INTERNAL FRICTION IN ROCKS 611 



different rocks was the same, a special series of experiments to 

 decide this question was made, employing copper, lead, marble, 

 Belgian black, and granite. In each instance the experiment was 

 carried to the point where the bulge or expansion of the diameter 

 amounted to 0.030. The cylinder was then removed, and by using 

 an electric arc light in a dark room a sharp shadow of the outline 

 of the bulged cylinder was cast upon sensitive paper, removed at 

 such a distance that the photograph enlarged the outline of the 

 cylinder approximately 18 times. The cylinder was then placed in 

 the Wicksteed machine, and the bulge increased to o.no, and a 

 similar photograph taken. By a comparison of the photographs it 

 was found that the outline of the deformed wall was essentially 

 identical in all cases. 



As has been mentioned, from two to rive experiments were made 

 in the case of each rock when inclosed in a 0.25-centimeter tube 

 and the same number with each rock inclosed in a tube having a 

 wall thickness of 0.33 centimeter. The mean of the closely con- 

 cordant results was then worked out in each case, and the figures 

 obtained are presented in Tables I and II. These represent the 

 data yielded by the experimental work. 



The necessary data having been thus secured, a curve was 

 plotted presenting these graphically in the case of each experiment. 

 In these curves the exact amount of the load required to produce 

 any required bulge or distension of the tube is shown from the 

 point when the first movement can be detected until the final rup- 

 ture of the tube takes place. The curves for the several experiments 

 with Carrara marble inclosed in the steel tubes with a 0.25- 

 centimeter wall are shown in Fig. 5 (p. 620). A curve represent- 

 ing the mean of the results obtained in the several experiments is 

 also given. In Fig. 6 (p. 621) this curve of the mean of the results 

 obtained from the marble inclosed in a 0.25-centimeter tube is 

 reproduced, and below it is the mean of the curves obtained from 

 tallow when inclosed in a o. 25-centimeter steel tube. 



Since the tallow, as has been shown, offers itself no measurable 

 resistance to deformation under the conditions of the experiment, 

 the curve in the tallow experiments shows merely the resistance 

 offered to deformation by the steel tube itself. 



