Chap. 3. PHILOSOPHY. 191

to be shewn is, that the fun acts upon them likewife: forthis purpofe it is to be observed, that is to the motion os thesatellite, whereby it would be carried round its primary atrest, be superadded the same motion both in regard tovelocity and direction, as the primary it self has, it willdescribe about the primary the same orbit, with as greatregularity, as if the primary was indeed at rest. Thecause of this is that law of motion, which makes abody near the surface os the earth, when let fall, todescend perpendicularly, though the earth be in so swifta motion, that if the falling body did not partake os it,its descent would be remarkably oblique; and that a bo-dy projected describes in the most regular manner the sameparabola, whether projected in the direction, in which theearth moves, or in the opposite direction, is the project-ing force be the same a . From this we learn, thatif the satellite moved about its primary with perfect re-gularity, besides its motion about the primary, it wouldparticipate of all the motion of its primary; have thelame progressive velocity, with which the primary is car-ried about the fun; and be impelled with the same velo-city as the primary towards the fun, in a direction parallelto that impulse of its primary. And on the contrary, thewant of either of these, in particular os the impulse to-wards the fun, will occasion great inequalities in the mo-tion of the secondary planet. The inequalities, which would,arise from the absence os this impulse towards the fun arc

ib

a Xhe second of the laws of motion laid down in Book I. Ch. r.