PRESSURE. To explain the use of this term in mechanics, we must remember its previous and common use. When we attempt to move matter, or when we sustain a weight, the effect is accompanied by a perception which is called pressure. The sense of touch is nothing but this perception; contact without pressure is not touch, and there are even weights too small to give the sense of touch. A small feather supported on the open palm is not felt, though it would fall to the ground if the palm were removed.
The word pressure is soon indissolubly connected with the notion of motion caused or prevented. Let the obstacle which is pressed suddenly break, and the hand which pressed must follow, unless the person who presses can take an instantaneous warning to cease his effort. Hence, whenever we see motion caused, prevented, or altered, we are apt to carry with us the notion that pressure is exerted. The weight in the scale of a balance is said to piess the scale; not that wo suppose the scale to have muscles to be acted upon, and nerves to carry news of the action to a living brain, but that we see a counteraction of the known tendency of the weight to fall, and know that if the counteraction were the work of a human agent, that agent would be conscious of the perception of pressure. Hence everything fitted to produce the sensation of pressure, such as a weight, the elasticity of a spring, fie., comes to be called a pressure, and the word loses its meaning of a perception conveyed, and takes that of an agent proper to produce that perception if the human being were situated so as to receive it.
This consideration is not unimportant, for a latent confusion between pressure as a perception and pressure as a cause proper to produce that perception has prevented many from a clear understanding of the New tonian doctrine of gravitation, has caused some to reject it altogether, and has made one or two write books against it. We have in mechanics the word FORCE, which really means nothing but the cause of motion produced,altered, or prevented; but this word force having been used in two distinct senses, namely, both in the sense of pressure and in that of acceleration produced by pressure, it is sometimes necessary to have recourse to the word pressure instead of force where precision of lan guage is required. The 'motion of the planets is continually altered :
there must be a producing cause of that alteration, a force, an agent proper to produce the sensation of pressure in a human being who should interfere with it, if such a thing could be; or, in common language, a pressure. If it is found that the laws of alteration in a planet's motion are such that the pressure must always be directed towards the sun, and if we thence say that the sun attracts the planet, or pulls the planet towards it (pull and pressure being the same terms, or at least only differing in their original meaning by this, that a pull is made towards the agent, and a pressure from him), a person who thus learns the notion of attraction may, without absolutely attributing consciousness to the sun, obtain a rather mysterious notion of attraction from something remaining of the original sense of the word pull or pressure. This it is of course desirable that he should get rid of ; but if, from failing to do so, he should be inclined to doubt of what is called attraction, he would do well to remember that by the same rule he should deny that a weight placed in the scale of a balance against the scale, or that a weight hung to a beam pulls the In.
Baring said thus much, nothing remains as to the manner of esti. mating isvatures, except to give a reference to the articles Fence, MAss, 31.0aleNTUIS, MOTION (LAWS OF), &C.
We thence collect that in problems of dynamics it is requisite to estimate pressure (in cases in which it cannot be balanced against a weight) by the velocity which it would produce or destroy in a given mass of matter, and in a given time : it being known that the weight of a mass means that pressure which, applied to the mass, would pro duce in one second 32.19 feet of velocity. An instance of this process occurs in CENTRIPETAL AND CENTRIFUGAL FORCES.