246 THE BEAUTIES OF NATURE chap. 



underlies Vesuvius, and during the eruption 

 of 1865 there was increased activity twice 

 a day, as we should expect to find in any 

 great fluid reservoir, but very different indeed 

 from what must have been the case if the 

 mountain was in connection with a central 

 ocean of molten matter. 



Indeed, unless the " crust " of our earth 

 was of great thickness we should be subject 

 to perpetual earthquakes. No doubt these 

 are far more frequent than is generally 

 supposed; indeed, with our improved in- 

 struments it can be shown that instead of 

 occasional vibrations, with long intermediate 

 periods of rest, we have in reality short 

 intervals of rest with long periods of vibra- 

 tion, or rather perhaps that the crust of the 

 earth is in constant tremor, with more 

 violent oscillation from time to time. 



It appears, moreover, that earthquakes 

 are not generally deep-seated. The point at 

 which the shock is vertical can be ascer- 

 tained, and it is also possible in some cases 

 to determine the angle at which it emerges 

 elsewhere. When this has been done it has 



