INTERNAL FRICTION IN ROCKS 603 



set the thickness is o. 25 centimeter. The interior diameter of the 

 tube in both sets is of such a size that it will just receive a column 

 of rock 2 centimeters in diameter. The inner surface of the tube 

 in every case was not only perfectly smooth, but highly polished. 

 The angle of the bevel, by which the thickness of the wall is reduced 

 at the middle of the tube, was adopted after a long series of pre- 

 liminary experiments, which proved it to be that which was 

 demanded by the conditions to be secured. Pistons fitting accu- 

 rately into either end of these tubes were then made of chromium 

 tungsten steel, suitably tempered by being heated, quenched in oil, 

 and then ground to the exact dimensions required. 



Large blocks of each of the rocks having been secured, rough 

 columns of them were bored out by means of a hollow-bit diamond 

 drill, care being taken in the case of each rock to have all the 

 columns bored out of the rock in the same direction, that is, parallel 

 to one another, so that any possible variations due to rift, grain, 

 or incipient foliation were avoided. These rough columns were 

 then reduced to the exact size required, by being ground down in a 

 lathe by means of revolving carborundum wheels of different degrees 

 of fineness, and were finally highly polished. When completed the 

 columns were of such a size that they would just pass into the steel 

 tubes at the ordinary temperature, the tube inclosing the column 

 with an absolutely perfect mechanical fit. The column was in each 

 case 4 centimeters long and 2 centimeters in diameter. While the 

 column was thus fitted accurately into the tube, it could, by the 

 exertion of a certain amount of pressure, be moved up and down 

 within the tube. The column of rock, when inserted into the tube, 

 was so placed that its center was exactly in the center of the thinner 

 portion of the tube, as shown in the diagram, the extremities of the 

 column being in this way supported by the walls of the thicker 

 portion of the tube at either end. 



The pressure to which the rock was submitted was obtained by 

 a Wicksteed testing machine set up in the Testing Laboratory of 

 the Macdonald Engineering Building of McGill University. This 

 machine has a capacity of 100 tons and, when loaded to its capacity, 

 is sensitive to a load of 4 pounds. Unfortunately, being graduated 

 to read only in tons and pounds, it was necessary to obtain the data 



