8 Mr. 11. Mallet on the Temperature attainable 



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 par- 

 tially immersed in cold water, and from whose action a torrent 

 of incandescent fragments of steel is discharged, is a like case. 



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 detrusion. No experiments have as yet, to the author's 

 knowledge, been made upon the amount of heat developable in 

 fragmentary pulverulent masses, such as sand, by the forcible 

 transposition of more or less of the particles ; nor do we know 

 with certainty the conditions under which external mechanical 

 pressure is transmitted through sand or like discontinuous 

 matter. As in rigid solids exposed to unequal mechanical pres- 

 sures there exist planes or surfaces within the mass such as have 

 been denominated by Moseley " planes of easiest shearing/' or 

 sliding, so in masses of pulverulent matter, whatever be the 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 sand or 

 other pulverulent matter forced to move over itself in opposite 

 directions at opposite sides of the plane; that is to say, we may 

 suppose the sand forced along such a plane much in the same 

 way that a mass of sandstone or of granite would be forced along 

 such a shearing plane as had been produced in it previously by 

 mechanical pressure. If this reasoning be admitted, we must 

 suppose that heat would be developed along such a plane and at 

 short distances from it in a way more or less analogous to that 

 produced by forcing one rough surface of stone over another. 

 What the coefficient of friction in this case would be can only be 

 determined by experiment ; but we may justifiably conclude that 

 the amount of friction per unit of surface would increase propor- 

 tionately to the pressure applied externally to the entire mass — 

 exposed to more or less of which, motion at any such sur- 

 face of friction must take place. Coulomb, Morin, and others 

 have found the friction of some sorts of rough stone upon other 

 rough stone to reach as much as three fourths the pressure ; and 

 should this coefficient increase proportionately under the enor- 

 mous pressures to which a discontinuous mass at several miles 

 depth may be subjected, we can readily see that the trans- 



