112 Our Surroundings 



and warmed by the furnace, it becomes very dry; that is, the 

 relative humidity becomes very low. This makes us feel chilly, 

 and we increase the fire. There are several devices on the market 

 today for adding the proper amount of moisture to the air in 

 rooms. 



Most persons are afraid of drafts in a room, but Dr. W. A. 

 Evans of Chicago, after a careful study of the relation of air 

 motion to ventilation, says that "a drafty room is a healthful 

 room." Of course he means a room where convection currents 

 keep good air of proper temperature in motion, and not a room 

 into which outside winds are blowing. 



Systems of Ventilation. There are two general methods 

 of ventilation : the gravity system and the forced system. In 

 the gravity system, fresh air is introduced into a building 

 through windows and doors, or 'by means of air shafts. In 

 the forced system, the air is blown into the building, or is drawn 

 in by suction, by means of fans, or by a combination of the 



The old time fireplace, while it did not give a uniform dis- 

 tribution of heat, provided excellent ventilation. The warm air 

 flowing up the chimney always caused convection currents in the 

 room where the fireplace was located. In rooms heated by stoves, 

 fresh air is commonly introduced by opening windows and 

 doors. A circulation is kept up since the fresh air moves in 

 to take the place of the warm air as it rises over the stove. 

 If a pail of water is near the stove, considerable moisture will 

 usually pass from it in the form of water vapor to render the air 

 healthful. 



A gravity system that works well in cold weather sometimes is 

 not satisfactory in mild weather, for then the temperatures of the 

 outside air and of the inside air are so nearly equal that an ade- 

 quate convection current is not produced. In such a case it may be 

 necessary to heat the air in the shaft or flue by means of a stove 

 in the basement. 



When steam or hot water is used for heating, a building may 

 be ventilated by the indirect radiation system, or by the direct- 

 indirect radiation system. In the first method, coils of steam or 



