SKCT. XXV. CENTRAL HEAT OF THE EARTH. 231 



estimated at the rate of 1 for every 50 or 60 feet, which corre- 

 sponds with the observations of MM. Marcet and De la Kive. 

 In consequence of the rapid increase of internal heat, thermal 

 springs, or such as are independent of volcanic action, rising 

 from a great depth, must necessarily be very rare and of a high 

 temperature ; and it is actually found that none are so low as 

 68 of Fahrenheit ; that of Chaudes Aigues, in Auvergne, is 

 about 136. In many places warm water from Artesian wells 

 will probably come into use for domestic purposes, and it is 

 even now employed in manufactories near Stutgardt, in 

 Alsace, &c. 



It is hardly to be expected that at present any information 

 with regard to the actual internal temperature of the earth 

 should be obtained from that of the ocean, on account of the 

 mo.bility of fluids, by which the colder masses sink downwards, 

 while those that are warmer rise to the surface. Nevertheless, 

 it may be stated that the temperature of the sea decreases with 

 the depth between the tropics ; while, on the contrary, all our 

 northern navigators found that the temperature increases with 

 the depth in the polar seas. The change takes place about the 

 70th parallel of latitude. Some ages hence, however, it may be 

 known whether the earth has arrived at a permanent state as to 

 heat, by comparing secular observations of the temperature of 

 the ocean if made at a great distance from the land. 



Should the earth's temperature increase at the rate of 1 for 

 every 50 feet, it is clear that at the depth of 200 miles the 

 hardest substances must be in a state of fusion, and our globe 

 must in that case either be encompassed by a stratum of melted 

 lava at that depth, or it must be a ball of liquid fire 7600 miles 

 in diameter, enclosed in a thin coating of solid matter ; for 200 

 miles are nothing when compared with the size of the earth. 

 No doubt the form of the earth as determined by the pendulum 

 and arcs of the meridian, as well as by the motions of the 

 moon, indicates original fluidity and subsequent consolidation 

 and reduction of temperature by radiation ; but whether the law 

 of increasing temperature is uniform at still greater depths than 

 those already attained by man, it is impossible to say. At all 

 events, internal fluidity is not inconsistent with the present 

 state of the earth's surface, since earthy matter is as bad a 

 conductor of heat as lava, which often retains its heat at a very 



