K. DOMESTIC AND HOUSEHOLD ECONOMY. 493 



duction. The particles of air iu contact Avitli the body be- 

 come warm, and are replaced by colder ones, creating a cur- 

 rent, which is insensible, because of less velocity than three 

 feet per second. In better conductors cooling takes place 

 more rapidly, water of 61 seeming much colder than air of 

 61. These three modes of cooling, however, supplement 

 each other, and act together. Thus a current of warm air 

 cools more rapidly than calm cooler air, not only by reason 

 of renewal of the air, but by favoring evaporation. 



The chief object of clothing is to surround the body artifi- 

 cially with a warm climate, poor conductors being conse- 

 quently selected. The cooling process is, liowever, simply 

 checked by the clothing. Even the thinnest, finest fabric, as a 

 veil, diminishes loss by radiation. But the inclosure of air is es- 

 pecially eflective, and consequently garments of porous heavy 

 material are warmer than those which are more compact. 

 Felt shoes, permeable to air, are warmer than those made of 

 leather or India rubber, while the latter soon become unendur- 

 able because of checked ventilation. The more hygroscopic 

 the material, the colder the clothing, because it is a better 

 conductor when moist. Linen and silk are for this reason 

 colder than wool, and also because the latter retains its elas- 

 ticity when moist, and keeps the air within its pores. And 

 our bed, which is, iu fact, our sleeping garment, is of special 

 interest. It must be warmer than our waking clothing, since 

 less heat is developed during sleep. Consequently, loss of 

 sleep is very exhausting. The feather bed possesses in the 

 highest degree feeble conducting power, elasticity, and per- 

 meability to air ; but, if too thick or soft, resembles more an 

 air-tight garment. The house, too, may be regarded as an 

 extended piece of clothing, so gradual is the transition from 

 bodily garments to it (the step from the wide garment of 

 the Arab to his felt tent being a small one); and, in hygienic 

 functions, they agree precisely in regulating our relations with 

 the surroundins: air. The ease with which a current of air 

 may be blown through a brick, pieces of mortar, wood, etc., 

 by glass tubes cemented to opposite sides, and the passage 

 of water (so much denser) through these substances, show 

 how imperfectly our walls, of whatever material, and however 

 thick, exclude the air from us. We do not perceive the free 

 passage of air through them because the current is too slow. 



