From the gravity of fluids arises their pressure, which is always proportioned to the gravity. For if the particles of fluids have equal magnitude and weight, the gravity or pressure must be propor tional to the depth, and equal in every horizontal line of fluid ; consequently, the pressure on the bottom of vessels is equal in every part. The pressure of fluids up wards is equal to the pressure down wards, at any given depth. For, suppose a column of water to consist of any given number of particles acting upon each other in a perpendicular direction, the first particle acts upon the second with its own weight only ; and, as the second is stationary, or fixed by the surrounding particle, according to the third law of motion, that action and reaction are equal, it is evident that the action, or gravity, in the first is repelled in an equal degree by the reaction of the second; and in like manner the second acts on the third, with its own gravity added to that of the first; but still the reaction increases in an equivalent degree, and so on throughout the whole depth of the fluid.
The particles of a fluid, at the same depth, press each other equally in all di rections. This appears to rise out of the very nature of fluids ; for as the particles give way to every impressive force, if the pressure amongst themselves should be unequal, the fluid could never be at rest, which is contrary to experience ; there fore, we conclude that the particles press each other equally, which keeps them in their own places. This principle applies
to the whole of a fluid as well as a part. For if four or five glass tubes, of differ ent forms, be immersed in water, when the corks in the ends are taken out, the water will flow through the various wind ings of the different tubes, and rise in all of them to the same height as it stands in the straight tube : therefore the drops of fluids must be equally pressed in all di rections during their ascent through the various angles of the tube, otherwise the fluid could not rise to the same height in them all.
From the mutual pressure and equal. action of the particles of fluids, the sur face will be perfectly smooth and parallel to the horizon. If from any exterior cause the surface of water has some parts high er than the rest, these will sink down by the natural force of their own gravitation, and diffuse themselves into an even sur face. See Hynaos•vrics.