WEATHER AND CLIMATE 143 



When coal or wood is burned in a stove, the warmth (heat) 

 liberated as a result of combustion is disseminated in three 

 different ways. The stove itself becomes heated largely by 

 conduction. In such cases the heat travels from molecule 

 to molecule till distributed throughout the body that is 

 heated. It is believed that in the conduction of heat in a 

 body a quickening of molecular motion is passed on through 

 it, and the degree of this molecular activity manifests itself 

 in the rising temperature at different places outward from 

 where the liberation or application of heat occurs. This 

 belief is in accordance with the molecular theory of the 

 structure of matter, and makes possible the definition that 

 heat is molecular energy. 



From a stove as a center convection currents are set up as 

 the air next the hot surface is warmed and expanded, and a 

 circulation of air is maintained. (See pages 20 and 123.) 

 A hot-air furnace is essentially a large stove located com- 

 monly in the basement and surrounded by a metal jacket 

 from which large sheet-metal pipes convey heated air as 

 convection currents direct to the rooms to be warmed. 

 Into the space between jacket and furnace cold air is con- 

 veyed directly from out-of-doors, or more commonly 

 through cold- air pipes from the various rooms to which the 

 heated air rises, thus completing a round of circulation of air 

 in these rooms. In part at least this air may be warmed 

 many times over. 



Unequal heating of the rooms of a house from use of a hot- 

 air furnace sometimes results by reason of the fact that 

 heated air cannot rise through the hot-air pipes into any 

 room from which the cold air is unable to escape freely 

 through openings of sufficient size. Distribution of the 

 heated air to different rooms in a house may be controlled by 

 use of dampers in the hot-air pipes. 



In hot-water heating plants the heat liberated at a furnace 



