DEW AND FROST 35 



sterility. If the atmosphere in the African deserts 

 could be saturated with moisture, they would be- 

 come as verdant as any other portion of the earth. 



The amount of moisture, or water, which the at- 

 mosphere is capable of containing, to become fully 

 charged, varies with the degree of temperature. If 

 it is eighty degrees Fahrenheit, it will contain ten 

 grains to' the cubic foot ; if it be twenty degrees 

 Fahrenheit, the amount of vapor would be little 

 more than one or one and a half grains. Such being 

 the case, a sudden fall of the temperature would re- 

 sult in the precipitation of the superfluous moisture. 

 The diurnal depression of the thermometer is fol- 

 lowed by dew, in consequence ; or, when cold enough 

 to freeze, by hoar-frost, which is merely frozen dew. 

 Every one is aware of the clearness of the air in the 

 winter, because its low degree of temperature does 

 not allow it to absorb much aqueous vapor. When 

 the air becomes heated in the spring, evaporation 

 goes on yery rapidly to supply the deficiency in the 

 atmosphere, and thus nature dries up the soil. 



The vapors contained in the atmosphere, and its 

 reluctance to part with heat, are among the reasons 

 why, in our latitude, the temperature does not sink 

 to the freezing-point every night. Late frosts in 

 the spring are not so injurious to vegetation on the 

 coast as inland, because, when the sun is up, and 

 the temperature begins to rise, a fine mist immedi- 

 ately ascends to supply the want of the atmosphere, 



