. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. Chap, xix.] PASCAL'S LAW. 189 by which the compressibility is measured is called a piezometer. Transmission of pressure by liquids.— Pascal's law. The law or principle, first enunciated by Pascal, is, that in a liquid, pressure exerted upon any point of its mass is transmitted equally in all directions ; and the pressure is at all times perpendicular to the surface on which it is exercised. Thus, suppose a mass of liquid, pressed upo
. The elements of physiological physics: an outline of the elementary facts, principles, and methods of physics; and their applications in physiology. Biophysics. Chap, xix.] PASCAL'S LAW. 189 by which the compressibility is measured is called a piezometer. Transmission of pressure by liquids.— Pascal's law. The law or principle, first enunciated by Pascal, is, that in a liquid, pressure exerted upon any point of its mass is transmitted equally in all directions ; and the pressure is at all times perpendicular to the surface on which it is exercised. Thus, suppose a mass of liquid, pressed upon by a piston at A (Fig. 92). Suppose, also, in the interior of the mass of liquid a molecule M, one of the infinite number of molecules of which, it may be conceived, the liquid consists; then if the molecule M retains its equilibrium when pressure is exerted at A, it must be because the tendency of M to move is resisted in Fig. 92.— T ,- -i .1 -i Transmission every direction by the pressure exerted Of Pressure, upon it by the surrounding molecules. The piston presses upon the molecules of the mass of liquid in immediate contact with it; these, in turn, press upon the neighbouring molecules, and thus the pres- sure is transmitted to the walls of the vessel, which re-act upon the molecules with a force equal to their own. Thus the pressure exerted at A is transmitted equally in all directions throughout the fluid, and each molecule of the fluid is equally pressed in all directions. An important application of this principle is illus- trated in Fig. 93. A closed vessel of water ABCD has, in its upper wall, an opening pq, in which is fitted a piston P. A piston of the same size p' is fitted on one side, and one of double the size p" on the other side. If a force be exerted at p, by the law already announced it is transmitted equally in all directions, and will, consequently, act with undiminished strength upon p' and P". If, therefore, P be pushed in with a given. P
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