324 



VENTILATION 



humidity. Turning to the table of relative humidity (Table 

 XI, page 218) we see that when the dry-bulb thermometer 

 reads 70 and the relative humidity is 20 per cent., that the 

 wet-bulb thermometer reads 20 lower, i.e. t the wet bulb then 

 reads 50. Now, the human body is constantly moist; more 

 or less evaporation is constantly taking place from the skin. 

 While the clothing, by enclosing an envelope of air 

 about the body, checks this evaporation somewhat, 

 still, if the air in the room is in as rapid motion as 

 it should be, we feel decidedly the chilling effect of 

 evaporation. One is more comfortable in a room 

 heated to 65 with the. humidity 50 per cent, than in 

 a room heated to 70 with the humidity 20 per 

 cent. This is especially true if the air be in motion. 

 For this reason many physicians now advocate the 

 use of the wet-bulb thermometer only, to determine 

 room temperatures. 



FIG. 231. 

 A home- 

 made wet- 

 bulb ther- 

 mometer. 



Any common house thermometer can be converted into 

 a wet-bulb thermometer by suspending a 3- or 4-oz. 

 bottle of water from the frame and wrapping the ther- 

 mometer bulb with a .wick of soft muslin (Fig. 231). 

 Such a thermometer will closely indicate the actual 

 temperature in which we are living. 



357. Large Amounts of Water must be Evapo- 

 rated. Many people, who have become convinced that a 

 higher indoor humidity is desirable than is usually obtained 

 during the winter months in the northern states, find diffi- 

 culty in evaporating the necessary amount of water. In fact, 

 much larger quantities must be evaporated than most people 

 realize. It is an easy matter to calculate the amount of water 

 which must be evaporated in order to raise the humidity to 

 the proper point under given conditions. 



Let us suppose that our problem is that of ventilating the 

 school room considered in Art. 354. We will suppose that 

 this room contains 30 pupils and is supplied with the accepted 

 amount of fresh outside air, namely, 1800 cu. ft. per hour for 



