210 ESSAY 



, the air being now free from fog. If the 

 atmosphere had been as still on this, as on the 

 former evenings, a greater difference would 

 doubtless have been seen. I conclude, there- 

 fore, that fogs do not in any instance furnish a 

 real exception to the general rule, that what- 

 ever exists in the atmosphere, capable of stop- 

 ping or impeding the passage of radiant heat, 

 will prevent or lessen the appearance at night 

 of a cold on the surface of the earth, greater 

 than that of the neighbouring air. 



It follows also, from what has been said in 

 this article, that the water deposited upon the 

 earth, during a fog at night, may sometimes 

 be derived from two different sources, one of 

 which is a precipitation of moisture from a con- 

 siderable part of the atmosphere, in consequence 

 of its general cold \ the other, a real formation 

 of dew, from the condensation, by means of the 

 superficial cold of the ground, of the moisture 

 of that portion of the air, which comes in con- 

 tact with it. In such a state of things, all 

 bodies will become moist, but those especially, 

 which most readily attract dew in clear weather*. 

 I have had no opportunity, however, of trying 



* The moisture observed at night by Musschenbroek in 

 Holland, and called by him dew, appears to me to have been 

 of this kind. See this Essay, p. 127. 



