402 ENGINEERING ON THE FARM 



it which opened through the floor to the room above (Fig. 

 307). With the neck of this jacket divided into two or more 

 parts a hot-air furnace is produced which will heat two or 

 more rooms, one for each pipe. Later developments gave us 

 hot water and steam systems, and these now find some com- 

 petition in the vapor system. There is also a combination 

 system of hot air with hot water, steam, or vapor. 



HEATING PRINCIPLES 



Expansion. Before considering the effectiveness of these 

 systems, it will be well to consider the fundamental prin- 

 ciples upon which all of them operate. When a body is 

 warmed, it expands and what was a pailful (of air or water) 

 becomes more. In the case of water we can see some of it 

 flow over the edge of the pail. The pail is still full, but the 

 water does not weigh so much as it did. If this pail of water 

 were placed in a tank of cold water, it would float — only for 

 a time, however, because it would give up its heat to the 

 colder water and both would become of the same tempera- 

 ture. The same is true of air. A heated body expands 

 because of the vibration of the particles of which the sub- 

 stance is composed. The hotter the substance, the more 

 vibration, and when each particle is using more room in 

 order to vibrate, the whole mass of particles requires a 

 larger space. 



Mobility of air and water. Putty and clay are very soft 

 and are called plastic. Air and water are called mobile, 

 because they are more than plastic; the particles of which 

 they are composed will easily slide over one another. An 

 automobile or train displaces the air, and the inflow of air 

 behind it causes what is known as suction and is visible 

 because of the dust picked up by this current of air. 

 Water does the same thing when a boat passes through it. 



Convection. This mobility of air and water makes them 

 ideal carriers of heat. It makes it possible to have a fire in 

 the cellar or in an adjoining building and conduct the heat 



