Understanding, at the fame Time they fill the Mind with the moft fublime and rational Pleasures. Cleon. The retrograde Motion in Mercury is but of fhort Duration indeed, as in the prefent 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 fhew the direct and retrograde Motions of Venus, or any of the other Planets; for the fuperior 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 Inftrument, will be the fame to a Day as we observe them in the Heavens. But we must at present defift from a farther Speculation of the Planets, and leave what remains till you are a little more prepared for aftronomical Calculations, and the Ufe of Tables for that Purpose. You DIALOGUE XV. Of the COMETS. OU have, dear Cleonicus, obliged me with a long and particular Account of the fix primary Planets as they are called, which conftitute the most confiderable Part of the Solar Syftem. But what fucceeds to these? When we are arrived to the out-moft Bounds of the Syftem, where go we then? Cleon. Where go we! much farther, my Euphrofyne, than we have yet been. Euphrof. What, to the Stars, I suppose. Cleon. No, no; now you over-fhoot 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 recollect 'tis the Comets we next furvey; did you not intimate thus, Cleonicus? when we laft converfed together. -- Cleon. Yes, I did; the Comets next prefent thems felves as the Subject of our Speculation. Euphrof. Pray, what is a Comet ? 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 threat'ning Comets, when by Night they rife, Euphrof. Did you ever fee 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 feen. Euphrof. You fay the Comets are many in Number, pray, can you tell how many? Cleon. No; all thofe which have been duly obferved, and fallen under the Notice of Aftronomers, are between 40 and 50; tho' 'tis poffible, if not probable, there are, belonging to this Syftem, 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 fuppofed to confift of a very folid, compact and durable Subftance, capable of the greatest Degree of Heat and Cold without Diffolution; and of an opake Nature, fhining 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 moft certainly fo; for they move in ftated Periods of Time about the Sun; of which Periods, fome are longer, fome fhorter, as in the other Planets. Euphrof. Are the Times of the Periods of these . Cometary Planets known by Aftronomers, like thofe of the other? Cleon. No, but of fome 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 fhorteft of all, viz. 75 Years. So that the fhorteft Period of the Comets is much longer than the longest Period of the Planets. Euphrof. Well, but if this laft 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 fay you, Cleonicus? Clean. 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 fo near, we shall both live to fee fo extraordinary a Phænomenon. Cleon. I hope fo too; I fhall fcarce regret Death fo much on any worldly Account as preventing me of fo defirable a Sight. * Euphrof. In what Kind of Orbits do the Comets move about the Sun? Cleon. In fuch as are of an oval or elliptical Form, like that of which you faw a Part in the Plate of the folar System. Euphrof. Here is the Plate you mention,—I see the Orbit and the Comet in it: The Sun is very near to one End of it, I fee, and the two Sides of the Orbit of the Comet run out far beyond the Planetary Syftem. Cleon. Yes, and fo it is in Truth; the Comets in one Part of their Orbits approach extremely near the Sun; and in another, are immenfely diftant 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 fometimes much hotter than Mercury; and at other Times much colder than Saturn; for I prefume, 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 increafes 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 whofe Orbit is determined. Euphrof. Pray, can Aftronomers tell the nearest and greatest Distance of Comets from the Sun? Cleon. Of fome few they can pretty nearly; particularly that Comet in 1680 came so near the Sun, that it was not a fixth Part of the Sun's Diameter diftant from his Surface; and therefore, its Heat muft then be two thoufand Times hotter than red hot Iron. And from thence it took its Course from the Sun to the Distance of above 11 Thousand Millions of Miles, which is at least 14 Times farther than the Orbit of Saturn, the Boundary of the Planetary Syftem.--The Light and Heat therefore of that Comet is then near two Hundred Times lefs than at Saturn, and above 17 Thousand Times less than with us; thus the Light and Heat of Saturn is much more intense, compared with the Comets, than ours in Comparison of Saturn's. Euphrof. Oh! Cleonicus, were it not for the Power of Numbers, we could never be able to form Ideas of fuch extreme Degrees of Cold and Heat! for its Heat, I fuppofe, if compared with ours, is as surprifing.- Cleon. And much more fo: For being, when nearest the Sun, about one Hundred fixty Seven Times nearer the Sun than the Earth, and fixty five Times nearer than Mercury itself: the Light and Heat therefore, at that Time at the Comet must be at least four Thousand Times greater than in Mercury; and no less than twenty eight Thoufand Times greater than in our torrid Zone. What think you of that? Euphrof Think! what fhould a Woman pretend to think of fuch Matters, as I believe I may venture to say, furpaffes the most philofophical Heads; and which they can give no juft Account of? Or, am I mistaken, Cleonicus? Cleon. Very little; for though it be poffible to produce a fmall circular Spot of Heat by Means of a burning Glafs, that shall have a Degree of Ardour equal to that at the Comet; yet this Spot can bear no more Proportion to the glowing Globe of the Comet in Point of Heat, than it does, perhaps, in Magnitude, which is infinitely fmall. Euphrof. Can you tell any Thing of the Magnitude of a Comet, Cleonicus? Cleon. I do not remember that there is any certain Menfuration of the Comets; though there is fufficient Reafon to believe that they are fome lefs, fome equal to, and fome bigger than the Moon; for they are but fmall, and it is their Tail that makes them fo confpicuous. Euphrof. But, pray, what is that you call the Tail of the Comet? Cleon. It is a prodigious Quantity of Fume and Vapours flying off from the Body of the Comet, as it becomes more and more heated in its Approach to the Sun. Euphrof. No Wonder there fhould be fuch an Evapo-. ration, confidering how fo cold a Body is gradually fo intenfely heated. What Particulars do you obferve of a Comet's Tail? Cleon. The following. (1.) They feem largest and moft fplendid immediately after they return from the Sun; because being then hottest, they emit the greatest Quantity of Vapours. (2) The Tail lies always towards thofe Parts, which the Body of the Comet leaves in its Defcent; which is agreeable to the Nature of Smoke and Vapour. (3) They appear broader on their upper Part than near the Head of the Comet; as all Vapours, the higher they rife the more they dilate themselves. (4) The fixed Stars are often feen through the Tails of Comets; which prove they confift of a very fine and pellucid Vapour. (5.) The Tails of Comets are extremely long; fome having been computed to be not lefs than eighty Millions of Miles in Length. Euphrof. Well; Nothing would more delight me, than to have but a probable Use of thofe Prodigies of Nature affigned! Can you fay, Cleonicus, to what End the Almighty has appointed those wondrous Bodies? Cleon. No, I can scarce conjecture at it; the Ancients (who were entirely ignorant of their Nature) efteemed them a Sort of Meteors kindled in the Air, and portending fome extraordinary Event. As Hudibrafs merrily expreffes it—— Portending Blood like blazing Star, |