228 RADIATION. 



A. 1 st Because less vapour is formed 

 on a frosty day ; and 



2ndly The vapour is frozen upon 

 the ground before it can rise from the 

 earth, and becomes HOAR-FROST. 



Q. Why are FOGS more general in AUTUMN 

 than in spring ? 



A. In spring the earth is not so hot as 

 it is in autumn. In AUTUMN the earth 

 is generally wanner than the air', and, 

 therefore, the vapour (issuing from the 

 earth) is condensed into fog by the chill 

 air. 



Q. Why are FOGS more common in VALLEYS 

 than on HILLS ? 



A. 1st Because valleys contain 

 more moisture than hills : and 



2ndly They are not exposed to so 

 much wind, (which dissipates the vapour). 



Q. How does WIND dissipate FOGS ? 



A. Either by blowing them away; 

 or else by dissolving them into vapour 

 again. 



Q. What is HOAE-FKOST ? 



A. There are two sorts of hoar-frost : 

 1. FROZEN DEW : and 2. FROZEN FOG. 



