METEOROLOGY. 



tinually giving out heat,^ither by radiation or by contact, or 

 it is receiving and absorbing heat from other bodies. Upon 

 the principle that heat tends to seek an equilibrium, by means 

 of radiation and absorption among bodies, the production of 

 dew and frost may be accounted for. 



During the absence of the sun, a great quantity of heat is 

 dissipated from the surface of the earth by ladiation: by this 

 means, when the night is clear, the temperature is considerably 

 lowered : when, however, the earth is overhung by a canopy 

 of clouds, they radiate in return, or reflect, and thus maintain 

 an almost uniform temperature. When the clouds are absent, 

 all the heat radiated by the earth is lost in the upper regions 

 of space, and the surface is reduced in temperature many 

 degrees below the atmosphere. 



" The stratum of air which lies in contact with the surface 

 of the ground is then cooled by contact, and a portion of the 

 watery vapor which it had possessed in the elastic form, is 

 deposited as liquid water. If the temperature of the air be 

 itself low, and the night very clear, the cooling may proceed 

 so far that the drops of dew at the moment of their deposition 

 shall be frozen, and thus form frost." [Kane.] The fact is 

 familiar to most observers, that dew and frost are formed only 

 in clear starlight and still nights, and then only on the 

 surface of good radiators. 



The cooling of the earth's surface by the loss of radiant 

 heat, is prevented by a covering of snow or any other sub- 

 stance which intercepts its passage, and no dew or frost is 

 formed. Thus plants may be protected against frost by 

 covering them with a blanket or layer of straw : the same end 

 may be attained by raising large fires by means of damp straw, 

 brush, <fec., so as to destroy the transparency of the air by a 

 cloud of smoke and watery vapor. This mode is practiced by 

 the Incas of South America, who seem to understand the 

 conditions under which dew and frost are formed. When 



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