CONVECTION. 231 



and trees arrest the process of radiation 

 from the earth : and 



2ndly Shrubs and trees radiate a 

 little heat towards the earth ; and, there- 

 fore, the ground beneath is never cold 

 enough to congeal the little dew which 

 rests upon it. 



Q. What is the cause of that HOAR-FKOST 

 which arises from FROZEN FOG ? 



A. The thick fog (which invested the 

 earth during the night) is condensed by 

 the cold frost of early morning, and 

 congealed upon every object with which 

 it comes in contact. 



CHAPTER XVII. 



5. CONVECTION. 



Q. Wliat is meant by the CONVECTION of HEAT? 



A. Heat communicated by being 

 carried to another thing or place ; as 

 the hot water resting on the bottom of a 



x 2 



