PHENOMENA OF "DEW." 273 



until dew appears on its surface. When this 

 takes place, by looking at the little thermometer 

 inside, the dew-point is easily read off. 



In nature, the deposition of dew takes place 

 under the following circumstances : So soon as 

 the sun sinks beneath the horizon, the earth, 

 no longer absorbing the rays of heat from that 

 source, begins rapidly to lose by radiation its 

 temperature acquired during the day. This 

 cooling process continues until the surface of the 

 ground and the bed of air overlying it has 

 reached the dew-point, and then almost every 

 object becomes covered with a deposit of dew. 



It is singular that gardeners have adopted 

 from experience, as a shelter ibr their flowers, 

 a plan which theory has subsequently shown to 

 be based upon purely scientific principles. It is 

 noticed, for instance, that it is chiefly on clear 

 calm evenings that dew is deposited, and rarely 

 on cloudy evenings. The cause of this is, that 

 the clouds prevent the loss of heat by radiation 

 from the earth, and consequently it is kept so 

 warm that the dew-point is never reached. It 

 has been found by the gardeners that a slight 

 screen will effectually preserve their plants from 

 severe cold, the radiation of heat being in this 

 simple manner effectually prevented. Bodies 

 which are good radiators, such as the hairy 

 foliage of some plants, since they lose heat more 



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