MELLONI ON THE NOCTURNAL. COOLING OF BODIES. 531 



mining the temperatures of the air and of the plants ; in fact, if 

 we substitute a vegetable leaf for the lamp-black in the arrange- 

 ment adopted in our experiments, the cold produced on the 

 thermometer is no more than from 1° to 2°, as in the observa- 

 tions above named. 



5. That cotton and woollen stuffs communicate to the ther- 

 mometers degrees of cold three or four times as great as those 

 obtained by means of lamp-black and vegetable leaves; that 

 such excess is diminished by condensing the matter round the 

 bulb of the thermometer, and is reduced to the fraction of a 

 degree in the case of cotton and woollen stuffs of fine and close 

 texture ; whence it follows, that the greater energy of these sub- 

 stances arises scarcely from their greater radiating power, but 

 from the air interposed between the threads of which they are 

 formed. 



6. That the degree of cold due to the nocturnal radiation of 

 bodies, does not vary with the varying temperature of the atmo- 

 sphere. 



We shall now endeavour to prove that certain nocturnal dif- 

 ferences of heat, humidity, and aqueous precipitation, do not 

 arise, as is tacitly admitted in Wells's theory, from the direct 

 action of the cold due to the radiation of plants and from the ex- 

 posed portions of the ground; and that almost all the facts 

 which precede and accompany the formation of dew, result from 

 the presence (of shorter or longer duration) of the air around the 

 radiating surfaces. Consider, in the first place, a large and fer- 

 tile meadow, well furnished with grass, where the phEenomenon 

 of dew is developed in all its glory. Suppose the air to be calm, 

 the sky pure and clear. In order to make the reasoning clearer, 

 omit the consideration of the higher regions of the atmosphere, 

 and let us divide the rest into two strata, — the lower, which 

 scarcely rises above the grass of the meadow ; the higher, which 

 extends upward from this limit 30 or 40 metres. And although 

 experience has shown us that the cold due to the nocturnal ra- 

 diation of plants, that is, the lowering of their temperature below 

 that of the surrounding medium, sometimes reaches 2°, let us 

 suppose it to be only 1°, and let it not be forgotten, that this 

 degree of cold is always the same, whatever be the temperature of 

 the atmosphere. 



If the air is at 20°, the higher portions of the grass will de- 

 scend to 19° a few minutes after sunset; the air in contact with 



