ACOUSTICS AND GRAVITATION. 45 



valve atmospheric pressure must thus be reduced, to be in equilibrium with 

 the internal pressure p &p. It would follow that the size of the pin-hole bears 

 direct relation to Ap, and this seems to be the case. 



It is more difficult to account for the association of pressure increments with 

 pressure decrements (in different orders of frequency, it is true, but for the 

 same direction of current) unless these sequences result in variations of the 

 mode of vibration of the telephone plate. I shall discuss this question in 

 Chapter VI. The observations that on intense telephonic vibration the pres- 

 sure decrements frequently become pressure increments, that only certain 

 notes are reversed on commutation, etc., all point out the same argument. 

 Moreover, if the magneto symmetric inductor replaces the motor-current 

 break, decrements do not occur and commutation of current is ineffective so 

 far as observed. 



