SEC. V.] OF NATURAL PHILOSOPHY. 293 SECTION V. Of the density and compression of fluids ; and of hydrostatics. THE DEFINITION OF A FLUID. A fluid is any body whose parts yield to any force impressed on it, by yielding, are easily moved among themselves. PROPOSITION XIX. THEOREM XIv All the parts of a homogeneous and unmoved fluid included in any un moved vessel, and compressed on every side (setting aside the consider ation of condensation, gravity, and all centripetal forces], will be equally pressed on every side, and remain in their places without any motion arising from that pressure. CASE 1. Let a fluid be included in the spherical vessel ABC, and uniformly compressed on every side : 1 say, that no part of it will be moved by that pressure. For if any part, as L), be moved, all such parts at the same distance from the centre on every side must necessarily be moved at the same time by a like motion ; because the pressure of them all is similar and equal ; and all other motion is excluded that does not arise from that pressure. But if these parts come all of them nearer to the centre, the fluid must be condensed towards the centre, contrary to the supposition. If they recede from it, the fluid must be condensed towards the circumfer ence ; which is also contrary to the supposition. Neither can they move in any one direction retaining their distance from the centre, because for the same reason, they may move in a contrary direction : but the sami part cannot be moved contrary ways at the same time. Therefore no part of the fluid will be moved from its place. Q,.E.T). CASE 2. I say now, that all the spherical parts of this fluid are equally pressed on every side. For let EF be a spherical part of the fluid ; if this be not pressed equally on every side, augment the lesser pressure till it be pressed equally on every side; and its parts (by Case I) will remain in their places. But before the increase of the pressure, they would remain in their places (by Case 1) ; and by the addition of a new pressure they will be moved, by the definition of a fluid, from those places. Now these two conclusions contradict each other. Therefore it was false to say that the sphere EF was not pressed equally on every side. Q,.E.D. CASE 3. I say besides, that different spherical parts have equal pressures. For the contiguous spherical parts press each other mutually and equally in the point of contact (by Law III). But (by Case 2) they are pressed on every side with the same force. Therefore any two spherical parts lot