370 Sir ISAAC N E W T O N’s Book IV.

motion of the earth, and it is impossible to bring their motionsto any regularity without allowing that motion ; and it, alone,suffices for explaining the irregularities of every comet, as wellas of every planet; we obtain from this a new confirmation ofthe motion of the earth, and find all the parts of this philo-sophy perfectly consistent.

Our author having shewed that the comets descend into theplanetary regions when they are visible to us, against the opi-nion of Des Cartes , he proceeds to trace them in their courses.It follows, from the general law of gravity already established,that they must move either in parabolic , or very excentic el-lipticals orbits, that have one focus in the centre of the fun.He then enquires, with his usual skill and a great deal oflabour, how a motion in a parabola may agree with the obser-vations that have been made upon the comets ; and, for thisend, shews how, from three observations, the parabolic tra-jectory which a comet describes may be determined : and,from several examples which he has given, there appears soperfect a harmony between his theory and the observations, asadds a new evidence to it, and shews its use in carrying on theknowledge of our system.

He insists particularly on the celebrated comet that appearednear the end of the year 1680, and in the beginning of 1681.He determines its trajectory, or curve, from three observationsmade by Mr. Flamjleed ; and then compares all the obser-vations, that were made by himself or others, with the mo-tion of a body in that curve, and finds the differences betwixtthe observed places of this comet and those computed for it inthe curve, for the fame time, to be very small. It was thefame comet that was seen in November 1680, and in Decem-ber , January , February and March following, tho’ they had

been