238 PHYSICAL SCIENCE BK. vi 



fall through age, for nothing is safe from the ravages 

 of time, which waste even the solidest and strongest 

 edifice. In old buildings parts fall without being 

 knocked off, merely because they have more weight 

 2 than strength. So in the earth's body as a whole 

 it comes to pass that portions are loosened by age, 

 and being loosened, fall, causing shock to the things 

 above them. This they do primarily while they are 

 leaving their place ; for nothing, especially if it is 

 large, can be wrenched off without movement of 

 that to which it adhered. But further, when the 

 objects have fallen, they meet the solid earth and 

 rebound like a ball. When a ball falls, it jumps up 

 and bounces repeatedly, just as often, in fact, as 

 it recoils from the ground for a new flight. If the 

 loosened objects within the earth are carried down 

 into stagnant waters, this accident of itself causes 

 a shock to the vicinity through the wave cast up 

 by the weight of the objects shot suddenly down 

 from a great height. 



XI 



SOME attribute these earthquakes to fire, but 

 give different explanations of its action. When 

 fire causes intense heat at various points beneath 

 the earth, it must roll up a great cloud of vapour, 

 which can find no exit, and which dilates the air 

 by its high temperature. If the pressure of the 

 vapour is excessive, it scatters all obstructions ; but 

 if it is comparatively moderate, it merely causes 

 movement of the earth. We observe water smoke 

 when fire is applied. What the fire does to this 

 water in a narrow pot, one may suppose is done 

 on a much greater scale when a violent and wide- 



