314 HUMAN BIOLOGY 



already given. The absolute optimum appears to be a 

 temperature of d^'^ or 66° and a relative humidity of about 

 80 per cent. Such conditions are Hke those which prevail in 

 cool greenhouses, the kind which have a springlike freshness 

 and in which one feels neither hot nor cold and can either 

 work or rest without discomfort. 



3. Air Movement. Thus far we have considered only still 

 air. That is by far the most important condition because 

 most civilized people spend much of their time where the 

 air moves only shghtly. Nevertheless the movement of the 

 air is so important that Dr. Leonard Hill and others have 

 made vigorous efforts to devise an instrument which will 

 measure the combined effect of temperature, relative humid- 

 ity, and movement of the air. The resultant instrument is 

 known as the katathermometer. A large wet-bulb ther- 

 mometer is heated to ioo°f., or approximately the tem- 

 perature of the body, and exposed to the air. Its rate of 

 coohng depends on all three atmospheric conditions. Thus 

 the time required for the katathermometer to drop from 

 100° to 95° gives an approximate measure of the coohng 

 power exerted by the air upon the human skin. That is one 

 of the best measures of the extent to which the air is com- 

 fortable and healthful. It is by no means a perfect measure 

 however, for hot, dry air may have as great a coohng power 

 as moist air of moderate temperature, but it is by no means 

 so healthful. 



The relation between movement of the air and tem- 

 perature is illustrated in Figure 6. Here the reading of the 

 dry bulb thermometer is indicated on the left, and of the 

 wet bulb thermometer on the right. The lower curved hne 

 above the words "Effective Temperature" indicates the 

 degree of heat or cold experienced at any given temperature 

 when the air is at rest and is saturated with moisture. The 

 greatest degree of comfort is found of course at 66° where 

 the effective temperature hne joints comfort hne. At both 

 higher and lower temperatures discomfort increases until 

 the hmits are reached and death ensues. 



The other long curved hues indicate the conditions when 

 the air moves with velocities such as 100, 200, or more feet 

 per minute. The faster the movement of the air the higher 



