Page:Popular Science Monthly Volume 23.djvu/813

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the interposed stratum of air. Hence, we should expect in many cases to find a loose garment warmer than a tight one; and we know that close-fitting gloves or shoes afford but a poor protection against the cold. This reasoning, however, supposes that the protecting layer of air is motionless; but more frequently an ample and flowing garment favors the circulation of air, and therefore seems to us to be cooler, and is for that reason preferred in summer and in hot climates. We are now brought to the important fact that the most serious obstacle to the propagation of heat in any body is the discontinuity of its elements. This is because heat is a mode of motion, and every derangement of molecular continuity impedes the transmission of vibrations. This principle is more or less unwittingly put to profitable use in the manufacture of clothing. We obtain very warm clothes from light, loose, and porous tissues, having a capacity to retain in the spaces between their fibers a large volume of air. I said, retain; I might more properly have said, let pass; for the air which our clothes inclose is not motionless, but circulates and undergoes constant renewal in filtering through the envelopes which we mistakenly believe are intended to isolate us from the surrounding medium. It is, in fact, an essential condition of a good garment that it shall not interpose an obstacle to ventilation. The warmest clothes let the air pass more readily than those which are considered cool. Dr. Pettenkofer demonstrated this fact by measuring the volumes of air which under the same pressure and in the same time passed through a series of tubes stopped by pieces of different kinds of cloth. The numbers representing the volumes were for the different goods: flannel, 100; linen, 58; silk, 40; strong cloth, 58; buckskin, 51; glazed skin, 1. Flannel is, then, a hundred times more permeable to the air than a glazed glove, and we know at the same time that it is infinitely warmer. The volumes of air transmitted are but little changed by doubling the goods. Our clothes are thus continually aerated by an exchange, the activity of which depends on the external temperature, the degree to which the atmosphere is agitated, and the porosity of the tissues; the essential point is that the change shall be so slow that the nerves of touch shall not be affected by it. The warmest coat is one of fur, and its warmth lies not in the skin only, but chiefly in the hairs, although their mass is relatively insignificant, and is almost wholly due to the air interposed between them. Furs are warmer in proportion as the hairs are finer, because, doubtless, the air that circulates through them is more thoroughly warmed. There are formed around the bodies of furred animals superimposed strata of air, the temperature of which diminishes from the skin to the ends of the hairs; and in winter the animals seem cold to the touch, while the zone of exchanges retires toward the skin as the cold becomes more intense. The body of the animal is, then, cooled principally by convection and by the ventilation which incessantly removes the heated