PRESSURE

Dr. Young defines pre ure to be " a force counteracted by another force, so that no motion is produced." {Lectures on Nat. Phil.) Thus, when a heavy body is supported on a table, or the ground, the force of terrestrial gravity, which, if the support were removed, would cause the body to descend towards the centre of the earth, being destroyed at every instant by, produces a pre ure. A pre ure and a moving foVce differ from one another only the resistance of the support, produces a pre ure. in this respect, that the infinitely small velocities which the pre ure tends to produce are ince antly destroyed by the resistance of the obstacle; whereas those that are actually produced at every instant by the moving forces are accumulated in the moving body, and produce a finite velocity after a finite time. The pre ures of two different bodies are, therefore, to each other as the ma es multiplied by the infinitely small velocities which they tend to produce in the same instant of time, and which they would produce if the bodies were free to move. The pre ure of a solid body is exerted in the direction of the resultant of all the forces by which the body is acted upon. In the interior of liquid and aeriform bodies, the pre ure is equal in all directions. {See Hydrostatics, Pneumatics.) Centre of Pre ure, in Hydrostatics, is that point of a plane or side of a ve el containing a liquid to which, if a force were applied equal to the total pre ure, and with the opposite direction, it would exactly balance the effort of the total pre ure. PRESSWORK, in Printing, is the operation of taking impre ions from types, . by means of the pre ; distinct from composing, which is arranging the types to prepare them for pre . [s. 986]