272 WORLD-POWER AND EVOLUTION 



to the more favorable humidity. The average outside temperature in 

 New York City from October to April, the months when houses are 

 usually heated, is almost exactly 40°, while the relative humidity is 

 about 75 per cent. When such air is taken into the house and heated 

 to 70°, its relative humidity is reduced to about 16 per cent, provided 

 no new moisture is added. Even with the addition of a little moisture 

 from people's breath, from pans in the furnace, or from other sources, 

 such air is as dry as that of the driest deserts and is very harmful to 

 the mucous membranes. When the same air is heated to 64°, its 

 relative humidity without the addition of other moisture becomes 

 about 80 per cent. This is altogether too dry, but it is much better 

 than 16 per cent. 



In a crisis like the war which is coming to an end as these lines 

 are written, it is necessary to conserve coal in every possible way. 

 It would seem as if the practice of having cooler air in our houses 

 would be one of the best ways to accomplish this. Normally the 

 people of the United States burn about 120,000,000 tons of coal each 

 year in their houses. If the inside temperature were kept at 64° 

 instead of 70°, the air taken in from outside would have to be warmed 

 24° instead of 30°, or only four fifths as much as now. That would 

 mean a corresponding decrease in the consumption of coal, or a saving 

 of 24,000,000 tons per year. In order to be comfortable under such 

 conditions we should have to dress more warmly than at present. 

 We should also need to take more exercise in the morning and again 

 toward night in order to set the blood in motion. 



At times when conservation of coal is not one of the most pressing 

 demands, which means in all ordinary times, the humidity as well as 

 the temperature should be right. That means that when the winter 

 air at an average temperature of 40° and a humidity of 75 per cent 

 is taken into our houses it should not only be heated to 64°, but should 

 be caused to evaporate enough water to raise its humidity to about 

 60 per cent. Strange as it may seem, the latent heat required for 

 the evaporation of water is so great that more heat is required to 

 bring the air to a temperature of 64° and a humidity of 50 per cent 

 than to bring it to 70° without humidification. The writer himself, 

 in an article prepared hastily at the request of the United States Fuel 



