366 .1/. Melloui on Dew. 



sarily diminishes the accumulation of dew. A strong wind 

 impedes its formation, by bringing fresh supplies of heat, 

 and especially by renewing incessantly the stratum of air 

 comprised between the summit of the plants and the surface 

 of the earth, and thus taking away from it the possibility of 

 gradually acquiring that high degree of humidity necessary 

 to the precipitation of the vapour, by reason of the small de- 

 gree of cold which the plants contract with regard to the sur- 

 rounding medium. 



The differences of the quantity of dew on different sub- 

 stances 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 ra- 

 diating substances, and gives rise to that aerial circulation, 

 whence result the gradual cooling and successive augmenta- 

 tion of humidity in the lower stratum of the atmosphere. 



Distribution of Dew in different Regions. 



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 diur- 

 nal 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 pro- 

 portions between the temperatures of day and night in dif- 

 ferent 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 tempe- 

 rature is less lowered than in the latter part of autumn, 

 when the plants still preserve a part of their foliage. We 



