30 



EAETHQUAKES. 



accompanying figure, s is a segment of a large sphere 

 with a centre near c. Slightly below this centre a heavy 

 weight B, which may be a lead ring, is pivoted. At the 

 time of an earthquake c is steady, and the earth's motions 

 are magnified by the pointer can in the proportion of 

 c A : A N. The working of this pointer or index is similar 

 to that of the pointer in the pendulum. 



Fig. 5. 



Closely connected with the rolling sphere seismo- 

 graphs, are Gray's rolling cylinder seismographs. 



These are two cylinders resting on a surface plate 

 with their axes at right angles to each other. Near to 

 the highest point in each of these cylinders, this point 

 remaining nearly steady when the surface plate is moved 

 backwards and forwards, there is attached the end of a 

 light index. These indices are again pivoted a short 

 distance from their ends on axes connected with the 

 surface plate. In order that the two indices may be 

 brought parallel, one is cranked at the second pivot. 



