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- 
tamed, and it is also possible in some cases 
to determine the angle at which it emerges 
elsewhere. When this has been done it has 
