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 



