110 GENEKAL SCIENCE 



allow the foul air to escape. At the same time the heat of the smoke 

 warms the air in the vent flue and causes an upward draft from the 

 floor which removes the foul air. This method is very effective for 

 heating rooms, but is not very highly recommended as a means of 

 ventilation. 



Convection and Ventilation. It will be seen from the work under 

 convection that all our methods of obtaining pure air make use of 

 this process. The subject of ventilation, and the amount of air 

 required for each person, will be studied carefully under the head 

 of ventilation. 



How the Air is Heated. The atmosphere is warmed by radia- 

 tion, by conduction, and by convection, just as a stove warms the 

 air in a room. Radiation is more effective in warming the air than 

 conduction, since the air must come in contact with the warm 

 earth to get its heat by the latter process. Air is not perfectly 

 transparent. Some of the radiant energy is, therefore, used in 

 heating the air near the earth air which is full of water vapor and 

 dust. The higher portions of the atmosphere are chiefly heated by 

 convection, since the air, becoming warmer and lighter nearer the 

 earth, is therefore displaced by the colder and heavier upper layers 

 of air. 



SYSTEMS OF HEATING 



The various systems employed for the warming of buildings, 

 aside from the use of stoves and fireplaces, may be classified as 

 follows : 



Furnace heating ( Natural-draft, or gravity system. 

 I Forced-draft, or fan system. 



Gravity, or low-pressure systems: 



(1) Direct radiation. 



(2) Direct-indirect radiation. 



Steam heating 



(3) Indirect radiation. 



Vacuum systems: 



(1) Paul system. 



(2) Webster system. 



