SPREAD OF EARTHQUAKES. 107 



regions in which volcanic outbursts are quite un- 

 known. 



Any shaking given to a surface of water, for 

 instance, by a stone thrown into it, gives rise, as 

 you know, to a set of waves, to an alternate rising 

 and falling of the liquid mass, which is sent on- 

 wards in all directions from the point of first move- 

 ment. If now a solid but flexible body covers the 

 liquid surface, this, too, will share in the motion 

 more or less, according to its liability to stretch or 

 bend. In enclosed masses of liquid, any pressure 

 exerted on them may be sent on through them to 

 very great distances, with scarcely diminished force, 

 and with astonishing speed ; but, especially in cases 

 where the resistance from above is small compared 

 to the force applied, the shock immediately makes 

 itself felt at the surface. You will now understand 

 why an earthquake is perceived almost at the same 

 moment over a wide extent of country. The energy 

 of the powers let loose in the depths of the earth 

 must be the more destructive, as you will at once 

 see, the less the thickness of the solid crust beneath 

 winch they are at work. The peculiar violence of 

 earthquakes in all volcanic districts, as well as the 

 volcanic activity itself, bears witness to a quicker 

 increase in such places, from the surface down- 

 wards, of the inner heat of the earth. 



Since, by reason of the gradual cooling of the 

 surface of the earth from its original state of red 



