202 EARTHQUAKES. 
whether the interior of the earth is really fluid, viscous 
or solid. 
On this question there is a diversity of opinions. 
It is generally admitted that the ‘‘density of the 
crust beneath the mountains is less than below the 
plains, and still less than that below the ocean-bed,”’ 
and that there is an ‘‘exeess of heavy materials in the 
water or southern hemisphere and beneath the ocean- 
bed as compared with the continental masses ;”’ it is be- 
cause of this condition—some have said—that the vast 
body of water is held in the southern portion of the 
globe ; hence, too, it is argued the interior is composed 
of heavier material than the surface, and may be 
metallic. 
Pressure, however, would make the nucleus heavier 
even if it were composed of the same—or no heavier 
material than the crust—unless indeed the effect of 
pressure is neutralized by some other agency ; and that 
is the only force capable of thus acting. The structure 
of the earth’s crust clearly indicates that the heavier 
substances are towards the center, and it is likewise 
well established that an excess of heat exists below the 
surface also. 
The fact of internal heat, greater than the surface 
temperature is proved by the hot vapors, ashes, molten 
rocks, water and fire which are thrown out of volcanoes ; 
from thermal springs, and from the average rate of in- 
creased heat—1° Fahr., for every fifty or sixty feet 
descent—obtained in artificial borings, wells and mines— 
which, at fifty miles would give a temperature of 46009, 
which is greater than the fusing point of platinum. 
From such data geologists have urged the inference 
that the earth’s interior is a molten mass, whose heat is 
slowly escaping outward by the processes of radiation, 
conduction and convection. 
The earth therefore is constantly losing heat, and, as 
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