Chap. 12.] Earth and Atmofpbere. 477 



is in that cafe drawn off towards the furface. Hence in 

 Switzerland it has been remarked, that the fnow ge- 

 nerally begins to melt at the bottom ; and if the heat 

 of the fun- is not ftrong, the fame thing may be ob- 

 ferved in the progrefs of a thaw in this country. 



The furface of the earth is capable of receiving a 

 great acceffion of heat from the fun's rays. But it has 

 been before remarked, that light has not the fame ef- 

 fecl: on a tranfparent medium, for thefe mediums af- 

 ford a free paffage to the rays of the fun, which ap- 

 pear to aft only as fire, when accumulated and con- 

 fined within the minuteft interftices of bodies. Hence 

 the tops of high mountains are always, even under 

 the equator, covered with fnow ; and hence at a cer- 

 tain height, which varies in almoft every latitude, it 

 freezes during the night in every feafon, as was ftated 

 in a preceding chapter. 



Heat is obferved to diminim as we afcend into 'the 

 atmofphere, nearly in an arithmetical proportion. In 

 the vicinity of Paris, lat. 48 50' the temperature of 

 the earth being 47, at the eflimated height of 1 1,084 

 feet, it was found by M. Charles, the aeroftatical ad- 

 venturer, to be at 21 or 11 below congelation; near 

 Dijon, lat. 47% on the 25th of April, the temperature 

 near the earth was 56, but at the height of 10,63 i 

 feet, it was found by M. Morveau to be 26 ; and 

 Lord Mulgrave, at the bottom of Hacklyt Hill, lat. 

 80, found the temperature of the lower air 50 j but 

 on the fummit of the hill, 1 503 feet, only 42. 



Water refembles air in being little arTe-fled by the 

 paflage of the fun's rays j but the bottom of every fea 

 or lake, being opake, the heat is ftill capable of being 

 excited or collected there. Between water and earth 

 there is, however, this difference, that land or earth 

 (particularly if dry) receives heat very readily from the 



rays 



