146 Scientific Intelligence. 
As regards the modus operandi of this agency, Mallet considers 
mainly three points, viz: 1. Evolution of heat by compression 0 
rocks within the limits of elasticity; 2. Friction, by sliding of 
Rock crushing. 
Concerning the first point, Mallet deduces from known data and 
assumed conditions largely within the limits of probable maxima, 
that in an unsupported shell, the crushing force by horizontal 
thrust would exceed at least 428 times that required for crushing 
(a free cube of) the hardest known rocks; and that no practically 
possible conditions will suffice to prevent crushing under such pres- 
sures. He then proceeds to investigate experimentally the ques 
to powder occurred, were noted, as well as the distance through 
which the crushing plunger had descended at either point. The 
elevation of temperature by crushing was in all cases very percep” 
tible, and in those of the harder rocks very considerable. It was 
not directly determined, but calculated by the aid of Joule’s equiv” 
alent number, and the (known or re-determined) specific heats and 
gravities of the rocks, ‘ 
_From the elaborate table in which these results are given and 
discussed, it appears that the mean work expended in crushing 
one cubic foot of the rocks experimented upon, varied trom 
is 
and of general considerations, he obtains for the “mean rock” of 
the first 100 miles of crust, the following data: 
