246 AIR. 



ascends also ; and these are called 

 convective currents. 



("Convective currents;" so called from the Latin 

 words, cum-vectus {carried with) because the heat is 

 " carried with" the current.) 



Q. Is AIR HEATED by the RAYS of the SUN ? 



A. No; air is not heated (in any 

 sensible degree) by the action of the sun's 

 rays passing through it. 



Q. Why then is the AIR HOTTER on a SUNNY 

 DAY, than on a CLOUDY one? 



A. On a fine day, the sun heats the 

 surface of the earth, and the air (resting 

 on the earth) is heated by contact; as 

 soon as it is heated it ascends, and other 

 air succeeding is heated in a simitar way, 

 till all is heated by convection. 



Q. If AIR be a BAD CONDUCTOR, why does hot 

 IRON get COLD, by being EXPOSED to the AIR? 



A. A piece of hot iron exposed to 

 the air, is made cold 1st By "convec- 

 tion;" and 2ndly By "radiation." 



Q. How is hot iron (exposed to the air) made 

 cold by CONVECTION ? 



A. The air around the iron (being 

 intensely heated by contact) rapidly 

 ascends, carrying some of iis heat with it : 



