INTERNAL FRICTION IN ROCKS 609 



nal friction than the water, but at the range of pressure to which 

 it was submitted in the present investigation this difference is not 

 noticeable and may therefore be neglected. The tallow, however, 

 being more convenient for purposes of experiment, was employed 

 in a further series of comparative experiments. 



There was one other possible source of error, namely, the fric- 

 tion between the walls of the tube and the thin feather edge of the 

 hollow-faced piston used in the experiments with the tallow. In 

 the experiments with a column of rock a flat-faced piston was of 

 course employed, and this source of friction was thus eliminated. 

 In order to ascertain the amount of this friction in the case of the 

 tallow, another steel tube was constructed, identical in all respects 

 with those used in this investigation. One end of it, however, was 

 closed so that it would be necessary to employ only a single piston, 

 and through the closed end a small copper tube was inserted, which 

 led to a powerful pump provided with an accurate pressure gage. 

 The whole apparatus having been filled with water supplied by the 

 pump, the steel tube with its cup-shaped piston was placed in a 

 75-ton Emery testing machine, and the piston slowly forced into 

 the fluid, the pressure required to do, this being noted at every 

 stage on the testing machine and also on the gage fitted to the 

 pump. In this way the pressure necessary to force the piston for- 

 ward was measured at each additional increment of load applied 

 to the piston by the Emery machine. As a result of a series of 

 trials, it was ascertained that the friction on the feather edges of 

 the piston amounted on an average to only 290 pounds, so that, in 

 view of the very heavy pressure employed in this investigation, the 

 error thus introduced is so small that it may be neglected. 



It having been ascertained that soft tallow was a material which 

 for the purposes of this investigation might be considered to move 

 without the development of internal friction, a series of experiments 

 were made with steel tubes identical in character and dimensions 

 with those employed to inclose the marble, but soft tallow was 

 substituted for marble. The two series of experiments were 

 carried out in exactly the same manner in every detail, except that 

 in the tubes filled. with tallow the load was raised by increments 

 of 500 pounds, instead of 1000 pounds, and the readings were taken 



