( 20 ) 
being impressed on the underlying iron, and he claims that this is 
due to the fact that the gases acting on the iron have occupied exactly 
the same space, and no more than the solid explosive previously oc¬ 
cupied and thus transferred its form, and hence he concludes that 
only the gases evolved by the very undermost layers of gun-cotton 
act on the iron while the others are lost. 
In this same magazine* I have given an illustration of similar 
impressions which I had observed previously to meeting with von 
Forster’s paper, and I advanced the opinion then and subse¬ 
quently in my Notes on the Literature of Explosives,! that it was 
due to projection ; the residual gun-cotton being driven into the 
metal by the explosion of a portion of the original mass just as 
any other body, interposed in the path of the explosive wave, 
would have been. Of course, we are met here by the difficulties 
that this hypothesis implies: (i) That the pressure exerted upon 
the residual mass of gun-cotton is transmitted more rapidly than 
the explosive reaction is propagated within the mass, and (2) it 
implies also a great rigidity or coherency for this mass. The last 
condition requires that which is a property of masses of matter 
when moving at high velocities as in the well known candle ex¬ 
periment, and in the cutting of steel by soft iron and the like. 
The difficulties presented in the first condition do not seem so 
great as those in Lieutenant von Forster’s hypothesis. 
Some months subsequent to this, Commander T. F. Jewell, U. 
S. Navy, read a paper before the American Association for the 
Advancement of Science j on the u Apparent resistance of a body 
of air to a change of form under sudden compression,” and pre¬ 
sented as an example of the action of this phenomenon, an iron 
plate upon which a disk of gun-cotton had been detonated. The 
letters U. S. N, and the figures 1884 had been indented jn the 
surface of the gun-cotton, and similar letters and figures were 
found indented in the iron plate. He held that this indentation 
was due to the fact that the air enclosed in the letters and figures 
in the disk acted, under the sudden and enormous pressure to 
which it was subjected, like a hard body and was thus driven into 
the iron. 
*Van Nostrand’s Eng. Mag. 32, 1; Jan., 1SS5. 
fProc. Nav. Inst., 11, 110; Feb., 1S85. 
t Proc. Am. Assn., 34, Si; 1886. 
