284 Professor Bertram Ilopkinson [Jan. 26, 



thick, the curious result which I exhibit here is obtained (Fig. 4). 

 Instead of a complete hole being made, a depression is formed on the 

 gun-cotton side of the plate, while on the other a scab of metal of 

 corresponding diameter is torn off, and projected away with a velocity 

 sufficient to enable it to penetrate a thick wooden plank, or to kill 

 anyone who stands in its path. The velocity in fact corresponds to 

 a large fraction of the whole momentum of the blow\ The scab 

 behaves much in the same way as the piece which we saw would be 

 shot off the end of a rod struck at the other end if the rod were 

 divided or weakened, so as to be unable to sustain the reflected tension 

 wave. The separation of the metal implies, of course, a very large 

 tension, which can only result from some kind of reflection of the 

 original applied pressure, but the high velocity shows that this tension 

 must have been preceded by pressure over the same surface, acting for 

 a time sufficient to give its momentum to the scab. 



Wishing to ascertain how and where the separation originates, I 

 caused a tw^o-ounce cylinder of gun-cotton to be detonated in contact 

 with a somewhat thicker plate. In this case no separation of metal 

 was visible ; the only apparent effects being a dint on one side and a 

 corresponding bulge on the other. On sawing the plate in half, how- 

 ever, I was gratified to find an internal crack, obviously the beginning 

 of that separation which in the thinner plate was completed (Fig. 5). 



The pressure exerted by the gun-cotton in the experiments which 

 I have just described is practically confined to the circular area of 

 contact between it and the metal, as is shown by the accurate agree- 

 ment of the print on the plate with that circle. The effects of that 

 pressure must, however, be largely conditioned by the fact that the 

 metal upon which it acts is attached to the surrounding portions of 

 the plate, and is held back by them. In order to get an idea of the 

 effect of this factor, I have tried the experiment of removing this 

 outside metal, leaving the steel cylinder opposed to the gun-cotton. 

 If such a short cylinder of steel be placed in contact with a gun- 

 cotton cylinder of equal diameter, the result of detonation was at 

 first sight merely to flatten it out slightly, and to produce a depres- 

 sion on one side with something of a bulge on the other. No 

 external crack was visible. But on sawing the piece in half a 

 remarkable system of cracks was disclosed (Fig. 6) ; the cracks spread 

 in all directions, as though tension had been acting in every direction ; 

 in fact, it appeared as though the steel cyHnder had begun to burst. 

 The tension necessary to produce these cracks, which, as you will see, 

 must have radial as well as axial components, must originate in some 

 kind of wave action which follows the blow. The problem is very 

 complicated, and I have not yet succeeded in finding a full explana- 

 tion of the phenomenon ; but there cannot be nmch doubt that the 

 longitudinal tensions are due to a wave generally similar to that 

 which we have been discussing in connection with the rod. To 

 account for the radial tensions which the cracks show also to have 



