48 INTERNATIONAL EXHIBITION, 1876. conclusion which maybe drawn from recent experimental research in this direction. 1. It has always been a matter of difficulty to distinguish a “blow” from a pressure. A blow is said to be a pressure suddenly applied; but as there must always be variation in the suddenness of application, we cannot easily define the point where the pressure ends and the blow begins. It has long been assumed that the effect produced by the explosion of a charge of gunpowder within the bore of a gun partakes of the character of a blow, but recent experiments appear to indicate that this is not the case. In the investigations carried out at Woolwich in 1869-76, the pressure was directly recorded by means of an instru ment on the Rodman principle. The apparatus, termed the “ Crusher- Gauge,” is screwed into the body of the gun, and admits of the explosion acting directly on the base of a small steel piston, which, in its turn, acts upon a small cylinder of pure copper. The latter, on the explosion of the charge, is compressed by the piston, and the amount of compression is a measure of the pressure exerted. It has been found that successive applications of pressure produced by again and again using the same copper cylinder with similar charges, pro duce no further compression over that due to the first charge than might be accounted for by variation in the pressure of similar charges. For example, in a 10-inch gun the effect of firing a 400-pound projectile with a 70-pound charge of pebble-powder was to reduce the length of the copper cylinder, say from 0.5 inch to 0.25 inch; but this cylinder was not perceptibly reduced in length lower than 0.25 inch, by subjecting it to the action of several other discharges of 70- pound charges and 400-pound shot. A similar copper cylinder was then placed under a falling weight and made to receive a blow which reduced its length from 0.5 inch to 0.25 inch. It was then subjected to several successive blows given by the weight falling again and again from the same height. The result was that it was battered into a form somewhat resembling a cent. It is evident, therefore, that the pressure produced by slow-burning powder is not dynamical, or similar to that due to a weight falling from a height. The following table gives the results of an experiment in which the same cylinders were subjected several times to the action of heavy charges fired from a 12^-inch rifled gun of thirty-eight tons. In some cases the form was unchanged after the second round.
