Chap. 2. PHILOSOPHICAL DISCOVERIES. 243

cases, its gravity is measured by the pressure os the quiescentbody upon the obstacle that hinders its motion. Seco?idly ywhen a body descends in the vertical or plumb-line, its mo-tion is continually accelerated, in consequence of the power otgravity’s acting incessantly upon it; or if it be projected upt-wards in the same right line, its motion is continually retarded,in consequence of the same power’s acting incessantly upon itwith a contrary direction : and, in such cases, the force ofgravity is measured by the acceleration or retardation os thethe motion produced in a given time, by the power continueduniformly for that time : but if the body descend or aseendalong an inclined plane, or move in a resisting medium, then, in.measuring this power, due regard must be had to the principlesof mechanics described in the preceding book. 'Thirdly , whena body is projected iff any direction different from the verticalline, the direction of its motion is continually varied, and acurve line is described, in consequence of the incessant actionof the power of gravity, which in such cafes is measured bythe flexure or curvature of the line described by it; for thepower is always the greater, cæteris paribus , the more it bendsthe way or course of the body from the tangent or direction inwhich it was projected. Effe<sts of the power of gravity, ofeach kind, fall under our constant observation, near the surfaceof the earth ; for the fame power which renders bodies heavywhile they are at rest, accelerates them when they descend per-pendicularly, and bends their motion into a curve line whenthey are projected in any other direction than that of theirgravity. Put we have access to judge of the powers that acton the celestial bodies by the effects of the last kind only : wesee bodies near the earth falling towards it; but this is aproof of the moon's gravity that cannot be had, till the presentstate of things comes to its dissolution. When a body is pro--

2 i 2 jected .