KINETIC MEASUREMENT OF THE ELASTICITY OF ROCKS. 



To test the apparatus, the experiment was made with a 

 tuning-fork of known frequency, giving the following result. 



Tuning-fork. Solo. 384 VS. June 8, 1904. 



Tr=40-ö95 grs. w=4-77 X lO--" grs./cm, gf = 979-8 in the laboratory. 



Here it is necessary to remark that, although the vibration 

 of the string is due to the periodic force imparted from the 

 specimen, the point of application of the force never corresjDonds 

 to a loop, but, on the contrary, there is a node in that vicinity. 

 Assuming the impressed force given by the vibrating rock, which 

 is connected with the string at a point whose distance from the 

 bridge is x=b, to vary as F cos pt, the amplitude of the motion 

 between the bridge and the rock is approximately given by 



IÖ1U r 



I a 



r< 



4 a' 



cos 



px 

 a 



•2A2 



ism"^^ i- "^ 9 cos""-^— I 



2 P^ ^ ft> 



a 



4a' 



pb 

 a 



where r is the amplitude of the rock measured at the point where 

 the string is connected, and / is the coefficient of friction of the 

 string. From this expression it is evident that the amplitude of 

 the forced vibration attains its maximum value when the point 

 of application of the impressed force is a node. 



In the above example, it is reasonable to assume that a 

 nodal point lies somewhere at a point near the origin of the 



