312 EAETHQHA.KES. 



a table, it will be found that a small piece of iron like a 

 nail may be pushed so near to it that the needle assumes 

 a position of extremely unstable equilibrium. If the 

 table now receives the slightest tap or shake this condition 

 is overcome, and the needle flies to the iron and there 

 remains. By making the support of the needle and the 

 iron the poles of an electric circuit it is possible to 

 register the time at which motion took place with con- 

 siderable accuracy. 



With crude apparatus like this a large number of 

 small earth disturbances have been recorded. 



Another form of apparatus, employed in Japan, has 

 been a delicately constructed circuit closer. The motions 

 of this instrument were recorded by causing an electro- 

 magnet to deflect a pencil which was tracing a circle on 

 a revolving dial. The revolving dial was a disc of wood 

 covered with paper fixed to the hour-hand axle of a 

 common clock. 



A third form of apparatus used in Japan consisted of 

 a small piece of sheet lead about the size of a threepenny 

 piece suspended by a short loop from a rigid support. 

 Projecting from the lead a fine wire, about two inches in 

 length, passed freely through a hole in a metallic plate. 

 By the slightest motion of the support the small pendulum 

 of lead was set into a state of tremor and caused its 

 pointer to come in contact with one or other side of the 

 hole in the metal plate and thus to close an electric circuit. 



A more refined kind of apparatus which has been 

 employed in Japan was similar to that used by the 

 Darwins at Cambridge. This was so arranged that any 

 deflection of the mirror was permanent until the instru- 

 ment was reset, and in this way the maximum disturbance 

 which had taken place between each observation was 

 recorded. 



