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of 1532 and 1661 has been noticed in the account of the firstedition of the Synopsis : whether he abandoned the idea of theiridentity without further examination, or whether upon trial liefound that an elliptic orbit for the revolution of 129 years wouldnot answer to the observations, is uncertain, as he tells us nothingon the subject. But if he now omits 0 all mention of this specu-lation, he notices another comet which had subsequently occurredto him as moving in a much longer period.

David Gregory died in 1709, and in 1715 an English editionwas published, in two volumes, of his Astronomy. The transla-tion is said by tradition to have been made by himself, and thereseems to be good reason for believing that it was so. It is de-scribed in the title as being “ done with corrections and addi-tions,” which would hardly have been hazarded by an editor;and tracesP of the author himself are to be found in the additionalmatter. Halley, however, was probably connected with the pub-lication ; for there was the intention of printing a third volume,to contain astronomical tables, derived “ from the true theory ofgravity,” and not “ drawn from a different principle,” like thosewhich are said to have previously been in use. When it is recol-lected that Halley must at this time have been engaged in draw-ing up tables which will well answer to this description, therecan be little doubt of his having contemplated this plan of pub-lishing them. At all events it must have been, not only with hisconsent, but by his direct authority, that the Synopsis was an-nexed to the second volume; for this reprint contains much whichwas not in any of the previous editions, and which he must havehimself inserted. Halley had at first expressed his belief “ that thecomet which Apian observed in the year 1531 was the same with that

0 Possibly the case is stronger thanthat of mere silent omission; for afterstating the arguments and facts respect-ing the comet of 1682 (the place in whichthe circumstances of 1661 and 1532 wereoriginally introduced), he begins the nextsentence by saying, “ut pari evidentia acin nostro hoc anni 1682, non constat re-diisse aliquem cometam. Vemm si quidargumenti ex requalitate periodorum etex phaenomenis similaribus peti possit,mirus ille cometa qui anno 1680 fnlsit,unus idemque fuit qui anno 1106 . . . esolaribus radiis primum emersit.”

P The eighth proposition of the thirdbook, on the Cassinian curve, is altered

and much enlarged. Hearne (in one ofhis MS. memorandum books) tells usthat Gregory cancelled the sheet (E e),which contained the original Latin , inconsequence of objections that Halley hadpointed out to him against what had beenprinted in it. There might, therefore, besome suspicion of another hand in thisplace, if the additional matter had notbeen introduced by the following sentence(p. 397)j which none but the author couldhave used : " In my second thoughts uponit, its different species and some proper-ties . . . not sufficiently known presentedthemselves to me.”

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