HUMIDITY 187 



that it is precipitated as snow. Very dry air takes the 

 moisture from our bodies more rapidly than that which 

 is humid. Thus we feel chilly in winter when in a dry 

 indoor air at 75 or 80 F. ; in summer at the same tem- 

 perature we feel oppressively hot in the moisture-laden 

 air of the dog days. In the one case, the air is so dry 

 that it takes the body's moisture rapidly enough to 

 chill it; in the other, the air is so full of moisture that 

 it will not take the moisture which, if evaporated, would 

 cool it. It is therefore obvious that a person's comfort 

 depends quite as much upon the humidity as upon the 

 temperature of the air. 



Relation of humidity to ventilation. In cold cli- 

 mates, such as that of the northern part of the United 

 States, the warming of the air for ventilation often gives 

 it a parching dryness which may equal that of the 

 Sahara desert. Because this excessive dryness causes 

 the moisture on the body's surface to evaporate rapidly, 

 the body is chilled unless the temperature of the room, 

 is kept as high as 75 or 80 F. The eyes and the 

 mucous membrane of the respiratory tract also become 

 irritated by undue drying of their surfaces. By adding 

 moisture to the air so as to raise its humidity from 25 

 or 50 per cent to 70 or 75 per cent, the air may be made 

 very comfortable at a temperature of 65 F. and its 

 irritating effects removed. The addition of moisture not 

 only makes the air more wholesome but saves expense in 

 heating. 



In moist climates, such as are found in coast regions, 

 where the ordinary winter temperature is above freezing, 

 the addition of moisture may not be necessary, except 

 in cold dry weather. It is undoubtedly on this account 

 that in England 60 to 65 F. is accepted as the normal 

 temperature for comfort indoors. 



