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Understanding, at the same Time they fill the Mind with the most sublime and rational Pleasures.

Cleon. The retrograde Motion in Mercury is but of short Duration indeed, as in the present Cafe no more than 23 Days; whereas he continues direct in Motion about 90 Days, which is more than three Times as long. And in the fame Manner we might shew the direct and retrograde Motions of Venus, or any of the other Planets; for the superior Planets, Mars, Jupiter and Saturn, may be placed on this Stellated Planetarium in their true Proportion of Distances, and then the Times of their appearing direct and retrograde among the Stars on the Instrument, will be the same to a Day as we observe them in the Heavens. But we must at present desist from a farther Speculation of the Planets, and leave what remains till you are a little more prepared for astronomical Calculations, and the Use of Tables for that Purpose.

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DIA LOG U É XV.
Of the COME TS.

Euphrosyne.
V OU have, dear Cleonicus, obliged me with a long

I and particular Account of the fix primary Planets as they are called, which constitute the most considerable Part of the Solar System. But what succeeds to these ? When we are arrived to the out-moft Bounds of the System, where go we then ?

Cleon. Where go we! much farther, my Euphrofyne, than we have yet been. · Euphrol. What, to the Stars, I suppose.

Cleon. No, no, now you over-Moot the Mark ; we must take a much longer Time to travel thither.

Euphrof. It can't be the Moon; for we left her far behind, hovering about the Earth. But hold I rea collect --'tis the Comets we next survey ; did you not intimate thus, Cleonicus ? when we last conversed together.

Cleon. Yes, I did ; the Comets next present thema selves as the Subject of our Speculation.

Euphrof. Pray, what is a Comer?

Cleon. A Comet is what the common People call a Blazing Star, because it appears like a large Star with a long Tail, blazing as it were, or streaming from it. It is a very extraordinary and amazing Sight.

Thus Virgil :
- The threatning Comets, when by Night they rise;

Shoot sanguine Streams and jadden all the Skies.
Euphros. Did you ever see a Comet, Cleonicus ?

Cleon. Yes; but they very rarely appear ; and tho' they are many in Number, yet their long Periods render their Return but feldom, and therefore a Comet can't be often seen.

Euphrof. You say the Comets are many in Number, pray, can you tell how many ? .

Cleon. No; all those which have been duly observed, and fallen under the Notice of Astronomers, are between 40 and 50; tho' 'tis possible, if not probable, there are, belonging to this System, many more.

Euphrof. But I want to know what a Comet is, or what it is that makes the Appearance of a Star and its blazing Tail ?

Cleon. They are supposed to consist of a very folid, compact and durable Substance, capable of the greatest Degree of Heat and Cold without Dissolution; and of an opake Nature, Mining only by Reflection of the Sun Beams, like other Planets.

Euphrof. By that you intimate the Comets to be a · Kind of Planets.

Cleon. They are most certainly so; for they move in stated Periods of Time about the Sun; of which Periods, some are longer, some shorter, as in the other Planets.

Euphrof. Are the Times of the Periods of these · Cometary Planets known by Astronomers, like those of the other?

Cleon. No, but of some few only; as that which appeared in 1680 hath its Period in 575 Years; that in 1661, has its Period in 129 Years ; and that of 1682 has its Period the shortest of all, viz. 75 1 Years, So that the shortest Period of the Comets is much longer than the longest Period of the Planets,

Euphrof. Well, but if this last Comet's Period be but 75 Years, and he appeared in the Year 1682, it must be expected that he will again appear in the Year 1758; What lay you, Cleonicus ?

Cleon. Say, 'tis true, it will undoubtedly then appear, to the great Pleasure and Surprize of all the curious Part of Mankind.

Euphrof. I hope, as the Time is so near, we shall both live to see so extraordinary a Phänomenon.

Cleon. I hope fo too; I thall scarce regret Death so much on any worldly Account as preventing me of so desirable a Sight. *

Euphrof. In what kind of Orbits do the Comets move about the Sun?

Cleon. In such as are of an oval or elliptical Form, like that of which you saw a Part in the Plate of the folar Systein,

Euphrof. Here is the Plate you mention, I see the Orbit and the Cornet in it: The Sun is very near to one End of it, I see, and the two Sides of the Orbit of the Comet run out far beyond the Planetary System.

Cleon. Yes, and so it is in Truth; the Comets in one Part of their Orbits approach extremely near the . Sun; and in another, are immensely distant from him. In short, they come nearer to, and go farther from the Sun, than any of the Planets by much.

Euphrof. Why then 'they must needs be sometimes much hotter than Mercury; and at other Times much colder than Saturn; for I presume, they have no Heat but what they receive from the Sun.

Cleon. They have not; and therefore they have the greatest Extremities of Heat and Cold by Turns ; for when they are in that Part of their Orbit nearest the Sun, they conceive a prodigious Degree of Heat, which gradually diminishes as they recede from the Sun, and till becoming cold, that Coldness gradually increases to an Extremity proportional to the Distance they go from the Sun.

H 3 * This Comet did actually appear in the Winter of the Year 1757, and in the Spring of the Year 1758, exactly according to Dr. Halley's Prediction, and is the first whore Orbit is determined.

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