MELLONI ON THE NOCTURNAL COOLING OF BODIES. 545 



air comprised between the summit of the plants and the surface 

 of the earth, and thus taking away from it the possibiUty of gra- 

 dually acquiring that high degree of humidity necessary to the 

 precipitation of the vapour, by reason of the small degree of cold 

 which the plants contract with regard to the surrounding medium. 



The differences in the quantity of dew on different substances 

 all arise, either from their difference of emissive power, or from 

 the diversity of their situation with regard to the heavenly vault, 

 or from the hygrometric condition of the surrounding space, or 

 from the greater or less obstacles which retard the descent of 

 the air, and thus more or less favour its frigorific reaction ; or, 

 lastly, from the proximity of the soil, which permits the return 

 of the air on the radiating substances, and gives rise to that 

 aerial circulation, whence result the gradual cooling and suc- 

 cessive augmentation of humidity in the lower stratum of the 

 atmosphere. 



To complete the study of our subject, it now only remains for 

 us to examine the intensity of the nocturnal radiation and the 

 distribution of dew in the different regions of the globe. 



Many observations have been made to determine the diurnal 

 temperature in different parts of the world, but very few with 

 the object of determining the nocturnal heat ; so that we are 

 almost entirely ignorant as to what are the true proportions be- 

 tween the temperatures of day and night in different latitudes 

 and seasons of the year. In accordance, however, with the 

 preceding remarks, it is seen that in calm and clear seasons 

 the difference between the temperature of the day and of the 

 night ought to be so much the greater as the vegetation is 

 richer and the night longer ; and we have already observed, 

 that in the nights of the early part of spring, vegetation 

 being but little developed, the temperature is less lowered than 

 in the latter part of autumn, when the plants still preserve a 

 part of their foliage. We shall now add, that in those countries 

 where the foliage is generally narrow and vertical like that of 

 New Holland, the nocturnal temperature ought to be less dimi- 

 nished, relative to the diurnal temperature, than in places of 

 the same latitude covered with plants analogous to those which 

 grow in other countries. 



But, laying aside everything depending on the alternations of 

 the seasons in our temperate climates, and on the differences of 

 vegetation in countries situated under the same latitude, it is 



