THE EFFECT OF CLIMATE AND WEATHER 3II 



of deaths declines steadily as the humidity increases, as 

 appears in the dotted lines, but when the 8 a.m. temperature 

 is above 70° it is very dangerous to submit to a surgical 

 operation if the relative humidity is either very low or very 

 high, whereas with a humidity of ^^ to 60 per cent the 

 chances of survival are excellent. In this particular case 

 other factors such as the change of temperature from day to 

 day are doubtless concerned, but abundant other evidence 

 shows that at high temperatures there is a decided optimum 

 of humidity which is far more favorable than either extreme. 



Optima Determined by Laboratory Experiments. The 

 conclusion that there are very definite cHmatic optima is 

 supported by numerous experiments as well as by the 

 statistical methods already described. At the Pittsburgh 

 Laboratory of the U. S. Bureau of Mines hundreds of 

 persons have been tested in experimental chambers where 

 the temperature, humidity and movement of the air can be 

 controlled with great accuracy. The following table sum- 

 marizes the results thus obtained at various temperatures 

 in saturated air with subjects who are very Hghtly clad. 

 The pulse rate, body temperature and metabolism are all 

 influenced. With ordinary clothing and for persons absolutely 

 at rest, the most comfortable temperature in satural^ed air 

 appears to be not far from 70°, but when work is done the 

 most comfortable temperature is lower. 



Another way of representing the same experiments appears 

 in Figure 5. There temperature is measured horizontally 

 from low on the left to high on the right; the amount of 

 moisture in the air is measured vertically, the bottom of 

 the diagram representing absolutely dry air and the top 

 300 grains of moisture per 100 pounds of air; the curved 

 fines show percentages of relative humidity. The heavy 

 "comfort fine" indicates that when people are normally 

 dressed and absolutely at rest, the most comfortable tem- 

 perature in motionless saturated air is 64°. A departure of a 

 single degree from this condition is at once perceptible. In 

 unsaturated air an equal degree of comfort is felt at all other 

 points along the comfort fine. Thus the feefing of comfort 

 at 68°F. and 60 per cent humidity is the same as at 64°f. 

 and 100 per cent, or at 76°f. and 10 per cent or any other 



