CONDUCTION OF HEAT. 33 



of wool and cotton would lead us to suppose. As air conducts 

 heat more feebly than wool, it follows that the more air a 

 piece of cloth retains, the less will be its conducting power ; 

 therefore, very closely woven cloth is a better conductor of 

 heat than it would be if its texture were more porous. It is 

 evident that the limit of porosity in this case must not be so 

 great as to allow air to circulate freely through the material. 

 We find that in the case of " cellular cloth " the heat-retaining 

 capacity of the cotton is much heightened by having the cloth 

 loosely woven, and that, with a given weight of clothing, we 

 obtain a somewhat similar effect by increasing the number of 

 thicknesses, between each of which a more or less perfect 

 layer of air is imprisoned. Thus, two rugs will be warmer 

 than one which is as thick as both of them put together, and 

 which is of similar material and structure. Probably most 

 men know from experience that the addition of an ordinary 

 cotton shirt worn over a flannel one will be almost, if not 

 quite, as effective in protecting the body from cold as that of 

 a second flannel shirt. In our own cases, as a protection from 

 cold, it is better to put a cotton shirt over a flannel shirt than 

 a flannel shirt over a cotton shirt, which would be apt to stick 

 to the skin, and would thus prevent the formation of a layer of 

 air between the skin and the shirt. By employing cotton 

 with horses' clothing in the manner indicated, economy in 

 weight and expense may be obtained without much loss of 

 heat-retaining power. 



When two bodies are in contact, each of them transmits 

 heat to the other independently of any difference there may 

 be between the respective temperatures. If one of these 

 bodies be colder than the other, the balance of heat trans- 

 mission gained by it, at any given moment, will be equal to 

 the difference of temperature between the two. Hence, the 

 rate of loss of heat by the warmer body will decrease 

 according as the temperature of the colder one becomes raised. 



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