SCIENCE OF COMMON THINGS. 163 



Cause of dew. When most dew is deposited. Difference between dew and rain. 



air does not ; in consequence of which the earth is 

 often five or ten degrees colder than the air (after sun- 

 set), although it may have been warmer than the air 

 during the whole day. 



1O5Q Why is the surface of the earth generally warmer than the air 

 during the day t 



Because the earth absorbs solar heat very freely, but 

 the air does not; in consequence of which it is often 

 many degrees warmer than the air during the day. 



1O6O Why is the surface of the ground colder in a fine night than in a 

 cloudy one ? 



Because on a fine, clear, starlight night, heat radiates 

 from the earth freely, and is lost in open space ; but 

 on a dull night, the clouds arrest the process of radi- 

 ation. 



1OQ1 Why is dew deposited inost readily on a fine, clear night f 



Because the surface of the ground radiates heat most 

 freely^ on a fine night, and (being cooled down by this 

 loss of heat) chills the vapor of the air into dew. 



1OS Why is there no dew on a dull, cloudy night? 



Because the clouds arrest the radiation of heat from, 

 the earth, and (as the heat cannot freely escape) the 

 surface is not sufficiently cooled down to chill the vapor 

 of the air into dew. 



1OQ3 Why is a cloudy night warmer than a fine, clear night? 



Because the clouds prevent the radiation of heat from 

 the earth ; in consequence of which the surface of the 

 earth remains warmer. 



1OQ-1 How do clouds arrest or prevent the radiation of heat from the 

 earth? 



The lower surfaces of the clouds turn hack the rays 

 of heat as they radiate or pass off from the earth, and 

 prevent their dispersion into space. 



1065 What is the difference between dew and rain? 



In dew, the condensation is made near the earth's 

 surface. In rain, the drops fall from a considerable 

 height. 



1066 What is the cause of loth dew and rain f 



