Pressure can only produce an effect in time; an instantaneous alteration of velocity (ea noted in laruLsE) being mechanically absurd. But if a pressure produce its whole effect in an imperceptible time, there Is all the appearance of an instantaneous destruction or creation of velocity. Suppose a hammer, for instance, to strike an anvil with a velocity of 20 feet in a second. The instant the contact begins, both hammer and anvil begin to be compressed, and the compression begins at the rate of 20 feet a second. But the resistance to compression is euormous, and is a pressure which, though it takes time to destroy any velocity, yet will destroy a velocity of 20 feet a second in a very small fraction of a second. The moment the velocity is all destroyed the effort of the anvil and hammer (both of which are compressed) endeavouring to restore themselves, the continuation in fact of the pressure which destroyed the velocity, will give a velocity to the ham mer in a contrary direction, or the hammer will rebound, as it is well known to do. In the appendix to Professor Whewell's • Elementary Treatise on Mechanics' (third or fourth edition), a mathematical inves tigation of such problems (by Mr. Airy) will be found, upon highly
probable hypotheses as to the constitution of matter. The following result of these hypotheses (which cannot be far wrong) will give an idea of the enormous pressures which are created in common cases of what is called impact :—If a column of iron three inches high be let fall on an anvil two feet high, from a height of eight feet, the com pression of both hammer and anvil is about one-thousandth of a foot, and the pressure exerted at the moment when it is greatest is that of a quiescent column of iron similar to the hammer in its action, but of 110 less than 4000 feet high.
PlIF.SSUItE, CENTR,E OF. When a fluid presses upon a surface, there is a point in that surface (known as the centre of pressure) at which, if a force be applied in the same line with the pressure of the fluid and equal to the whole of that pressure, but in a contrary diree• tion, it will balance or counteract the whole pressure of the fluid. It is often of great importance to find the centre of pressure, and for the methods of doing so we must refer to HYDROSTATICS.