Chemistry and Physics. 503 



tion of longitudinal waves along steel rods, we shall now consider 

 .very briefly some entirely new investigations made by Hopkinson 

 on the effects produced by detonating small cylinders of gun- 

 cotton in contact with steel plates. The gun-cotton is converted 

 into gas at small volume, high temperature, and enormous pres- 

 sure, in roughly three or four millionths of a second. The only 

 tiling which restrains the expansion of the gas is the inertia of 

 the surrounding air, and the pressure accordingly drops with very 

 great rapidity. It is estimated that the pressure falls from 120 

 tons per square inch to atmospheric value in about 1 / 25000 of a 

 second. The same pressure is, of course, exerted by the gas upon 

 any rigid surface with which the gun-cotton is in contact, and the 

 force so produced has the characteristics of a blow, namely, great 

 intensity and short duration. If a cylinder of gun-cotton weigh- 

 ing one or two ounces is placed in contact with a plate of mild 

 steel one half an inch thick, or less, and if the explosive is then 

 detonated, the effect will be to punch a clean hole through the 

 plate, of approximately the same diameter as that of the cylinder 

 of gun-cotton, just as if a projectile had passed through the plate. 

 On the other hand, if the steel plate had a thickness of three- 

 quarters of an inch, a very curious result would be obtained. A 

 depression would be formed on the side of the plate next to the 

 explosive, while a disc of steel of corresponding diameter would 

 be torn off from the opposite face of the plate and projected with 

 very high speed. The speed, in fact, corresponds to a large frac- 

 tion of the whole momentum of the blow. By detonating a two- 

 ounce cylinder of gun-cotton in contact with a still thicker plate 

 of steel, a depression and a complementary bulge were produced 

 on the respective faces of the plate. When the plate was sawed 

 in two in a plane containing the centers of the dent and of the 

 lump, the presence of an internal crack was brought to light, 

 thus showing the beginning of that separation which was com- 

 plete in the case of the plate three-quarters of an inch thick. All 

 of these phenomena can be accounted for by simple mechanical 

 principles involving the reflection of longitudinal or sound waves 

 in the metal. 



In conclusion, a few words with regard to the behavior of large 

 projectiles and armor-plate may not be without interest. Modern 

 shells are made of a special steel of great strength and consider- 

 able ductility, the region of the point only being subsequently 

 hardened by thermal treatment. When a shell of this construc- 

 tion strikes normally against a plate of wrought iron, or even 

 mild steel, it ploughs straight through the plate, pushes a plug of 

 metal before it, and emerges unscathed. A rim or lip is formed 

 on the incidence face of the plate, which is analogous to the rim 

 arising when a hole is blown in a lead plate by means of a gun- 

 cotton primer. To bring a 14-inch shell, having a speed of 2,000 

 feet per second, to rest would require at least 2*5 feet of wrought 

 iron. Modern armor plate is made as hard as possible on the 

 outside surface, the back being left tough and ductile. When a 



