ft. Mallet-—Temperature attainable by Rock-crushing. 268 
to, may give rise to motion and crushing with velocities eve 
exceeding those with which aérolites traverse our atmosphere. 
e well-known experiment of cutting a hard steel file in two 
by the rapid rotation of a thin disk of soft sheet iron pressed 
against it is anotherexample. The heat developed at the work- 
ing-point, so far as it is communicated to the disk, is rapidly 
carried off and dissipated by its rotation, and it thus remains 
cool enough to be touched by the hand, although the heat de- 
veloped by it and accumulated at and near the working-point 
in the file is sufficient to raise that to the temperature at which 
cast steel becomes softened and approaches fusion. 
The cutting of steel railway bars across when at a very low 
red heat by a rapidly revolving circular saw, which revolves 
partially immersed in cold water, an se action a tor- 
Od 
i ro 
rent of incandescent fragments of steel is discharged, is a like 
cas 
e. 
Besides the heat transformed from the work of compression 
and crushing, a large amount of heat must also be generally 
produced by transformation of the work expended in friction 
and oer No experiments have as yet, to the author's 
) 
pressure is transmitted through sand or like discontinuous mat- 
ter. As in rigid solids exposed to unequal mechanical pressures 
shape or size of the particles, provided these be small in relation 
to the whole mass, and their mutual adhesion (if any) small also, 
such planes must by unequal mechanical pressure be brought 
into existence. Along any such plane we may imagine the 
