106 volcanoes; earthquakes. 



cially in volcanic regions, gives us reason to believe 

 that, the underground forces are in a state of cease- 

 less activity, which from time to time results in 

 volcanic outbursts. It is commonly observed, too, 

 that, while frequent shocks occur as the forerunners 

 of an outbreak, it is followed by a long period 

 of rest, until by degrees the hollows in the earth 

 may become again filled with water and steam, and 

 these, by the gradual rise of temperature, may again 

 reach the bursting pitch of elasticity. It will often 

 have been remarked, that the most violent shocks 

 occur in volcanic districts at those times, when, by 

 the gradual stopping up of the crater-mouths, a 

 limit has been set to the continual escape of the 

 great masses of vapour. At all events this fact 

 agrees well with the property, which, as you know, 

 belongs to hot water — that while the formation 

 of steam continues, it cannot get above a certain 

 temperature, whatever the heat of the surround- 

 ing matter may be, and that, therefore, it cannot 

 attain an elasticity greater than that corresponding 

 to this temperature, — and that, if the escape 

 of steam is quite stopped, the water at last 

 must take the temperature of the neighbouring 

 bodies. 



The same underground forces, which in volcanic 

 districts throw up lava and ashes, and heave up the 

 ground, may, by means of the fluids which are 

 within the earth, transmit a part of their action to 



