BAROMETER. 355 



Finally, all the variations of the barometer 

 are greater at the level of the sea than in high 

 situations, corresponding with the greater fluc- 

 tuations of temperature of the lower atmosphere. 



Deiv and Frost. 



After the setting of the sun, the surface of the 

 earth is cooled down by radiation from 12 to 15 

 F. below the temperature of the superincumbent 

 atmosphere, when it abstracts caloric from the 

 stratum of transparent aqueous vapour immedi- 

 ately over it, by which it coalesces into dew, or 

 crystallizes into frost, according to the tempera- 

 ture of the surface. 



If the atmosphere be full of vapour, there is 

 a large deposition of dew during summer, on 

 perfectly clear nights : but during spring and 

 autumn, instead of heavy dews, we have large 

 white frosts, that are generally followed in two 

 or three days by rain ; of which they are regarded 

 by the people as a sure indication. As a ge- 

 neral rule, the quantity of dew is in proportion 

 to the amount of invisible vapour in the air ; and 

 to the difference between the temperature of 

 day and night. It is profuse in hot climates, and 

 more abundant on continents than islands, cce- 

 teris paribus: while on the ocean, far from land, 

 there is little or no dew ; because the difference 

 between the temperature of day and night at sea is 



