276 HEAT. 



Superficial observation often leads to very erroneous 

 conclusions. Seasoned wood will sometimes burn with 

 great rapidity, and, producing an intense heat for a short 

 time, will favor an overestimate of its superiority. Green 

 wood, on the other hand, kindles with difficulty, and 

 burns slowly and for a long time; hence, where the 

 draught of the chimney can not be controlled, it may be the 

 most economical, because a less proportion of heat may 

 be swept upward than by the more violent draught pro- 

 duced from dry materials. Where the draught can be 

 perfectly regulated, however, seasoned wood should be 

 always used, for convenience and comfort, and for economy. 



Where wood is to be drawn to a distance, the preceding 

 estimate shows that the conveyance of more than half a 

 ton of water is avoided in every cord by seasoning. 



CHAPTER II. 



RADIATION OF HEAT. 



The passage of heat through conducting bodies has 

 been already explained. There is another way in which it 

 is transmitted, termed radiation^ in which it is thrown off 

 instantaneously in straight lines from hot bodies, in the 

 same way that light is thrown off from a candle. A 

 familiar instance is furnished by the common or open fire- 

 place, before which the face may be roasted with the 

 radiated heat, while the back is chilled with cold. A 

 screen held in the hand will intercept this radiated heat, 

 showing that it flies in right lines like tiie rays of light. . 



Radiated heat is reflected by a polished metallic surface, 



