THERMAL PHENOMENA OF THE BODY 197 



conducting air; or, of two woolen garments of the same 

 thickness, one of which is rather loosely and the other tightly 

 woven, the loosely woven garment will be much the warmer 

 because so large a proportion of its thickness consists of the 

 poorly conducting air rather than of the rather rapidly 

 conducting solid woolen fibers (see p. 423) Or, again, 

 air of 70 F. is very comfortable ; it feels neither cold nor 

 warm to the skin ; but water of 70 F feels distinctly cool. 

 This is because heat is conducted away from the skin more 

 rapidly by water than by air. For this reason we may feel 

 chilly when our clothing has become drenched with rain. 



2. Moist air is a better vehicle of heat transfer than dry air. 

 This becomes obvious when one is exposed to damp air at a 

 temperature of less than 70, and the familiar difference be- 

 tween dry and damp winds in winter illustrates the same fact, 

 for a damp wind at 50 F. chills the skin more than a dry 

 wind at 40 F. The student is cautioned, however, against 

 supposing that dampness always favors the output of heat 

 from the body; it favors only one method of heat output, 

 namely the transfer of heat. On the other hand, dampness 

 hinders the output of heat by evaporation. Hence at those 

 temperatures (above 80) where the output is chiefly by 

 evaporation, a damp atmosphere is close, warm, and muggy; 

 where the output is chiefly by heat transfer (below 70), a 

 damp atmosphere is chilly. 



12. The evaporation and not the secretion of perspiration 

 cools the body. The student should understand clearly that 

 it is the evaporation of the perspiration, not the secretion of 

 it, which abstracts heat from the body. Perspiration may be 

 secreted in large quantities, but if it does not evaporate, 

 as happens on a very moist, humid, muggy day, when the 

 atmosphere already contains about as much aqueous vapor as 

 it can hold, it takes little or no heat from the skin. Nor 

 is the efficiency of the perspiration as a cooling agent meas- 

 ured by the amount of visible or " sensible " perspiration, for 



