PRESSURE

, is properly the action of a body which makes a continual effort or endeavour to move another; such as the action of a heavy body supported by a horizontal table; in contradistinction from percussion, or a momentary force or action. Pressure equally respects both bodies, that which presses, and that which is pressed; from the mutual equality of action and reaction.

Pressure, in the Cartesian Philosophy, is an impulsive kind of motion, or rather an endeavour to move, impressed on a fluid medium, and propagated through it. In such a pressure the Cartesians suppose the action of light to consist. And in the various modifications of this Pressure, by the surfaces of bodies, on which that medium presses, they suppose the various colours to consist, &c. But Newton shews, that if light consisted only in a Pressure, propagated without actual motion, it could not agitate and warm such bodies as reflect and refract it, as we actually find it does; and if it consisted in an instantaneous motion, or one propagated to all distances in an instant, as such Pressure supposes, there would be required an infinite force to produce that motion every moment, in every lucid particle. Farther, if light consisted either in Pressure, or in motion propagated in a fluid medium, whether instantaneously, or in time, it must follow, that it would inflect itself ad umbram; for Pressure, or motion, in a fluid medium, cannot be propagated in right lines, beyond any obstacle which shall hinder any part of the motion; but will inflect and diffuse itself, every way, into those parts of the quiescent medium which lie beyond the said obstacle.| Thus the force of gravity tends downward; but the Pressure which arises from that force of gravity, tends every way with an equable force; and, with equal ease and force, is propagated in crooked lines, as in straight ones. Waves on the surface of water, while they slide by the sides of any large obstacle, do inflect, dilate, and diffuse themselves gradually into the quiescent water lying beyond the obstacle. The waves, pulses, or vibrations of the air, in which sounds consist, do manifestly inflect themselves, though not so much as the waves of water; for the sound of a bell, or of a cannon, can be heard over a hill, which intercepts the sonorous object from our sight; and sounds are propagated as easily through crooked tubes, as through straight ones. But light is never observed to go in curved lines, nor to inflect itself ad umbram; for the fixed stars do immediately disappear on the interposition of any of the planets; as well as some parts of the sun's body, by the interposition of the Moon, or Venus, or Mercury.

Pressure of Air, Water, &c. See Air, Water, &c.

The effects anciently ascribed to the fuga vacui, are now accounted for from the weight and Pressure of the air.

The Pressure of the air on the surface of the earth, is balanced by a column of water of the same base, and about 34 feet high; or of one of Mercury of near 30 inches high; and upon every square inch at the earth's surface, that Pressure amounts to about 14 3/4 pounds avoirdupois. The elasticity of the air is equal to that Pressure, and by means of that Pressure, or elasticity, the air would rush into a vacuum with a velocity of about 1370 feet per second. At different heights above the earth's surface, the Pressure of the air is as its density and elasticity, and each decreases in such sort, that when the heights above the surface increase in arithmetical progression, the Pressure &c decrease in geometri- cal progression: and hence if the axis BC of a logarithmic curve AD be erected perpendicular to the horizon, and if the ordinate AB denote the Pressure, or elasticity, or density of the air, at the earth's sursace, then will any other absciss

The Pressure of water, as this fluid is every where of the same density, is as its depth at any place, and in all directions the same; and upon a square foot of sursace, every foot in height presses with the force of a weight of 1000 ounces or 62 (1/2)lbs avoirdupois. And hence, if AB be the depth of water in any vessel, and BE denote its Pressure at the depth B; by joining AE and drawing any other ordinates FC, HI; then shall these ordinates FG, HI, &c, denote the Pressure at the corresponding depths AG, AI, &c; also the area of the triangle ABE will denote the whole Pressure against the whole upright side AB, and which therefore is but half the Pressure on the bottom of the same area as the side. Moreover, if a hole were opened in the bottom or side of the vessel at B, the water, from the Pressure of the superincumbent fluid, would issue out with the velocity of 8√AB feet per second nearly; AB being estimated in feet.

Centre of Pressure, in Hydrostatics, is that point of any plane, to which, if the total Pressure were applied, its effect upon the plane would be the same as when it was distributed unequally over the whole; or it is that point in which the whole Pressure may be conceived to be united; or it is that point to which, if a force were applied equal to the total Pressure, but with an opposite direction, it would exactly balance, or restrain the effect of the Pressure, so that the body pressed on will not incline to either side. Thus, if ABCD (2d fig. above) be a vessel of water, and the side BC be pressed upon with a force equivalent to 20 pounds of water, this force is unequally distributed over BC, for the parts near B are less pressed than those near C, which are at a greater depth; and therefore the efforts of all the particular Pressures are united in some point E, which is nearer to C than to B; and that point E is called the centre of Pressure: if to that point a force equivalent to 20 pounds weight be applied, it will affect the plane BC in the same manner as by the Pressure of the water distributed unequally over the whole; and if to the same point the same force be applied in a contrary direction to that of the Pressure of the water, the force and the Pressure will balance each other, and by opposite endeavours destroy each other's effects. Supposing a cord EFG fixed at E, and passing over the pulley F, has a weight of 20 pounds annexed to it, and that the part of the cord FE is perpendicular to BC; then the effort of the weight G is equal, and its direction contrary, to that of the Pressure of the water. Now if E be the centre of Pressure, these two powers will be in equilibrio, and mutually defeat each other's endeavours.

This point E, or the centre of Pressure, is the same with the centre of percussion of the plane BC, the point of suspension being B, the surface of the water. And if the plane be oblique, the case is still the same, taking for the axis of suspension, the intersection of that plane and the surface of the sluid, both produced if necessary. See Cotes's Lectures, pa. 40, &c.

The centre of Pressure upon a plane parallel. to the horizon, or upon any plane where the Pressure is uniform, is the same as the centre of gravity of that plane. For the Pressure acts upon every part in the same manner as gravity does.

PRIMARY Planets, are those which revolve round the sun as a centre. Such are the planets Mercury, Venus, Terra the Earth, Mars, Jupiter, Saturn, and Herfchel, and perhaps others. They are thus called, in contradistinction from the secondary planets, or satellites, which revolve about their respective Primaries. See Planet.|