APPENDIX E 267 



In September and October, or again in April and May, the thermome- 

 ter at breakfast time frequently stands at about 60° not only out of 

 doors, but in the house. Yet we feel comfortable without a fire. 



Another test would be possible on a rainy day when the thermometer 

 outside stands at about 65°. Let the inside temperature fall to 64°. 

 Without telling the family what you have done ask them whether the 

 house is too warm or too cool. Ten to one they will say it is all right. 

 Yet if the outside temperature were 10° and the inside temperature 

 64°, they would all be shivering and urging you to open the draft of 

 the furnace. Even if the outside temperature were only 30° they 

 would still feel cool with the thermometer at 64° inside. It is all a 

 matter of moisture, and thus of sensible temperature as opposed to 

 actual temperature. 



On a rainy day such as you have chosen for your experiment, the 

 outside air has a relative humidity of nearly 100 per cent. When 

 such air is brought into the house and heated to 64°, its capacity for 

 moisture increases so much that its relative humidity would fall to 

 60 per cent if no new moisture were added. As a matter of fact, a 

 little is added by people's breath and by evaporation from their bodies, 

 as well as from plants or other sources. Hence when you try your 

 experiment the house may have a relative humidity of nearly 75 per 

 cent and a temperature of 64°. In other words, you have reproduced 

 the optimum conditions, which are much like those in the cooler kinds 

 of greenhouses. Suppose now that the outside air has a temperature 

 of 30° and is bright and clear, so that the relative humidity is 70 per 

 cent. If the air is heated to 70° in the house, its humidity will be 

 12 per cent, or perhaps 25 per cent if allowance is made for additions 

 of moisture from other sources. Such air makes people feel cooler 

 than does the air at 64° and 75 per cent, for it causes rapid evapora- 

 tion from the skin and still more rapid evaporation from the delicate 

 mucous membranes of the nose and throat. 



These considerations lead us to ask whether it is possible to con- 

 struct a table showing what combinations of temperature and humidity 

 give approximately the same feelings of warmth. A brief account 

 of two sets of experiments will illustrate the way in which this has 

 been attempted. First, in Shanghai and Canton, China, in the summers 



