SCHOOL HYGIENE 431 



to the health of those living in it, as explained in Chapter II. 

 Experiments which were carried on at Yale University showed 

 that when the air was kept at the proper humidity, the 

 pupils were able to do better mental work, and were freer 

 from colds than when the humidity was low. Experiments 

 show that the proper per cent of humidity for the school- 

 room is about fifty. 



How humidity is determined. One may naturally ask at 

 this point, " How can one tell what the per cent of humidity 

 in a room is? " This is determined by an instrument called 

 a psychrometer. This consists of two ordinary thermom- 

 eters, one of which has a piece of wet cloth fastened over the 

 bulb. We have often noticed that when our hands are wet 

 on a cold day they are colder than when dry. This is be- 

 cause the water evaporates and when it does so, it makes 

 things colder around it; that is, it requires heat and takes 

 it from the surrounding bodies. On hot days sidewalks 

 are often sprayed with water to cool off the surrounding 

 air. 



In a similar way, the water in the cloth on the bulb of the 

 thermometer evaporates and lowers the temperature of the 

 thermometer, so that if the readings of the two thermom- 

 eters are taken, the one with the wet cloth will be found to 

 be lower. The difference between these two thermometers 

 varies from day to day. The amount of water that evapo- 

 rates, and hence the lowering of the temperature, depends 

 on the amount of moisture in the air. If the air is saturated, 

 no water will evaporate, and hence there will be no difference 

 in the readings of the two thermometers. On the other 

 hand, if the air is very dry a large amount of water will 

 evaporate, and the reading of the wet bulb will be much 

 lower than that of the dry bulb. In general, then, a small 

 difference between the two bulbs means a high per cent of 

 humidity, while a large difference means a low per cent of 

 humidity. Tables have been worked out by which, if one 



