286 On the Physical Geology of the United States, §c. 
globe may be supposed so much greater than the mean depth of 
the ocean, that the contraction of the solids would be greater than 
that of the water, so as to produce the effect of a rise of water 
with reference to the land. And again, for another reason, viz. 
the diminution of the temperature of the land and water inde- 
pendently of the consideration of the relative masses, through 
the whole range of temperature from boiling to freezing water, 
would be entirely inadequate to account for the apparent diminu- 
tion in the level of the ocean, even if its mean depth was much 
greater than it is. 
The third cause, viz. that of an undulatory motion of the fluid 
interior of the earth, combined with a lateral tangential force,* 
harmonizes with many of the facts that have been observed. It 
will be discussed in another place. 
The fourth cause, viz. that of the secular refrigeration of the 
globe, is that usually adopted, and it seems to accord and harmo- 
nize with the facts known. 
The numerous investigations on the temperature of the earth 
at the various depths to which man has penetrated by mining, 
and by boring for salt wells and Artesian springs in different 
countries, have established the fact, that the earth becomes 
warmer as the depth increases, at the rate of one degree of Fah- 
renheit’s thermometer for forty-five to sixty-five feet in depth. 
If the temperature increases in the same proportion toward the 
centre of the earth,} the rocks at no great distance below the sur- 
face would be in a melted state. 
The form of the earth is also found to be such as would be the 
form of equilibrium of a fluid body revolving with the velocity of 
the earth. ; 
The varied mathematical researches of Cordier, Fourier, Pois- 
son and Svanberg upon the refrigeration of heated bodies, the 
temperature of the earth and of space, tend to show that the earth 
is in acooling state, and that it radiates into space more caloric 
than it receives from the sun, although it has reached what may 
be called an asymptotic condition. | 
M. Fourier has shown that the radiation of the earth must have 
been such in times past as to reduce the temperature of the exte- 
* Vide Prof. Rogers’s paper, Transactions of the Association of American Geol- 
ogists and Naturalists, Vol. I. 
t It really increases more rapidly at increased depths. 
