Page:Popular Science Monthly Volume 10.djvu/690

670 and frequency of respiration; they were 102° and 100 per minute. He did not use any varnish, to avoid any possible suppression of evaporation from the skin, but enveloped the shorn animal in a wet cloth. The temperature of the room being at 66°, the animal lost so much heat that, after five hours, its interior temperature had fallen to 75°, and its respiration to 50 per minute.

A fur is so arranged that its fine hair, projecting into the air, intercepts all the heat, which flows from the surface by radiation and conduction, and distributes this heat through the air, which circulates between the single hair-cylinders; the finer the hair of the fur, the more of the outgoing heat is taken up by the air, which, however cold the temperature may be, reaches the nerves of the skin as a warmed air. Furred animals, in winter, when touched superficially, give a very cold sensation; it is only near the skin that their hair feels warm. In severe cold, certainly little of our animal heat comes as far as the points of the hair, from which it would radiate or be conducted into the air; the current of air in the fur cools the hair from its point toward its roots, and a severer cold penetrates only a little farther into the fur, without necessarily reaching the skin of the same. This takes place only when the temperature is uncommonly low, and the air in violent motion. Travelers in high latitudes all agree that extreme degrees of cold can be borne very well when the air is calm, but scarcely so when there is a brisk wind.

This tends to show that in very severe cold the outflow of heat, by the skin into the air contained in the fur or within the dress, takes place through one route only—that of conduction; when a fur is worn, no heat comes to the surface for radiation, as soon as the points of the hair have the temperature of the surrounding air. Evaporation also sinks to a minimum, because at 68° Fahr. under freezing-point all formation of aqueous vapor already ceases, and nearly all the heat in the fur and the dress is employed to heat the arriving air, whose velocity increases according to the difference of temperature. In a well-furred animal the changes of temperature in the surrounding air only change, if I may say so, the latitudes of the cold and warm zones in the fur; the place where the temperature of the body and the air equalize each other moves between the roots and points of the hair, and for this reason such a well-furred animal is not warmer in summer than in winter. Its blood keeps always at the same temperature, because in summer a great part of its heat leaves at the points only of the hair by radiation and conduction, while in winter the heat departs already near the roots of the hair.

Air-proof fabrics ought to have only a very limited use. In India-rubber or gutta-percha textures we feel highly uncomfortable when we have to undergo much exercise, or have to give off more heat than usual. They become inconvenient, not because they stop the change of air entirely—which they cannot do in fact, on account of the