Is an exact Scheme of the principal known Comets, in just Proportion, to the Globe of the Earth represented at A, with the Nucleus, and Part of the Tail of the Comet of 1680, B, as it was observed in its Assent from the Sun, _viz._ _a a_ the Comet's natural Atmosphere, _z z z_, the _Denser Matter_ winding itself into the Axis of the Train _x x_, the inflam'd Atmosphere and Tail dilated near the Sun. C, represents the Ball of the Comet of 1682, D, that of 1665, E, that of 1742, and F, the Head of the Comet of 1744. And again, that you may have some Notion of the apparent Magnitudes of all these Planets and Comets, _&c._ as they appear at the Earth, [Illustration: Plate VI.] [Illustration: Plate VII.] [Illustration: Plate VIII.] [Illustration: Plate IX.] [Illustration: Plate X.] [Illustration: Plate XI.] PLATE X. Represents the Sun and Moon in the just Proportion of their mean Diameters, with two of the Comets A and B, and the five erratick Planets, as they are observed at the Earth, in a middle State of their Distances from it. For a more full and particular Description of all the Parts of the solar System, and of the home Elements of Astronomy in general, I refer you to my _Clavis Cœlestis_, &c. where every thing concerning the Planets, Comets, and Stars; and their real and apparent Motions, are at large represented, explained, and accounted for, for the Benefit of such as have not made the Mathematicks their regular Study. Now, to convince you that the Planets are all in their own Nature no other than dark opaque Bodies, reflecting only the borrowed Light of the Sun, I must recommend to your Observation, this natural and simple Experiment, which almost any Opportunity of seeing the _Moon_ a little before the Full, will put into your Power to make; but best and easiest when the Sun is in any of the North Signs, _i. e._ in _Summer_. At such a time, the Sun being near setting, the Moon will appear in the eastern Hemisphere; and if there be any bright Clouds northward, or southward near her, you will plainly perceive, that the _Light_ of the one is of the same Nature with that of the other; I mean the Light of the Moon, and that of the Cloud. To me there never appeared any Difference at all; and I am perswaded, were you to make but two or three Observations of this kind, which is from Nature itself, a sort of ocular Demonstration, you cannot fail of being convinced, that the Moon's Light, such as it is, without Heat, can possibly proceed from no other Cause than that which illumines the Cloud: For if the Clouds, whose Composition we know to be but a thin light Fluid, formed of condensed Vapours only, is capable of remitting so great a Lustre, how much more may we not allow the Moon, which, Length of Time, and many other Circumstances, have long confirmed to be a durable and solid Body.