SEC. V.] OF NATURAL PHILOSOPHY. 297 less gravity, descend or ascend, are comparatively and apparently heavy or light ; and their comparative and apparent gravity or levity is the excess .or defect by which their true gravity either exceeds the gravity of the water or is exceeded by it. But those things which neither by preponder ating descend, nor, by yielding to the preponderating fluid, ascend, although by their true weight they do increase the weight of the whole, yet com paratively, and in the sense of the vulgar, they do not gravitate in the wa ter. For these cases are alike demonstrated. COR. 7. These things which have been demonstrated concerning gravity take place in any other centripetal forces. COR. 8. Therefore if the medium in which any body moves be acted on either by its own gravity, or by any other centripetal force, and the body be urged more powerfully by the same force ; the difference of the forces is that very motive force, which, in the foregoing Propositions, I have con sidered as a centripetal force. But if the body be more lightly urged by that force, the difference of the forces becomes a centrifugal force, and is tc be considered as such. COR. 9. But since fluids by pressing the included bodies do not change their external figures, it appears also (by Cor. Prop. XIX) that they will not change the situation of their internal parts in relation to onf another ; and therefore if animals were immersed therein, and that all sen sation did arise from the motion of their parts, the fluid will neither hurt the immersed bodies, nor excite any sensation, unless so far as those bodies may be condensed by the compression. And the case is the same of any system of bodies encompassed with a compressing fluid. All the parts of the system will be agitated with the same motions as if they were placed in a vacuum, and would only retain their comparative gravity ; unless so far as the fluid may somewhat resist their motions, or be requisite to conglutinate them by compression. PROPOSITION XXI. THEOREM XVI. £