5. But when the space is enclosed, and both the bodies be fluid, they will, if they be pressed together, penetrate one another, though differently, according to their different endeavours. For suppose a hollow cylinder of hard matter, well stopped at both ends, but filled first, below with some heavy fluid body, as quicksilver, and above with water or air. If now the bottom of the cylinder be turned upwards, the heaviest fluid body, which is now at the top, having the greatest endeavour downwards, and being by the hard sides of the vessel hindered from extending itself sideways, must of necessity either be received by the lighter body, that it may sink through it, or else it must open a passage through itself, by which the lighter body may ascend. For of the two bodies, that, whose parts are most easily separated, will be the first divided; which being done, it is not necessary that the parts of the other suffer any separation at all. And therefore when two liquors, which are enclosed in the same vessel, change their places, there is no need that their smallest parts should be mingled with one another; for a way being opened through one of them, the parts of the other need not be separated. Now if a fluid body, which is not enclosed, press a hard body, its endeavour will indeed be towards the internal parts of that hard body; but being excluded by the resistance of it, the parts of the fluid body will be moved every way according to the superficies of the hard body, and that equally, if the pressure be perpendicular; for when all the parts of the cause are equal, the effects will be equal also. But if the pressure be not perpendicular, then the angles of the incidence being unequal, the expansion also will be unequal, namely, greater on that side where the angle is greater, because that motion is most direct which proceeds by the directest line. [Sidenote: When one body presseth another and doth not penetrate it, the action of the pressing body is perpendicular to the superficies of the body pressed.] 6. If a body, pressing another body, do not penetrate it, it will nevertheless give to the part it presseth an endeavour to yield, and recede in a strait line perpendicular to its superficies in that point in which it is pressed. Let A B C D (in fig. 1) be a hard body, and let another body, falling upon it in the strait line E A, with any inclination or without inclination, press it in the point A. I say the body so pressing, and not penetrating it, will give to the part A an endeavour to yield or recede in a strait line perpendicular to the line A D.