38 EQUILIBRIUM OF TEMPERATURE. 



cooling by conduction in solid bodies, and obtains expressions 

 which agree with his experimental results. 



We are now in a condition to advert with advantage to the 

 equilibrium of the temperature of the earth. There can be no 

 doubt of the existence, in this globe of ours, of a central heat. 

 At a depth under the surface of the earth, not in general ex- 

 ceeding fifteen yards, the thermometer is perfectly stationary, 

 not being affected by the change of the seasons ; but at greater 

 depths, the temperature progressively rises. M. Cordier, to 

 whom we are indebted for a most profound investigation of this 

 interesting subject, considers the two following conclusions to 

 be established by all the observations on temperature which 

 have been made at considerable depths. 1 st. That below the 

 stratum where the annual variations of the solar heat cease to 

 be sensible, a notable increase of temperature takes place as we 

 descend into the interior of the earth. 2ndly. That a certain 

 irregularity must be admitted in the distribution of the subter- 

 raneous heat, which occasions the progressive increase of tem- 

 perature to vary at different places. Fifteen yards has been 

 provisionally assumed as the average depth which corresponds to 

 an increase of one degree Fahrenheit. This is about 116 degrees 

 for each mile. Admitting this rate of increase^ we have at a 

 depth of 30i miles below the surface, a temperature of 3500, 

 which would melt cast iron, and which is amply sufficient to 

 melt the lavas, basalts, and other rocks, which have actually been 

 erupted from below in a fluid state. But this central heat has 

 long ceased to affect the surface of the earth. Fourier demon- 

 strates, from the laws of conduction, that although the crust of 

 the globe were of cast iron, heat would require myriads of years 

 to be transmitted to the surface, from a depth of 150 miles. 

 But the crust of the globe is actually composed of materials 

 greatly inferior to cast iron in conducting power. The tempe- 

 rature of the surface of the globe now depends upon the amount 

 of heat which it receives from the sun, compared with the heat 

 radiated away from its surface into free space. There is reason 

 to believe that no material change has occurred in the quantity 

 of heat received from the sun during the historical epoch. The 

 radiation from the surface of the earth has its limit in the tem- 

 perature of the planetary space in which it moves, which Fourier 

 deduces, from calculation to be 58, and which Schwanberg, 

 from a calculation on. totally different principles, estimates at 

 58.6j a very close coincidence. This low temperature 



