PRINCIPLES OF VENTILATION 321 



for the body. In such cases, an envelope of highly heated, 

 highly humidified air accumulates within one's clothing. 

 Moreover, when many people are quietly seated in a room 

 containing quiet air, as in the case of a schoolroom or a church, 

 there is a strong tendency toward the accumulation of a layer 

 of impure exhaled air at the height of the " breathing zone," 

 i.e., at the height of their faces. Authorities now agree that 

 the air in any room should be kept moving with such rapidity 

 that the air motion is perceptible to all. 



A careful study of the relation of air motion to ventila- 

 tion has led Dr. W. A. Evans of Chicago to declare that, " A 

 drafty room is a healthy room a windy city is a healthy 

 city." 



353. Importance of Proper Humidity. It is now a generally 

 accepted theory that just as there is a best average tempera- 

 ture from which there should be no great variation for any long 

 period of time, so there is a best humidity from which there 

 should be no great or sudden variation. Dr. Hill's students, en- 

 closed in their cage, soon raised both the temperature and the 

 humidity to such a point as to cause great discomfort. The 

 heat from their bodies caused a rapid rise in the temperature 

 of the confined air, while the moisture from their breath and 

 from perspiration soon raised the humidity nearly to the point 

 of saturation. High temperature, excessive humidity, still air, 

 and offensive odors were probably the chief causes of their dis- 

 comfort. Exactly in the same manner, the air in a crowded, 

 ill- ventilated room is likely to be at too high a temperature, 

 the humidity is likely to be excessively high, the air is almost 

 certain to have but little motion, and soon offensive odors 

 become noticeable. 



Dr. Hill has stated the principle of good ventilation in a single 

 sentence, thus: "The question of ventilation is primarily one 

 of keeping the temperature, relative moisture, and movement 

 of the air in proper state, so that the heat-regulating mechan- 

 ism of the body works without strain, and the nervous system 



is stimulated by pleasant cutaneous [skin] conditions and the 

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