206 G. F. Becker — Impact Friction and Faulting. 



found that the formulas for the distribution of energy in a sys- 

 tem of sheets could be reduced to terms of the total energy and 

 the coefficient of restitution which brings friction under the 

 head of the general elastic problem and accomplishes a main 

 purpose of the inquiry. Various experiments designed to test 

 the analytical results were then described. It only remains to 

 draw some general conclusions as to the character of friction 

 and to point out the application of the results to the geological 

 problems which led to the investigation. 



Character of Friction. — The term friction seems to be some- 

 what variously applied in literature, sometimes indicating the 

 whole resistance offered by a body to the passage of another 

 over its surface and sometimes only the surface resistance. It 

 appears to me more convenient to restrict it to the latter sense, 

 because in this case it is measured by what is known as the 

 coefficient of friction. When one body tends to move over 

 another, the contact being frictional, the resistance of the pas- 

 sive mass, as in any other case of motion, is the resistance which 

 it offers to acceleration, or its reaction against the active force. 

 The active force in this case is a shearing stress and its effect is 

 a shearing strain which is distributed throughout the entire 

 passive mass. The resistance as a whole, therefore, is a coun- 

 ter shearing stress, while friction considered as a surface resist- 

 ance is the value of this counter-stress for a particular plane. 

 If the passive mass is nowhere strained beyond its elastic limit 

 no relative motion can take place under ordinary circumstances 

 and the electric resistance of the minute inequalities of the fric- 

 tional surface is what is known as statical friction. If the stress 

 is sufficient to strain the minute inequalities beyond their elastic 

 limit, or to bend and compress them out of the way, relative 

 motion takes place. The conceivable case of surfaces wholly 

 elastic but only partially restitutional, which would allow of 

 relative motion, does not seem to occur, though in some in- 

 stances it is approached. The imperfectly restitutional resist- 

 ance of the minute projections is kinetic friction, and as a mat- 

 ter of experience is always attended by permanent deformation. 

 It must in all cases be accompanied by a strain of the entire 

 passive mass, though this strain may be elastic and commonly 

 is so. But if the passive mass is itself a sj^stem presenting 

 planes in which resistance is inelastic, motion and permanent 

 deformation must occur on them ah-o. The supposed entire in- 

 elasticity, like perfect elasticity, is ideal. The fact that any 

 surfaces, however inelastic, produce sound when undergoing 

 friction shows that the material is capable of molar vibrations. 

 -It is fair to conclude that the coefficient of restitution also never 

 reaches its limiting value, unity, which is of course also infera- 

 ble from experiments on impact. If the resistance on the planes- 



