628 FRANK D. ADAMS AND J. AUSTEN BANCROFT 



ratio of the amount of this movement to increment of loads 

 increases rather rapidly, and, after deformation amounting to 

 about 0.06 has been brought about — which requires a load of 

 38,750 pounds — the ratio of increase of load to amount of deforma- 

 tion of the column becomes constant, as when the marble is 

 deformed in the tubes with thinner walls. It will be seen, however, 

 that for the experiments in the thicker-walled tube this ratio of 

 increase is much less than when the wall was thinner, i.e., 0.25 

 centimeter being 0.0051 diametrical increase for each increase of 

 1,000 pounds in the load, instead of .0065, as in the first series of 

 experiments. 



This demonstrates that the moving rock possesses internal 

 friction and that with the increase of the lateral resistance the 

 amount or coefficient of friction rapidly increases, and at a con- 

 stant ratio. 



The investigation was then extended to the other rocks of the 

 series enumerated on pp. 598 and 599. The conditions and method 

 of conducting the experiments were in every case identical with 

 those just described with Carrara marble. Two sets of standard 

 steel tubes, having wall thicknesses of 0.25 centimeter and 0.33 

 centimeter, respectively, were employed, and the true curves were 

 plotted representing the mean of a series of experiments in each 

 case (see Figs. 9 and 10). 



''work done" in the deformation of rocks 



If Px be the load to which the specimen is subjected and Py 



be the resistance to movement offered by the inclosing walls of the 



steel cylinder, the data were first examined to ascertain whether 



the formula 



Px— Py = a, constant 



represents the movement, and it was found that this was not the 



case. They were then studied to see whether each rock possessed 



a constant factor K, which might be termed its modulus of plasticity, 



as in the formula 



Px— KPy = a, constant 



It was found that, if the data are calculated so as to take into 

 consideration the bulge of the cylinder and are plotted to show 



