30 ELEMENTS OF GENERAL SCIENCE 



may average below 30 per cent; and there is a record of 

 relative humidity as low as 3 per cent at Yuma, Arizona, 

 though this very low humidity did not last more than a 

 few hours. 



29. Humidity in houses. In the winter the air of our 

 houses is heated so that it is much warmer than the air out 

 of doors. Usually no water, or very little, is added, and 

 therefore the air seems very dry ; that is, the relative humid- 

 ity is very low. It. is usually below 50 per cent and some- 

 times as low as 20 per cent, 



During the winter those persons who spend most of their 

 time in the house are usually living in a desert climate as 

 far as atmospheric moisture is concerned. Various devices 

 have been suggested to remedy the condition of low relative 

 humidity. Some people allow a dish of water to stand in 

 the living room. Small tanks of water are placed upon the 

 backs of radiators, and most hot-air furnaces have tanks 

 containing about two gallons of water so placed that the, 

 water will evaporate and pass into the rooms. Experiments 

 to determine whether these devices are effective have failed 

 to show much gain in humidity from their use. The gain 

 from the furnace tank was shown to be about 1 per cent. 

 Even with specially constructed furnaces which evaporate 

 from 12 to 15 quarts per day the results were not so good 

 as desired. In one experiment it was possible by special 

 effort, in very cold weather, to evaporate from 20 to 24 quarts 

 per day. In this last case the relative humidity was raised to 

 35 per cent at a temperature of 20 C. (<J8 F.). No device has 

 yet been invented which is entirely satisfactory in keeping 

 dwelling houses and schoolhouses as humid as they should 

 be. Steam jets in the boiler room from which air passes into 

 schoolrooms have proved fairly satisfactory. 



30. Humidity and health. Although a relative humidity 

 of 35 per cent seems low, it has been noted that such a 

 condition is highly acceptable in winter time, since most 



