PHYSICAL GEOGRAPHY. 
53 
abundant evidence to believe that a central heat ex- 
ists, which exerts an important influence in main- 
taining the present temperature of the globe. This 
would seem to be proved by the existence of nu- 
merous active volcanoes ; of rocks of igneous origin 
in various parts of the earth ; and of numerous hot 
and warm springs, that preserve a uniform temper- 
ature for centuries. It is also rendered highly prob- 
able by numerous experiments made on the tem- 
perature of mines in Great Britain, France, Sax- 
ony, Switzerland, and Mexico, all of which go to 
prove that the temperature increases in proportion 
to the depth. M. Arrago also has shown, that in 
Artesian wells, which are borings by which water 
at different distances from the surface rises to, and 
even above the ground, from the pressure below, the 
temperature rises as we descend. " From numer- 
ous considerations," says Higgins, " it may be dedu- 
ced that the temperature of the interior of the earth 
increases with the depth ; and if this increase con- 
tinued to the centre, all the mineral masses must be 
in a state of fusion at a very inconsiderable depth 
beneath the lowest rocks with which geologists are 
acquainted. Some philosophers have attempted to 
assign a limit to the increase of temperature ; but, 
even according to their supposition, there are parts 
where the rocks are in a state of fusion, although 
the temperature cannot be raised higher. Strange, 
then, as it may appear, we are walking over a vast 
caldron of intumescent rocks, and are separated 
from it by only a thin crust, which is sometimes 
broken." From the result of all the observations 
hitherto made, we may safely conclude that the 
temperature increases as we descend at the rate 
of one degree of Fahrenheit for every eight fath- 
oms; consequently, at a depth short of 100 miles, 
the materials of the globe are in a state of incande- 
scence. 
The influence of climate upon the vegetable and 
E 2 
