HOAR-FROST. 229 



Q. What w the cause of the GROUND hoar- 

 FROST, or frozen DEW? 



A. Very rapid radiation of heat from 

 the earth ; in consequence of which, the 

 surface is so cooled down, that it freezes 

 the dew condensed upon it. 



Q. Why is HOAR-FROST seen only after a very 



CLEAR NIGHT ? 



A. Unless the night has been very 



clear indeed, the earth will not have 



thrown off heat enough by radiation, to 



freeze the vapour condensed upon its 



surface. 



Q. Why does HOAR-FROST very often COVER 

 the GROUND and TREES, when the water of rivers is 

 not frozen ? 



A. Hoar-frost is not the effect of cold 

 in the air, but the cold of the earth 

 (produced by excessive radiation) ; in 

 consequence of which, the dew (con- 

 densed upon it) is frozen. 



Q. Why is the HOAR-FROST upon GRASS and 

 VEGETABLES much thicker than that upon lofty 



TREES? 



A. Because the air (resting on the 

 surface of the ground} is much colder 

 after sun-set, than the air higher up; 



