Rocks and the Velocity of Seismic Waves. 



Do 



of the vertical scale divided in millim., and placed at a dis- 

 tance of 2*73 m., was observed by a telescope provided 

 with a filar micrometer. By this means, the deflexion of 1" 

 was easily measurable. 



Denoting the length and the thickness of the prism by b 

 and c respectively, the distance between the fulcrums by a, 

 and the angle of deflexion by 8, we obtain for the modulus 

 of elasticity E 



3Wa 2 



E = 



Mc*8 



where W stands for the weight suspended in the middle of 

 the prism. 



The elastic heterogeneity of rocks called for the necessity 

 of examining the constants in different directions ; for this 

 purpose, the prism was placed on its different faces on the 

 fulcrum and the moduli for two mutually perpendicular 

 directions were generally measured. These are denoted by 

 Ei and E 2 in the table of the elastic constants, and the mean 

 'of these two by E. 



Modulus of Rigidity. — The modulus of rigidity was deter- 

 mined by measuring the amount of torsion produced by a 

 given couple. Itwould lead too far were I to attempt to describe 



the details of the instrument. The rectangular prism R was 

 placed horizontal and firmly clamped at both its extremities 

 to two solid pieces, I l5 I 2 of iron. In order to prevent crack- 

 ing by too firmlv clamping, four small pieces of brass plate 



