668 THE POPULAR SCIENCE MONTHLY. 



being deducted, the impediment by the second layer is about the 

 same for different materials, but very considerable for each of them : 



For Linen 32$ 



" Shirting 33 " 



" Silk 32 " 



" Flannel 29 " 



" Wash-leather 30 " 



" Gutta-percha sheeting 36 " 



From this follows the practical truth, that we can produce a very 

 different effect on our body by the same number of clothes, according 

 to the tightness and looseness in the make. Just call to mind tight 

 shoes and gloves in winter-time ! 



This fact leads to a series of other facts, which contain the explana- 

 tion why wadding, as long as it is loose and elastic, keeps you warmer 

 than when it is once flattened. This is the air contained within the 

 clothes. 



One generally considers clothing as an apparatus for keeping the 

 air from us. This conception is utterly erroneous ; quite the reverse, 

 we can bear no garments which do not allow of a continual ventilation 

 of our surface. Just those textures which are most permeable to the 

 air keep us warmest. I have examined different materials for their 

 permeability to air, which can be easily ascertained. One closes a 

 series of perfectly equal glass tubes with different textures, and ob- 

 serves how much air passes through the clothing substances at the 

 same pressure during the same time. Taking the quantity of air pass- 

 ing through flannel as 100 



Linen allowed 58 



Silk " 40 



Buckskin " 58 



Kid " 1 



Chamois " 51 parts of air 



to pass through them. 



If our clothing kept us warm in proportion to its power of exclud- 

 ing the air from our body, kid would keep us a hundred times, and 

 chamois warmer by one-half than flannel, and so on, while every one 

 knows by experience that it is quite the reverse. 



If there are several layers of the same material, ventilation loses 

 but very little at the second layer, because the velocity of the air in 

 its passage through the first layer remains about the same on its fur- 

 ther progress, the following layers being like a continuation of the 

 preceding ones, as if they were tubes of the same calibre, retarding 

 the original velocity of a fluid by the amount only of unavoidable 

 friction. 



Thus, a current of air travels incessantly through our clothing. 

 Its force, as in ventilation generally, depends on the size of the open- 

 ings, the difference between the outside and inside temperature, and 



