SECT. XXVI. CENTRAL HEAT OP THE EARTH. 245 



on account of the mobility of fluids, by which the colder 

 masses sink downward, 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 fifty 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 encom- 

 passed by a stratum of melted lava at that depth, or it 

 must be a ball of liquid fire 7600 miles in diameter, in- 

 closed 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 subse- 

 quent consolidation and reduction of temperature by ra- 

 diation ; 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 caloric as lava, which often retains 

 its heat at a very little depth for years after its surface 

 is cool. Whatever the radiation of the earth might 

 have been in former times, certain it is that it goes on 

 very slowly in our days ; for M. Fourier has computed 

 that the central heat is decreasing from radiation by 

 only about the j^^th part of a second in a century. If 

 so, there can be no doubt that it will ultimately be dis- 

 sipated ; but as far as regards animal and vegetable life, 

 it is of very little consequence whether the center of 

 our planet be liquid fire or ice, since its condition in 

 either case could have no sensible effect on the climate at 

 its surface. The internal fire does not even impart heat 

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