EFFICIENCY OF CLOTHING 453 



If the garment were of wool, (^ = 68 X 10 '^j we have 



68 X 19 X 36 X 24 X 10 " X 10=^ X 10^ x 10 

 ^ 75 X 10' X 10^ 



= 1,488-3 Calories per day. 



The coefficient of utiHty of elothing materials may be determined 

 by covering with them a number of similar copper spheres lilled 

 with warm water and fmding how long they take to cool through 

 10° C. If the time taken to cool 10° C. by an unclothed sphere 

 with an external temperature of 12° C, be taken as 0, and the 

 time necessary for the same sphere (clothed) to cool to the same 

 extent l)e t, then t/Q = U (coefficient of utility). 



TABLE LXXIV (From Bergonie) 



The efficiency of clothing depends in great measure on the 

 arrangement of the fibres of the cloth so as to enmesh air and thus 

 form a stationary layer between the body surface and the outer 

 air. The following table (Table LXXV.) from Lefevre gives an 

 idea of the protective value of clothing. His subjects were })laced 

 in a rectangular box through which a measured amount of air was 

 passed. The temperature of the air just before it reached the 

 body and immediately after leaving the body was taken. If M, 

 the mass of air passing in t minutes, is heated by 6°, then the heat 



0-237 X 6 X li ^ , . , . , 



lost per mmute = Calories, where 0-237 is the 



specific heat of air. 



It will be noticed that heat production increases with the 

 lowering of the temperature of the wind and also with increasing 

 the speed of the wind. It is o})vious that, under similar conditions, 

 the heat lost (and the heat produced) by the clothed is less than 

 that of the naked subject. 



The colour of clothes has also something to do with their pro- 



