432 



SCIENCE OF HOME AND COMMUNITY 



reads the two thermometers, it is possible to find at once 

 the per cent of humidity. (See Appendix.) 



In order to get an accurate reading, the thermometers 

 should be fanned for a few minutes, or even better they may 

 be attached to a board which can be rotated in the hand by 

 means of a handle. This is called a sling psychrometer. 



Humidifiers. While many of the older buildings have 

 made no arrangement for adding water to the air, a number 

 of satisfactory systems have been devised and are being 

 introduced into the better and newer build- 

 ings. In one system, steam is allowed 

 to escape through small holes in pipes 

 placed in the cold air duct. Instead of 

 steam, water may be forced in as a spray 

 through nozzles. 



Another plan is to have large pans of 

 steaming hot water in the cold air duct. 

 In still another plan, a rotating cylinder is 

 placed in the fresh air duct. This is cov- 

 ered with a coarse meshed cloth, and as 

 the cylinder revolves the lower edge 

 passes through a trough filled with water. 

 Thus the cloth is kept moist and dripping 

 all the time. As thermostats are used to regulate the tem- 

 perature, so humidostats are used to control the humidity. 



FIG. 171. Paper tow- 

 els, a sanitary substi- 

 tute for the danger- 

 ous public towel. 



DEMONSTRATION 33 



Purpose. To see if the ventilating system of your school 

 furnishes the essentials of good ventilation. 



Apparatus. Touch paper, down, two thermometers. 



Directions, i. Amount of air entering the room. Throw a 

 tuft of down in front of the air inlet and estimate how far it 

 travels in a second. Try several times and take the average. 

 Measure the dimensions of the air inlet. Find its area. Multi- 

 ply this by the velocity of air just found so as to compute the 



