40 EARTHQUAKES. 



electro-magnet causes a pointer to make a mark on the 

 paper receiving the record of the motion. This mark 

 indicates the part of the earthquake at which the circuit 

 was closed. 



The duration of the earthquake is estimated from the 

 length of the record on the smoked paper and the rate of 

 motion of the drum. The nature and period of the dif- 

 ferent movements are obtained from the curves drawn on 

 the paper. 



Mr. Grray has since greatly modified this apparatus, 

 notably by the introduction of a band of paper sufficiently 

 long to take a record for twenty-four hours without repe- 

 tition. The record is written in ink by means of line 

 siphons. In this way the instrument, which is extremely 

 sensitive to change of level, can be made to show not only 

 earthquakes, but the pulsations of long period which have 

 recently occupied so much attention. 



