47 5 Temperature of tie [Book V. 



The phenomena, which prefent themfelves for our 

 immediate confederation, will therefore be thofe which 

 are, ftrictly fpeaking, aerial or atmofpherical. The 

 temperature of the atmofphere will therefore, with 

 propriety, be confidered, and the igneous meteors 

 with which it is occafionally charged, and of which 

 the air appears not only to be the vehicle but the pa- 

 bulum. 



The variations of temperature which we experience 

 are chiefly produced in the atmofphere, at no great 

 diftance from the furface of the earth. This is evi- 

 dent from a fimple and well known fad, that the 

 earth, at a certain depth beneath the furface, always 

 preferves nearly the fame temperature, and the degree 

 of heat at thofe depths generally approaches the mean 

 annual heat of the climate. Even where there is a 

 communication with the external air, the earth, at the 

 depth of 80 or 90 feet, commonly varies but little in 

 its temperature; and where there is no fuch commu- 

 nication the variation muft be ftill more inconfiderable. 

 Thus the temperature of fprings does not vary with 

 the feafon; and thus the cave of the obfervatory at 

 Paris, which is about ninety feet below the pavement, 

 preferves the conftant temperature of about 53 de- 

 grees, never varying above half a degree in the coldeft 

 years. Van Swinden has remarked, that the moil 

 extreme cold, even exceeding o in Fahrenheit's fcale, 

 if it endures for only a few days, penetrates no fur- 

 ther than twenty inches, even when the ground is not 

 covered with fnow, and not more than ten inches when 

 there is a coat of fnow on the furface of the earth. 



The earth may, therefore, be confidered as the great 

 repofitory of heat; but when its furface is rapidly 

 cooled, the interior parts experience a diminution of 

 their heat in fome meafure proportionable, as the heat 



is 



