Page:Popular Science Monthly Volume 10.djvu/684

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When I cover one part of my body I change the degree of abstraction of heat by all three routes known to you, but without obstructing any one of them entirely.

To speak in the first instance of radiation, it will be clear to you that our surface is prevented from radiating heat directly toward the colder objects in our neighborhood, and that it can only radiate toward the covering materials, which receive this heat. By the laws of conduction and radiation the heat, which has radiated from the body into the clothes, has to travel through them by radiation and conduction, till, arrived at their outer surface, it can radiate thence toward colder objects, just as it would from the naked surface of the body. Thus by our clothes we keep the heat radiating from us somewhat longer in the immediate neighborhood of our surface. The lightest covering even makes itself perceptible by impeding radiation, the thinnest veil keeps warm in some degree. It is just the same with the earth itself. On a calm, clear night the earth's surface becomes so chilled by radiation into the colder space, that the moisture of the air precipitates itself on it as dew, and at times as hoar-frost, and even as ice, just as the moisture in a warm room does on a windowpane cooled from the outside; but, when a veil of clouds overhangs the earth during the night, the earth never cools itself so much as to allow of any dew forming.

There are substances, called diathermal, which allow the rays of heat to go straight through them without any absorption, for instance, the crystals of common salt, but all the materials of our clothes are such as absorb the rays of heat which come to them from one side, and only part with them after they have reached the outer surface. The transit of heat through what we may call our artificial surface depends essentially on the conductive power of the material and its thickness, i. e., on the length of time and way which the heat has to go through in order to travel from our surface to the outer surface of the garment.

Thus the whole immediate neighborhood of our body is continually warmed in an even degree by our radiating heat, and our sensitive skin is spared the numerous disagreeable or injurious effects of a rapidly-changing temperature.

The heat does not remain in our clothes, it is continually on an outward move, faster or slower, and, to a certain degree, also warms the stratum of air between our clothing and our skin, so rich in nerves and blood-vessels. This air, as we shall see presently, continually changes, and must change if we are to feel comfortable. In the cold of winter, and in the open air, we lose our bodily heat out of our well-selected garments without any sensation of cold, only because we have removed the place of exchange between the temperature of our warm blood and the cold winter air from our sensitive surface to a substance without life and sensation; instead of our skin, our dress