HEATING AND VENTILATION 



207 



greatest enemies of mankind. Doubtless you have heard 

 of the fresh-air treatment for this disease. Doctors insist 

 that tubercular patients shall have all the fresh air it is 

 possible for them to get; this increases their 

 power to resist and overcome the disease. 



Ventilation was no problem at all when 

 houses were loosely built and heated by 

 fireplaces only. The heated air which 

 rushed up the chimneys was quickly re- 

 placed by fresh air which came in under the 

 doors and through other crevices. There 

 were plenty of "drafts," which, though un- 

 comfortable, probably did more good than 

 harm. But now that man lives mostly in 

 snugly built and usually overheated houses, 

 he must give serious attention to his fresh- 

 air supply. It is usually estimated that 

 about two thousand cubic feet of fresh air 

 per hour is desirable for each individual. 



In private houses ventilation usually con- 

 sists in simply opening the windows, and 

 this may suffice if it is done often enough, 

 and if circulation is provided by also opening some doors. 

 But in the case of public buildings, especially in the 

 case of public schools, there is need for the installation, 

 of special systems of ventilation. Very likely you. have, 

 noticed in your own schoolroom a large radiator in the 

 wall through which fresh air blows into the room, and 

 ducts through which bad air escapes. Large fans operated 

 by electricity are commonly used to drive the air through 

 such ventilating systems. Or the fans may be used to 

 draw the bad air out of the rooms. The fresh air which is 



FIG. 79. The 

 thermostat. 



