EARTH TEEMOES. 315 



of apparatus to determine whether the motions of each 

 were synchronous. So far as I went this appeared only 

 to be sometimes the case : — but this is a question difficult 

 to determine, unless a recorder of time be added to the 

 apparatus. 



The greatest objection to observations of this sort is 

 that the sensibility of the instrument is not constant. 

 After a current has been running for several days it is no 

 longer sensible to slight shocks, it appears as if its resist- 

 ance had been increased. To overcome this it is neces- 

 sary to resharpen the carbon points and bore out the 

 pivot holes every three or four days. Farther, the battery 

 varies. This might to some extent be overcome by using 

 a battery with large plates. These two causes tend to 

 reduce the sensitiveness of the galvanometer-like recorder 

 — the deflection of the needle gradually becoming less 

 and less, and therefore day by day needing a greater 

 swing to bring it into contact with the iron. For reasons 

 such as these this instrument, to be used successfully, 

 appears to require considerable attention. 



Another form of microphone employed by the author 

 consisted of an aluminium wire standing vertically on a 

 metallic plate, its upper end passing loosely through a 

 hole in an aluminium wire standard. 



The upper end of the vertical wire was loaded with 

 lead. This contrivance possesses all the sensitiveness of 

 an ordinary microphone, whilst, if it receives a sudden 

 impulse, there is a sudden break in the current, and 

 the vertical wire is thrown from one side to the other of 

 the hole in the standard. 



After many months of tiresome observation with 

 instruments of this description, and after eliminating all 

 motions which might have been produced by accidental 

 causes, the general result obtained showed that in Tokio 



