ON DEW, &c. 133 



to the production of dew ; but still the greatest 

 dew, I have ever witnessed, occurred while the 

 barometer was sinking. A corresponding ob- 

 servation is made by Mr. de Luc, who says, 

 that rain may be foretold, when dew is uncom- 

 monly abundant, in relation to the climate and 

 season*. 



To the greater or less quantity of moisture in 

 the atmosphere, at the time of the action of the 

 immediate cause of dew, are likewise to be re- 

 ferred several other facts respecting its copious- 

 ness, the explanation of which is, perhaps, not 

 so apparent, as in the preceding examples. 



In the first place ; dew is commonly more 

 plentiful in spring and autumn, than in summer; 

 the reason is, that a greater difference is ge- 

 nerally found between the temperatures of the 

 day and the night, in the former seasons of the 

 year, than in the latter. In spring, this cir- 

 cumstance is prevented often from having a 

 considerable effect, by the opposite influence of 

 northerly and easterly winds ; but, during still 

 and serene nights in autumn, dew is almost 

 always highly abundant. 



In the second place ; dew is always very co- 

 pious, on those clear and calm nights, which 

 are followed by misty or foggy mornings j the 



* Rech. sur les Mod. de 1' Atm. $ 725. 



