296 THE MATHEMATICAL PRINCIPLES [BCOK II Con. 4. And therefore if another body of the same specific gravity, in capable of condensation, be immersed in this fluid, it will acquire no mo tion by the pressure of the incumbent weight: it will neither descend nor . ascend, nor change its figure. If it be spherical, it will remain so, notwith standing the pressure ; if it be square, it will remain square; and that, whether it be soft or fluid : whether it swims freely in the fluid, or lies at the bottom. For any internal part of a fluid is in the same state with the submersed body ; and the case of all submersed bodies that have the same magnitude, figure, and specific gravity, is alike. If a submersed body, re taining its weight, should dissolve and put on the form of a fluid, this body, if before it would have ascended, descended, or from any pressure as sume a new figure, would now likewise ascend, descend, or put on a new figure ; and that, because its gravity and the other causes of its motion remain. But (by Case 5, Prop. XIX; it would now be at rest, and retain its figure. Therefore also in the former case. COR. 5. Therefore a body that is specifically heavier than a fluid con tiguous to it will sink ; and that which is specifically lighter will ascend, and attain so much motion and change of figure as that excess or defect of gravity is able to produce. For that excess or defect is the same thing as an impulse, by which a body, otherwise in equilibria with the parts of the fluid, is acted on: and may be compared with the excess or defect of a weight in one of the scales of a balance. COR. 6. Therefore bodies placed in fluids have a twofold gravity • the one true and absolute, the other apparent, vulgar, and comparative. Ab solute gravity is the whole force with which the body tends downwards ; relative and vulgar gravity is the excess of gravity with which the body tends downwards more than the ambient fluid. By the first kind of grav ity the parts of all fluids and bodies gravitate in their proper places ; and therefore their weights taken together compose the weight of the whole. For the whole taken together is heavy, as may be experienced in vessels full of liquor ; and the weight of the whole is equal to the weights of all the parts, and is therefore composed of them. By the other kind of grav ity bodies do not gravitate in their places ; that is, compared with one another, they do not preponderate, but, hindering one another's endeavours to descend, remain in their proper places, as if they were not heavy. Those things which are in the air, and do not preponderate, are commonly looked on as not heavy. Those which do preponderate are commonly reckoned heavy, in as much as they are not sustained by the weight of the air. The Common weights are nothing else but the excess of the true weights above the weight of the air. Hence also, vulgarly, those things are called light which are less heavy, and, by yielding to the preponderating air, mount upwards. But these are only comparatively lig s &mA not truly so, because hey descend in racuo. Thus, in water, bodies *>icfc. by their greater or