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 sup i>ort were removed, would cause the body to descend towards the centre of the earth, being destroyed at every instant by the resistance of the support, produces I V pre ure and a moving force differ from one another only In this respect, that the infinitely small velocities which ihe pre ure tends to produce nre 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 nre, 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 u|ion. In the interior of liquid and aeriform bodies, the pre ure is equal in all directions. (Set Hydrostatics, Pneumatics.) Centre of Pre ure, in Hydrostatics, I l>oint of a plane or side of a ve el containing a liquid n which, if a force were applied equal to the total pre and with the Opposite direction, it would exactly balain ■ effort of the total pre ure. PRKSSWORK, in Prinlinz, is the operation of taking impre ions from tyi>es. . i,y means of the pre ; distinct from composing, which is arranging the types to pre] them for pre . [s. 993]