Sir I s a a c Newton’s Book II.

23-2

fhall move in a parabola, and having once pafled aboutthe fun, fhall afcend for ever without returning any more:but the fun will be placed in the focus of this parabola.

. With a velocity ftill greater the body will move in anhyperbola. But it is moft probable, that the comets movein elliptical orbits, though of a very oblong, or in thephrafe of aftronomers, of a very eccentric form, fuch asis reprefented in fig. 107, where S is the fun, C the co-met, and ABDE its orbit, wherein the diflance of Sand D far exceeds that of S and A. Whence it is, thatthey fometimes are found at a moderate diflance from thefun, and appear within the planetary regions; at othertimes they afcend to vaft diftances, far beyond the very or-bit of Saturn, and fo become invifible. That the cometsdo move in this manner is proved by our author, from com-putations built upon the obfervations, which aftronomers hadmade on many comets. Thefe computations were perform-ed by Sir I s a a c Newton himfelf upon the comet, whichappeared toward the latter end of the year 1680, and atthe beginning of the year following a ; but the learnedDr. Halley profecuted the like computations more at largein this, and alfo in many other comets b . Which computationsare made upon propofitions highly worthy of our author’s un-parallel’d genius, fuch as could fcarce have been difcoveredby any one not poftefted of the utmoft force cf invention;

» Princ. philol. Lib. III. prg. 499, poo. t Ibid. pag. poo, and pio, &c.

4. Those