will be at its Aphelion at A, in half the Time of the Period, viz. 37 Years, which was in the Year 1720; therefore if to this we add the other Half, it will bring it again to its Perihelion B, which will be 1757, or rather in the Spring of the Year 1758; but for the particular Time to a Week or a Month, we cannot answer; it is fufficient that at prefent we can predict a Comet will nearly about that Time return. Euphrof. Well, Cleonicus, I cannot fay but your Cometarium gives me an agreeable Foretafte of the exquifite Pleasure that fuch a Phænomenon will afford me when it happens in Reality. But my Curiofity is not yet quite fatisfied; methinks I fhould be glad to fee the Infide of this Inftrument,-could you oblige me in this respect without too much Trouble? Cleon. Nothing is more easy-I only take off the Wire and Index; it is then but a common Box--I unlock, and lift up the Lid, and you see the Simplicity of the Mechanifm at once. Euphrof. There is but a fmall Matter of it, indeed,only two circular and two oval Wheels, as I may call them; I should have thought there must have been more Work neceflary to have produced fuch an inequable, yet regular and extraordinary Motion, as that of a Comet. Clean.. On the long Picce of Wood AB, are placed two circular Wheels, EF and IH, the firft of thefe is turned by the Worm or endlefs Screw G, by the Handle K: The Axle CD of this Wheel goes through the Lid, and carries the Hand H over the Circle of the Comet's Period. (See Plate X.) The fecond Wheel IH above, and the elliptic Wheel LM, below the Bar AB, arc both fixed on the fame Axle, and therefore must both move together. The other elliptic Wheel NO, is conneed with the former by a Cat-gut Line in a Groove made in the Perimeter of each oval Wheel. And the Axle P, fixed in the Focus of this Wheel, does alfo go up through the Lid or Cover, and carries the Wire and Comet upon it, as you faw on the Top of the Inftrument. Now it is to be obferved, that the Edge of the fecond Wheel NO, is conftantly applied to that of the frit See Plate XI. firft Oval LM, and is moved by it always at unequal Distances from the Center, or rather Focus Q, and therefore the Motion muft needs be very unequable, while that of the circular Wheels is conftantly the fame. Whence by this Contrivance, an equal Motion, you fee, produces a very unequal one; which is a Sort of mechanic Paradox. Euphrof. Truly it is, Cleonicus, and could never have thought which Way it was to be done; and I cannot but admire the fingular Structure, and curious Mechanism of the Parts-To fee an uniform Motion in the circular Wheels-A different Motion in every different Part of the first oval Wheel, and yet in the fame Part the Motion is uniformly and always the fame-And laftly, to fee so very unequal a Motion in every Part of the fecond Oval; I fay, to observe so many variable and diffimilar Effects in fo fmall and fimple an Inftrument and at the fame Time explanatory of the most unusual and extraordinary Phænomena of the Syftem, is what inftructs and entertains me beyond any Thing I could have imagined.—But I fear I have kept you too long upon a Subject, which, however new and engaging to me, may in fome Degree prove tedious to you, whe understand it fo well. ; VOL. I. The Young GENTLEMAN and LADY'S PHILOSOPHY. Ο PART II. DIALOGUE I. Of the MOON. Euphrofyne. UR laft Converfation, Cleonicus, concluded our Speculations of the larger Caeleftial Bodies of the Syftem, viz. the Primary Planets and Comets; the Order of our Method, I prefume, brings us now to thofe of the Secondary Clafs. Cleonicus. Yes, as we have confidered the Sun the Center of the Great Syftem, about which the Chorus of the largest Planets move;, fo now we fhall proceed to confider the Bodies of fome of those Planets as the Center of a leffer Syftem of Secondaries: And of these we have but three, viz. the Earth, Jupiter, and Saturn. Euphrof. To the Earth you affign one Moon; to 7piter, four; and to Saturn, five, if I remember right. Cleon. 'Tis very right: Of these we will difcourfe in Order, beginning with the Moon pertaining to the Earth ; which as it is the neareft of all the heavenly Bodies to us, fo it appears the moft perfect, either by the Eye or Glafs; and exceeds even the Sun itself in its apparent Magnitude. No Wonder, therefore, Mofes fhould call it a great Light. Euphrof. I remember Milton's Defcription of the Creation of the Moon, which is very beautiful. Having defcribed that of the Sun, he fays, -Lefe |