SUCTION, the act of sucking or draw ing up a fluid, as air, water, milk, or the like, by means of the mouth and lungs. There are many effects vulgarly attribut ed to suction, which, in reality, have very different causes. As when any one sucks water, or any other liquor, up through a pipe, it is commonly thought, that by that action the person draws the air up into his mouth, and that the water, which is contiguous to it, follows it by a kind of attraction, as if the air and water hung together ; and others fancy, that the air moves into the mouth of the suck er, and the water moves up after the air to prevent a vacuum, which, they say, nature abhors : whereas the true cause of this phenomenon is only that the air or atmosphere presses, with its whole weight, uniformly on the surface of the liquor in the vessel : and, consequently, prevents any one part of the water to rise higher than the other there : and if a pipe be put in, of any tolerable large bore, and be open at both ends, the water will rise within the pipe to the same height as without, and, indeed, a little higher, because the pressure of the air within the pipe is a little taken off by bearing against the side of the pipe. Now when any one applies his mouth to the upper end of the pipe, and sucks, his lips so strongly inclose the pipe, that no air can get between them and it; and, by the voluntary motion of the muscles, the cavity of his thorax, or breast, is opened and enlarged: by which means the air, included there, hath now a much larger space to dilate itself in, and, consequent ly, cannot press so strongly against the upper end of the pipe, as it did before the cavity of the thorax was so enlarged, and when the weight of the whole atmo sphere kept its spring bent. And that
weight or pressure being now taken off by the lips of the man that sucks, the equilibrium is destroyed, the air gravi tates on the surface of the water, but cannot do so on the upper orifice of the pipe, because the juncture of the lips takes it off ; and the spring of the air in cluded in the thorax being weakened by the dilation of its cavity, it cannot press so hard against the upper orifice of the pipe, as the water will do against the lower, and, consequently, the water must be forced up into the pipe. It is much the same thing in the suction of a com mon pump ; the sucker, being tight, takes off entirely the pressure of the at mosphere on the surface of the water within the barrel of the pump ; and, con sequently, the atmosphere, by its weight, must force the water up, to make the equilibrium.