by Rock-crushing and its Consequences, 1 1 



of this*. But had the rock-specimen crushed in the author's 

 experiments been a cubic foot in place of an inch and a half 

 cube, the time of crushing must have been greater than '249 

 second (or nearly a quarter of a second), or the crushing would 

 have been performed with nearly a velocity of 2 feet per second, 

 and that would be less than j^-q of the velocity with which 

 the same would have been crushed if circumstanced as in the 

 shell of our globe. And if we extend our view from the crushing 

 of a cubic foot or two to that of a cubic mile or more, we see 

 that there would be very little of the total heat evolved lost by 

 dissipation, there being scarcely any time in which that could 

 occur, the possible rate of crushing of a cubic mile being less 

 than half a second. 



In the case of a very large mass of rock crushed simultane- 

 ously, or nearly so, as every portion of the rock evolves heat pro- 

 portionate to the crushing-work done upon it, so the heated 

 portions of crushed material situated near the exterior of the 

 entire mass act as "jacketing" so as to preserve the deeper- 

 seated portions of the heated mass almost absolutely from any 

 dissipation of heat for a considerable length of time, that time 

 depending, ceteris paribus, on the conductivity of the crushed 

 material and the difference in temperature between the crushed 

 material and the uncrushed rock adjacent to it. There are 

 some experimental grounds for concluding that the absolute 

 crushing-force per unit of volume in any given rock increases as 

 the absolute volume of the mass simultaneously crushed becomes 

 greater ; thus it has been found by Rondelet that large cubes 

 of stone required a proportionately larger crushing-force than 

 smaller ones ; much stress, however, cannot be laid upon this, 

 as we cannot assume with any certainty what are the precise 

 conditions under which rock-surfaces in our earth's shell are 

 forced together, and the distribution of the crushing-pressure 

 may be indefinitely varied. 



In the author's experiments the cubes crushed by pressure on 

 two opposite faces were free upon the other four ; it cannot be 

 doubted that, had only two opposite faces been free and the 

 pressure applied simultaneously upon the four other faces, two 

 and two respectively, the pressure necessary for crushing or the 

 work thereof would have been considerably increased. Further, 

 if none of the faces were free, and all those except the two 

 opposite faces to which the crushing pressure is supposed applied, 

 had the motion outwards of any of their particles opposed by an 

 initial pressure, such as that of an insistent mass of rock, the 

 work to be done to produce crushing would be necessarily in- 



* See the author's " Earthquake-wave E xperiments," Phil. Trans. 18G2, 

 vol. clii. 



