324 EARTHQUAKES. 



Since February 1883 I have observed a tromometer 

 in Japan, and such results as have been obtained accord 

 with results obtained in Italy. The increase in micro- 

 seismical activity with a fall of the barometer is very 

 marked. Other peculiarities in the behaviour of the 

 instrument will be referred to under ' Earth Pulsations.' 



Cause of microseismic onovements. — As to the cause 

 of tromometric movements, we have a field for speculation. 

 Possibly they may be due to slight vibratory motions 

 produced in the soil by the bending and crackling of 

 rocks produced by their rise upon the relief of atmospheric 

 pressure. If this were so we should expect similar move- 

 ments to be produced at the time of an increase of 

 pressure. Eossi suggests that they may be the result of 

 an increased escape of vapour from the molten materials 

 beneath the crust of the earth, consequent upon a relief 

 of pressure. The similarity of some of the sounds which 

 are heard with the microphone to those produced by 

 boiling water are suggestive of this, and Eossi quotes 

 instances when underground noises like those which we 

 should expect to hear from a boiling fluid have been 

 heard before earthquakes without the aid of microphones. 

 One instance was that of Viduari, a prisoner in Lima, 

 who, two days before the shock of 1824, repeatedly pre- 

 dicted the same in consequence of the noises he heard. 



A possible cause of disturbances of this order may be 

 small but sudden fluctuations in barometric pressure, which 

 are visible during a storm. During a small typhoon on 

 September 15, 1881, when in the Kurile Islands, I ob- 

 served that the needle of an aneroid worked back and forth 

 with a period of from one to three seconds. This con- 

 tinued for several hours. With every gust of wind the 

 needle suddenly rose and then immediately fell. At times 

 it trembled. These movements were observed in the open 



