284 METEOROLOGY AND ALLIED SUBJECTS. 



The complete agreement between tliis result and tliat computed by the 

 empirical formula above given is a good joroof of tlie accuracy of tlie 

 latter up to altitudes of ten or eleven thousand feet, and it will be inter- 

 esting to make a similar study for the Alps and other mountainous 

 countries. {Z. 0. G. M., XIV, p. 105.) 



Jamin has undertaken to elucidate the formation of dew, and to re- 

 move the difliculty experienced by some in acknowledging that it is pos- 

 sible to have a cooling of the leaves of plants down to 8 or 10 degrees 

 below the temperature of the surrounding air. In the formation of dew, 

 radiation and evaporation are the two factors to be considered. For 

 the radiation we have the law of Dulong and Petit, for bodies which 

 are surrounded by air, which is as follows : 



Since the surrounding inclosure is the celestial space itself, whose 

 power of emission is certainly very near zero, therefore the term ma^ is 

 negligible. 



In consequence of the radiation of bodies, which is represented by 

 ma^+^, the temperature lowers; the lost heat is replaced by that of the 

 air coming in contact with the surface, as is expressed by the term 

 ^ipcfi.233^ The air becomes cooler, falls to the earth, and in consequence 

 of the radiation is continually being cooled still lower. Gradually the 

 lower cold stratum of air abstracts heat from the upper stratum, which 

 is still warm, so that the cooling process takes place from below and 

 upwards. 



The above describes the effect of radiation only as it occurs when the 

 sky is perfectly free from clouds; when the heavens are beclouded this 

 constitutes an enveloping surface, that can be considered as being of 

 the same temperature with that of the radiating surface of the earth. 

 In this case the first two terms of the above equation disappear, and a 

 thermometer cannot then indicate anything but the temperature of the 

 air. 



The evaporation co-operates with radiation, and is always active on 

 the moist surface of plants ; this effect is represented by the same law 

 as that which expresses the action of the psychrometer. 



Every moist body, in consequence of evaporation, assumes a lower 

 temijerature ; the process in general is analogous to that of radiation. 

 The body cools down and absorbs heat from the surrounding air so long 

 as the evaporation continues. But while the process of radiation has 

 almost no limit, the process of ev.aporation has one which is attained 

 Yvhen the surrounding cooled air has reached the limit of saturation; 

 from this point on the cooling through evaporation ceases. Any forma- 

 tion of dew, due to a further cooling, occurs in consequence of the radi- 

 ation, so that the evaporation contributes thereto only as preparatory, 

 by stimulating the cooling process which the radiation itself brings 

 about; the formation of dew itself then is only a further consequence of 



