentralSolid as it appeard to DHook through aTelescope. Figu XI I. Senex feulpt there are no folid Orbs, as the Ancients fuppofed; and that there is no fubtle Matter, as the Cartefians imagin'd; but that all the vaft Spaces between and beyond the Planetary Syftem, are an immenfe Void or Vacuity, as to fenfible refifting Matter; and admit ordinarily of nothing but of the Rays of Light, unless it be near the Comets with their Atmospheres and Tails; and near the Planets with their Atmofpheres; all which Particles put together are almost nothing in comparison of those vaftly prodigious, thofe immenfely numerous Vacuities which are interfpers'd between them, as we have already seen from an exact Calculation, Page 19. (16.) The Fixed Stars, vifible to the naked Eye of the Acutest Observers, are in Number confiderably under 2000, and those seen only through Telescopes, about 10 or 20 times that Number. Their Distance is found by Mr. Huygens's conjectural Method to be about 2,200.000, 000.000. And the Distance of fome, from Dr. Hook's and Mr. Flamfted's Annual Parallax, See my Awhich I look on as much more certain, about the fron. Lect. III, IV. third Part of that diftance, or 700.000,000.000 Miles. However, 'tis certain that this Distance is vaftly great, if compar'd with that of the Planets and Comets, when they are remotest from the Sun; and that in particular the famous Comet of 575 Years Period, which goes fometimes about 14 times as far off the Sun as Saturn, i. e. about 11.200,000.000 Miles, is not then near enough to them to be altered by that of the Fixed Stars Gravitation towards, or Influences from them. Thefe Stars feem to be of the the fame Nature with the Sun, as fhining with their own Native Light; and continuing fixed in the Centers of their feveral Planetary and Cometary Systems, as the Sun does. (17.) The Fixed Stars feem to be really of very different Magnitudes, yet not of fuch very different Distances from this System as is now generally fuppofed. As to the first Affertion, it feems very agreeable to the reft of the visible Bodies in the World, whether Planets or Comets, which are of very different Magnitudes alfo; and feems confirm'd from the fecond Affertion; for if the Fixed Stars vifible to the naked Eye be at no very different Distances from our Syftem, that vaft apparent Inequality of Light which they fend hither, and according to which they are ranked under fix or seven different Magnitudes on our Globes and Planispheres, will be next to a Demonstration, that they are themfelves really of very different Magnitudes alfo. Nor in that Cafe will it be proper to place the Telescopick Stars at any vaftly farther Distances, fince they do not at all feem different from the other, only ftill gradually fmaller. Now that the Fixed Stars, visible to the naked Eye, are not at any very different Distances from our Syftem, is moft probable, because the best Method we yet have of knowing those Distances, I mean their Parallax, determine that the Diftance of thofe Three which have been try'd, tho' all of different apparent Magnitudes, is very nearly the fame. I do not deny that fome Fixed Stars may be vaftly farther off than others; and that there may be Systems of Worlds fcattered every where in the Uni verfal Immenfe Void: But I fay, that if far ther |