664 THE POPULAR SCIENCE MONTHLY. 



When I cover one part of my body I change the degree of ab- 

 straction 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 con- 

 duction and radiation the heat, which has radiated from the body into 

 the clothes, has to travel through them by radiation and conduc- 

 tion, 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 window- 

 pane 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 rap- 

 idly-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 



