M. Melloni on the Theory of Dew. 13^ 



0^ and of 25.° For my own part, I am convinced that blackened 

 or varnished thermometers descend with a constant quantity, 

 whatever be the temperature of the night. Now we may 

 conceive that tufts of cotton or wool, placed on the upper part 

 of thermometrical bulbs under the action of a clear sky, after 

 having become cold by radiation, will communicate the cold 

 they have acquired to the surrounding air, which, becoming 

 heavier, will descend in the interior afterwards to fall to the 

 ground ; but this condensed air will always require a certain 

 time to disengage itself from the obstacles which arrest it 

 among the threads. The latter, therefore, will be surrounded 

 with a colder air than at the beginning of the experiment ; 

 and, as their sinking of temperature below the ambient 

 medium must continue invariable, it must necessarily happen 

 that they will cool the more. This increase of cold will cause 

 an additional depression of temperature in the medium ; the 

 latter, in its turn, will occasion an additional cooling in tlie 

 radiating bodies, and so on consecutively, until the weight 

 acquired by the condensed air release it from the obstacles 

 which opposed its escape from the envelope. 



What takes place with small quantities of cotton and wool 

 artificially placed round thermometers, ought to occur natu- 

 rally in many circumstances. In point of fact, plants with 

 downy leaves are much colder than such as have smooth 

 leaves. The temperature of grass and that of other low 

 plants which cover fields, descends, in consequence of this 

 fHgorific reaction of the air, much below that of elevated 

 bodies, on account of the neighbourhood of the ground which 

 supports the ambient medium, and forces it to remain in the 

 presence of radiating surfaces. In reality, the stratum of 

 air in which meadow-grass is placed, is by no means immove- 

 able ; on the contrary, it whirls about exactly like the water 

 in a vessel placed upon the fire ; the aerial particles, condensed 

 by the cold of the tops of the grass, descend to the surface of 

 the meadows, are warmed by contact with the earth, ascend 

 again towards the higher parts of the grass, and so on ; but it is 

 evident that, notwithstanding this state of agitation, they at 

 last become colder, and that, in order to maintain itself con- 

 stantly in a colder state than they, the gi'ass must cool more 



