Isaac Newton

“ Thus in water, bodies which, by their greater or less gravity, descend or ascend, are comparatively and apparently heavy or light, and their compararive 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 preponderating descend, nor, by yielding to the preponderating fluid, ascend, altho' by their true weight they do increase the weight of the whole, yet comparatively, and in the sense of the vulgar, they do not gravitate in the water. ”

“ For bodies further off are not attracted by the magnet so much as by the iron plate. If in this manner particles repel others of their own kind that lie next them, but do not exert their virtue on the more remote, particles of this kind will compose such fluids as are treated of in this proposition. If the virtue of any particle diffuse itself every way in infinitum, there will be required a greater force to produce an equal condensation of a greater quantity of the fluid. But whether elastic fluids do really consist of particles so repelling each other, is a physical question. ”

“ Therefore a body that is specifically heavier than a fluid contiguous 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 excels 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 ”

“ Whence neither will a motion of the parts of the fluid among themselves, be changed by a pressure communicated to the external superficies, except so far as either the figure of the superficies may be somewhere alter'd, or that all the parts of the fluid, by pressing one another more intensely or remissly, may slide with more or less difficulty among themselves. ”

“ Absolute 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 gravity, the parts of all fluids and bodies gravitate in their proper places ”

“ 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 retaining 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 assume a new figure, would now likewise ascend, descend, or put on a new figure ”

“ And therefore in an inflexible or rigid vessel, a fluid will not sustain a stronger pressure on one side than on the other, but will give way to it, and that in a moment of time; because the rigid side of the vessel does not follow the yielding liquor. ”

“ If all the parts of a sphærical fluid, homogeneous at equal distances from the centre, lying on a sphærical concentric bottom, gravitate towards the centre of the whole; the bottom will sustain the weight of a cylinder, whose base is equal to the superficies of the bottom, and whose altitude is the same with that of the incumbent fluid. ”

“ 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 consider'd as a centripetal force, fiut if the body be more lightly urg'd by that force, the difference of the forces becomes a centrifugal force, and is to be consider'd as such. ”