150 HEAT OF HIGHER LAYERS OE AIR, 



The nightly cooling of the ground acts, there- 

 fore, but little on the higher layers of the atmo- 

 sphere, and these yield up the heat, which they 

 have once taken in, but very slowly. This shows 

 why it is, that, although soon after sunset the 

 temperature of the ground falls below that of the 

 air, yet a thermometer hung up a few feet above 

 the ground, and shielded by a screen from the 

 nightly radiation towards the sky, being freely in 

 contact with the surrounding air, shows in clear 

 nights a temperature 5 or 6 degrees Cent. (9 or 

 11 P.), and even more, higher than another ther- 

 mometer, which at the same time is lying upon 

 the ground. 



The various materials which clothe the soil, 

 have altogether a far stronger radiating power 

 than the air, but between them severally there are 

 great differences. Water and plants are particu- 

 larly strong radiators. At seasons of low mean 

 temperature, and in clear nights, you will often 

 see stagnant pools, of small extent and depth, 

 become covered with a film of ice, the grass 

 clothed with rime, and the young sprigs and 

 leaves of trees nipped by frost, without the tem- 

 perature of the air falling below the freezing 

 point. 



Plants are particularly liable to be frost-bitten 

 in still nights, for, when the air is in motion, a 

 great part of their lost heat is restored to them 



