INTERNAL FRICTION IN ROCKS 



607 



change of shape. If it were possible, therefore, to ascertain the 

 amount of the pressure (or lateral resistance) exerted by the inclos- 

 ing tube, it would be possible by subtracting this from the total 

 load employed to determine the load which was required to over- 

 come the internal friction of the rock under the conditions of the 

 experiment. 



In order to determine 

 the amount of pressure 

 required to effect the pro- 

 gressive deformation of 

 the tube, i.e., the amount 

 of pressure exerted by the 

 tube on the inclosed rock 

 during the successive 

 stages of deformation, a 

 series of steel tubes, 

 identical in every respect 

 with those employed in 

 the experiment just de- 

 scribed, were taken and 

 were deformed in a pre- 

 cisely similar manner, ex- 

 cept that these tubes were 



filled with soft tallow, instead of being occupied by a column of 

 marble. This material was selected as being one which moves 

 with the development of an amount of internal friction which is 

 so small that it was negligible in the present case. In carrying 

 out the experiment with tallow, we found it necessary to slightly 

 alter the shape of the steel pistons, the ends inserted in the 

 steel tube being turned so as to present a somewhat concave face, 

 as shown in Fig. 4, the outer margins having a thin feather edge. 

 When pressure is brought to bear upon these pistons, this thin 

 edge expands slightly, thus pressing against the walls of the tube 

 and preventing the tallow from escaping between the piston and 

 the wall. It was found that in this way the deformation of the 

 tube could be readily effected. 



Fig. 3. — Photograph of columns of steatite 

 before and after deformation. The smaller 

 divisions of the scale below are millimeters. 



