16 ART, 10. S. KUSAKABE. 



These three values of the frequency, i.e. n ^ n" and n" are 

 nearly equal to one another provided there be no blunder or 

 mistake in the observation or in the redaction of the result. 

 Taking the mean of these three values as the frequency of the 

 free vibration of the specimen, with a small correction due to 

 the stiffness of the string, the modulus of elasticity is calculated 

 by the formula given in the third chapter. 



V. EXPERIMENTAL RESULTS. 



For the complete discussion of the elastic nature of rocks, 

 as many different elastic constants as the number of symmetry 

 planes, which can be drawn in the rock, must be determined. 

 As we have, however, no simple means of examining these sym- 

 metry planes, a single modulus of elasticity was determined 

 relating to two mutually perpendicuhir directions, on the supposi- 

 tion that the material was isotropic. 



The above enormously complicated method was applied to 

 one hundred and fifty eight specimens of different kinds of 

 several ages, and cost the author immense labor during one 

 complete year. For all this labor and trouble expended to obtain 

 but a poor result, he is consoled with Boyle's thought that, 

 " men are oftentimes obliged to suffer as much wet and cold 

 and dive as deep to fetch up sponges as to fetch up pearls." 



The table at the end of the text contains the results arranged 

 in the order of geological age ; for the same geological age, those 

 with the larger modulus come before those with the smaller. 

 The velocity for longitudinal waves, calculated by the formula 



