20-i G. F. Becker — Impact Friction and Faulting. 



Still more satisfactory is an experiment with slips of paper.* 

 If a considerable number of slips of uniform paper are arranged 

 in a pile held down by a weight, and a blunt edge, such as 

 that of a ruler, pressed upon them near the weight and drawn 

 forward at right angles to the direction of the edge the frac- 

 tion of an inch, the edges of the sheets will be found to occupy 

 a curve. If the operation is repeated a number of times the 

 curvature becomes very sensible. This curve can be plotted 

 from the experiment with great ease and will be found to coin- 

 cide most remarkably both as to curvature and the position of 

 the asymptote with the logarithmic curve. 



If the sheets were perfectly flexible the pressure throughout 

 the system under the narrow area in contact with the ruler 

 would be confined to nearly the same area at each contact.. 

 The weight of the slips of paper, it is true, causes an increment 

 in pressure from the surface downward, but this is inconsidera- 

 ble as compared with the combined pressure of weight and 

 ruler. If the slips are not perfectly flexible, the pressure is 

 spread over a wider and wider area from the top downward. 

 This however makes no difference in the result, since the fric- 

 ticnal resistance varies directly both as the surfaces under pres- 

 sure and the intensity of the pressure per unit of area, in other 

 words, as the total pressure which is necessarily the same (neg- 

 lecting the weight of the slips) at each contact, however the 

 pressure may be disseminated. For a single edge many edges 

 or points may be substituted. If for instance a kw dozen 

 grains of sand are strewn over the upper sheet and a bit of 

 plank be pressed upon the system and moved, the sheets will 

 still arrange themselves on the same curve. Even a single point 

 (a pencil point) will bring about the same result. On the other 

 hand, if the weight is uniformly distributed over the whole 

 upper surface and drawn away, the system will break at one or 

 a few contacts only. The experiments indicate what might 

 have been supposed d priori the reason of this failure. Films 

 of air occupy the by no means infinitesimal intervals between 

 the sheets and, unless these are broken by localized pressure, 

 such as is produced by a point, edge or narrow weight, these 

 films are shifted irregularly and prevent the uniform action of 

 friction. The phenomena are then comparable rather with 

 those of lubricated surfaces rather than with dry friction. An 

 almost self-evident modification of the experiment confirmatory 

 of this explanation may be effected by setting the blunt-edged 

 instrument in the line of motion instead of at right angles to 

 it, so that the pressure is uniform from end to end of the 

 sheets beneath the edge. If the edge slips too easily, or is too 

 smooth to communicate the necessary amount of energy to the 

 * Described at length in Geol. Cornstock Lode, p. 165. 



