44 HEAT. 



posure. That the water is not frozen by evaporation, is 

 evident from the fact that it does not freeze in windy 

 nights, when evaporation is greatest. 



87. THE FORMATION OF DEW. Dew 

 formation of does not "fall." Its deposition is an- 

 other consequence of the cooling of the 

 earth by radiation. The air, however transparent, al- 

 ways contains moisture, absorbed and invisible. Cold, 

 causes the air, like every thing else, to contract, and 

 presses out of it, as it were, the water which it con- 

 tains. Now, when at night the earth has become 

 cooled by radiation, the warmer air which comes in 

 contact with it is cooled, and thus made to deposit its 

 moisture in the form of dew. When the temperature 

 is sufficiently low, the dew takes the form of frost. 



88. WHY CLOUDS PREVENT DEW. Clouds 

 prevent he send back the heat radiated from the earth, 

 formation of -fry a new ra di a tion, and thus prevent the 

 cooling which is essential to the produc- 

 tion of dew. No dew is found therefore, on cloudy 

 nights, ' when, if it came from above, like rain and 

 snow, we should expect most. 



89. ARTIFICIAL PREVENTION OF DEW 



How can the . 



formation of AND FROST. It is only necessary to sub- 



stitute for clouds the artificial canopy of a 

 muslin handkerchief, or any other cover- 

 ing, at a little distance from the earth, to prevent the 

 deposition of dew and frost. Gardeners practised this 

 method of protecting their tender plants from frost, 

 long before philosophers explained it. 



