a <<" 
WILLIAM G. STEVENSON. 203 
a cooling body, it is constantly shrinking. The surface 
temperature, however, remains nearly uniform—being 
largely regulated by the solar heat received, by the 
amount of heat radiated into space and by atmospheric 
influences,—so that the contraction of the exterior is 
more gradual than that of the interior portion of the 
globe. This process of contraction causes heat, and 
thus feeds the internal fires ; but as the internal portions 
contract more rapidly than the external portions, they 
shrink away from the external shell and leave hollow 
places, like a shell within a shell—with a hollow sphere 
intervening. The strength of the outer shell is not suf- 
ficient to withstand the gravity of its own rocks—and it 
breaks in here and there—with earthquake shocks—and 
leaves fractured and distorted rocks as evidence of its 
fall. Faulting thus causes earthquakes. Sucharesome 
of the geological data and inferences relating to our 
globe. 
In opposition to the theory of the internal fluidity of 
the earth, are some physical and astronomical argu- 
ments which have commanded much attention and 
gained the support of many of our ablest physicists and 
ceologists. 
Mr. Hopkins in 1839 contended against the liquidity 
of the earth’s interior, because, such a condition is, ac- 
cording to his calculations, incompatible with the plan- 
etary motions of precession and nutation. These move- 
ments cannot possibly occur as they do if the earth 
consists of ‘‘a central ocean of molten rock sur- 
rounded with a crust of twenty or thirty miles in 
thickness,’ His calculations required a crust not less 
than eight hundred or one thousand miles in thickness 
in order to permit of an explanation of these present 
motions. 
M. Delauney reasoned that ‘‘if the interior were a 
mass of sufficient viscosity,’ it might act as a solid and 
87 
