TERRESTRIAL HEAT. 



If we penetrate to depths more or less considerable, we 

 shall find that the mean temperature M of the strata will be very 

 nearly the same as at the surface. The extreme temperatures 

 T and t, will, however, undergo a considerable change, T de- 

 creasing, and t increasing. Thus the extremes gradually approach 

 each other as the depth increases, the mean M remaining nearly 

 unaltered. 



24. A certain depth will therefore be attained at length, when 

 the maximum temperature T, by its continual decrease, and the 

 minimum temperature t, by its continual increase, will become 

 respectively equal to the mean temperature M. At this depth, 

 therefore, the periodical variations at the surface disappear ; and 

 the mean temperature M is maintained permanently without the 

 least change. 



This mean temperature, however, though nearly, is not precisely 

 equal to the mean temperature at the surface. In descending, M 

 undergoes a slight increase, and at the depth where T and t become 

 equal to M, and the variation disappears, the mean temperature is 

 a little higher than the mean temperature of the surface. 



25. The depth at which the superficial vicissitudes of temperature 

 disappear varies with the latitude, with the nature of the surface, 

 and other circumstances. In our climates it varies from 80 to 

 100 feet. It diminishes in proceeding towards the equator, and 

 increases towards the pole. The excess of the permanent tem- 

 perature at this depth above the mean temperature at the surface, 

 increases with the latitude. 



The same thermometer which has been kept for sixty years in 

 the vaults of the Observatory at Paris, at the depth of eighty-eight 

 feet below the surface, has shown, during that interval, the tem- 

 perature of 11 '82 cent., which is equal to 53 Fahr., without 

 A-arying more than half a degree of Fahr., and even this variation, 

 small as it is, has been explained by the effects of currents of air 

 produced by the quarrying operations in the neighbourhood of the 

 Observatory. 



26. We must therefore infer, that within the surface of the earth 

 there exists a stratum of which the temperature is invariable, and 

 so placed that all strata superior to it are more or less affected by 

 the thermal vicissitudes of the surface, more so the nearer they 

 are to the surface, and that this stratum of invariable temperature 

 has an irregular form, approaching nearer to the surface at some 

 places, and receding further from it at others, the nature and 

 character of the surface, mountains, valleys, and plains, seas, 

 lakes, and rivers, the greater or less distance from the equator or 

 poles, and a thousand other circumstances, imparting to it varia- 

 tions of form, which it will require observations and experiments 



