188 ESSAY 



may be added for those times, which will raise 

 the effect of the dew in diminishing the ap- 

 pearance of cold during them to about 6, on 

 the supposition, which cannot be far from the 

 truth, that dew had been attracted as copiously 

 by the grass, as by wool which lay upon it. 



The less difference commonly observed be- 

 tween the temperatures of grass and of air, in 

 the morning, than what occurs in the evening, 

 is likewise to be, in part, attributed to a greater 

 quantity of dew appearing in the former, than 

 in the latter season. 



A more remarkable fact, deriving an explana- 

 tion from the same source, is the greater dif- 

 ference which takes place in very cold weather, 

 if it be calm and clear, between the tempera- 

 tures of the air and of bodies on the earth, at 

 night, than in equally clear and calm weather 

 in summer; since, in very cold weather, any 

 diminution of the temperature of a portion of 

 air, in contact with a cold body, will be at- 

 tended, in consequence of the well known re- 

 lations of the atmosphere to moisture, with a 

 much less formation of water, than an equal di- 

 minution would be in summer, supposing the 

 air, before it touches the cold body, to be at 

 both times equally near to its point of repletion 

 with moisture. 



IV. In very calm nights, a portion of air, 



