PRINCIPLES OF VENTILATION 325 



each pupil. If it is in the month of January, it will be re- 

 membered that the average outdoor temperature is 25F. and 

 the average humidity is 80 per cent. Each cubic foot of 

 outside air contains 1.3 grains of water vapor. But if the 

 inside air is to have a humidity of 50 per cent, at the tem- 

 perature of 70, it must contain % or 7.98 grains, or 3.99 

 grains per cu. ft. This means that we must add 2.69 grains of 

 water vapor to each cubic foot of air. If each of the 30 pupils 

 is to be supplied with 1800 cu. ft. of fresh air per hour, the room 

 must be supplied with 54,000 cu. ft. But each cubic foot of 

 air requires the evaporation of 2.69 grains of water. We 

 therefore see that this room requires during the month of 

 January, on the average, the evaporation of 145,260 grains, 

 or 21 lb., or about 2^ gal., of water per hour (1 gal. of water 

 weighs about 8 lb.) in order that the relative humidity may 

 be maintained at 50 per cent. Verify these data and the com- 

 putation. A room less well ventilated will, of course, require 

 less water evaporated. Why? 



Exercise 73. Determining the Relative Humidity of Indoor Air 



During the months when artificial heat is being used, the student 

 should frequently determine the humidity of the schoolroom air. 

 If a hair hygrometer is used, it should occasionally be checked 

 by using the wet and the dry-bulb thermometers and be properly 

 adjusted whenever found to be inaccurate (see Arts. 247 and 248). 



358. How the Necessary Amount of Water may be Evapo- 

 rated. Where stoves are used to heat the room, it is usually 

 possible to place a pan of water having a large surface on the 

 stove and thus secure sufficient evaporation. In such cases, 

 however, the percentage of humidity in the room is likely to 

 vary much. 



In buildings heated by steam, it is generally possible to 

 permit the live steam to escape from the sj^stem, thus furnish- 

 ing the required water vapor. 



Where hot-water heating is used, small stoves to evaporate 

 the water appear to be the only adequate means of humidifying 

 the air. 



