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upon me before I was aware, and so I must stay ’till the Constellation os rappear in the Morning, before I can be able to rectify the Places of the Te-lefcopical Stars, by which I observed the Comet to pass; which I hope I maydo about a Fortnight hence; about which Time also I expect to see both theold or fir ft Comet with a Telescope; and the second or laft Comet with my Eye -for, if the Motion of them be regular, as I fee not the least Cause to doubt,I hope to be able to design their Places among the fixed Stars, without err-ing much more than I am able to fee at once with a Telescope; and there-fore I hope it will be no difficult Matter to find either of them, unless theJirJi may be gone so far as to disappear by reason of Distance, which is in-deed the greatest Part of my Fear: for, if it continue to move those Ways Ihave imagined it, whether we take the Supposition of the Motion of theEarth, and imagine the Comet to be moved in a Circle, one Side of whichtouches, or rather goes within the Orb of the Earth on one Side, and withoutthe Orb of Saturn, or at least that of Jupiter on the other, whose Plane i sinclined to that of the Ecliptick about 20 Deg. or whether we suppose theEarth to stand still, and the Comet to be moved in a great Circle whose con-vex Side is turned towards the Earth (which supposing no certain Paralla*has been observed, may be supposed of any Bigness, keeping only the salveProportion between the nearest Distance of it from the Earth and the Radiu 5or Diameter of that Circle) it must appear again very near the fame Placeabout a Fortnight hence. And I am apt to think the Body of the Comet > sof a Constitution that will last much longer than either a Month or a Year,nay than an Age ; and if I can be so lucky to meet with it again, I hope t°

trace it to its second appearing.-But I weary you with my Conjectures >

and I doubt not but that before this, you have perfected the Theory 01Comets, so as to be able to predict much more certainly what we are to ex-pect of these Comets for the future ; whereof if at your Leisure youplease to afford me a Word or two, you will much oblige me, t 'Sc.

Posthumous In one of Mr. Hook’s Discourses of Comets, containing a brief ExplicationHook f.o'of ^veral Opinions of the Antients, and some of the Moderns, of the Natureand 5 .’ ’ 104 of Comets j he takes Notice of a late Information from Prance of a Person,.D. Anthclm, a Carthusian of Dijon, pretending to have a true TheoryComets, and to be able to predict them; which, fays he, I think, may Nmuch more exactly done, than what Anthehn has, by the Way I have pub-lished in my Cometa, which was invented by Sir Christopher Wren ; by which*from any four Observations truly made, . one may certainly find the Lin'-’Distance, Motion, Inclination to the Ecliptick, its Place among the six^Stars, the Length of its Tail, Brightness, &c. so long as it shall appear t0the naked Eye ; for so long that Theory will hold pretty near, &e.

All the considerable Astronomers who have written of Comets, since GaB\do conclude them not to be sublunary, but far removed above the Moon, ^ethereal. Such were almost all those who writ of that great and very brig llCComet, which appeared to the World in the Year 1618, and such are th°^who have writ of Comets, that have appeared since; and more particularsof those two great ones, which appeared in the End of 1664, and in theginning of the Year 1665, many of which are comprised in the Stheatt^Cometicum, printed in 1667.

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