MELLONI ON THE NOCTURNAL COOLING OF BODIES. 543 



may be completely defended against the violent attacks to which 

 it has been subjected of late ; that, nevertheless, the radiating 

 substances are cooled much less than was supposed, in conse- 

 quence of the bad arrangement of the instruments formerly em- 

 ployed in these kind of researches. On the other hand, both Wells, 

 his partisans and his opponents, appear to have paid no atten- 

 tion to the important part played in this phaenomenon by the 

 well-known fact of the invariable difference between the tempe- 

 rature of the air and that of the radiating body ; so that every- 

 body has completely overlooked the reaction of the medium, which 

 exerts so remarkable an influence over the distribution and the 

 intensity of the cold produced by the nocturnal radiation of the 

 soil. We have endeavoured to supply this deficiency; and, 

 taking for our point of departure the feeble degree of cold which 

 is incontestably produced in vegetables and every other radiating 

 substance exposed to the free air in a calm and clear night, we 

 have arrived at a clear explanation of, — 1st, the great difference 

 of temperature between the air which envelopes the low plants 

 of meadows and fields, and the superimposed air ; 2nd, the 

 greater degree of cold in the interior than at the surface of 

 meadows ; 3rd, the great humidity which always prevails in the 

 stratum of air wherein the plants are immersed, from the first 

 moment of the precipitation of dew ; 4th, the favourable influence 

 of a perfect calm in the atmosphere ; 5th, the accumulation of 

 dew during the whole of the night ; 6th, the more copious 

 formation of dew from midnight to daybreak, than from sunset 

 to midnight ; 7th, its abundance on plants which have smooth 

 leaves ; 8th, its small quantity on trees, in comparison with what 

 is deposited on the grass ; 9th, its transportation, or progressive 

 invasion from a lower to a higher region ; 10th, its different pro- 

 portions in different seasons; 11th, and finally, all the circum- 

 stances, without exception, which precede and accompany, at 

 any period whatever of the year, the appearance of dew on the 

 earth's surface. 



The principle of the invariable lowering of the temperature of 

 bodies exposed to the free air during calm and clear nights, 

 below the temperature of the atmosphere, constitutes therefore 

 the fundamental base on which rests the theory of the phaeno- 

 menon which we are studying. 



Let us recapitulate. Dew is not an immediate effect of the 

 cooling produced by the nocturnal radiation of vegetables on the 



