386. THE SAND-BLAST. [Nature, Vol. xcm. p. 188, 1914.] AMONG the many remarkable anticipations contained in T. Young's Lectures on Natural Philosophy (1807) is that in which he explains the effect of what is now commonly known as the sand-blast. On p. 144 he writes:— " There is, however, a limit beyond which the velocity of a body striking another cannot be increased without overcoming its resilience, and breaking it, however small the bulk of the first body may be, and this limit depends on the inertia of the parts of the second body, which must not be disregarded when they are impelled with a considerable velocity. For it is demonstrable that there is a certain velocity, dependent on the nature of a substance, with which the effect of any impulse or pressure is transmitted through it; a certain portion of time, which is shorter accordingly as the body is more elastic, being required for the propagation of the force through any part of it; and if the actual velocity of any impulse be in a greater proportion to this velocity than the extension or compression, of which the substance is capable, is to its whole length, it is obvious that a separation must be produced, since no parts can be extended or compressed which are not yet affected by the impulse, and the length of the portion affected at any instant is not sufficient to allow the required extension or compression. Thus if the velocity with which an impression is transmitted by a certain kind of wood be 15,000 ft. in a second, and it be susceptible of compression to the extent of 1/200 of its length, the greatest velocity that it can resist will be 75 ft. in a second, which is equal to that of a body falling from a height of about 90 ft." Doubtless this passage was unknown to 0. Reynolds when, with customary penetration, in his paper on the sand-blast (Phil. Mag. Vol. XLVI. p. 827,1873) he emphasises that "the intensity of the pressure between bodies on first impact is independent of the size of the bodies." After bis manner, Young was over-concise, and it is not clear precisely what circumstances he had in contemplation. Probably it was the longitudinal impact of bars, and at any rate this affords a convenient example. We maypth of the water over the plate, so far as may be convenient.