Page:Popular Science Monthly Volume 23.djvu/809

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able in it, even with the air at a lower temperature than that in which we were previously chilly. In the former case the walls and the furniture were still cold and abstracted so much caloric as to provoke radiation from the body. The loss of heat becomes less and the sensation of cold disappears as soon as the objects around have become tolerably warm. This also explains why it is dangerous in winter to stay long near a wall or a window where one side of the body is exposed to be cooled by excessive radiation.

For a similar reason we feel too hot in a room full of people, even when the air is only moderately warm. The presence of a considerable number of persons prevents radiation, and the excess of heat can be carried off only by currents of air, or by a more abundant transpiration. We fan ourselves to expedite the cooling by convection and evaporation, by bringing more air in contact with the skin; and if we leave the room when we are nearly smothered, to go out "to take a breath" in an empty room, we shall be astonished to find by the thermometer that the temperature of the two rooms is nearly the same; only that radiation is free in the empty one. The agreeable refreshment the shadow of the woods gives us is due to the relatively low temperature produced in the trees by their faculty of evaporation, and the facility it affords for promoting radiation from the skin. The body is also cooled by convection, or by giving off its heat to the air that bathes it, and this loss is more sensible in proportion as the air is cooler and more frequently renewed. The atmosphere is always in motion, even when apparently most calm; and thousands of its movements escape our senses, because they are not strong enough to impress our organs. These ceaseless motions, it must be clear, contribute greatly to the cooling of our bodies; but the effect is most marked in the open air, when we are exposed to the action of the winds. In our climate, the average velocity of the atmospheric currents is about ten feet a second, or seven miles an hour. Supposing that the extent of the surface of the body exposed to the currents is one square metre, there pass over a man walking out for an hour about eleven thousand cubic metres of fresh air. In hot climates we seek the shade, not only because the air under it is fresher, but also because it has more motion, in consequence of the differences in density arising from the unequal heating. Notwithstanding all the devices that have been contrived for the reduction of temperatures, it is evident that civilization is possessed of more varied and efficacious means of contending against the cold than of mitigating the effects of the heat. It is for this reason that the European finds it so difficult to acclimate himself under the tropics. The Hindoo reduces his internal calorification by eating little; but he is at the same time defective in energy, and has extremely little capacity to work. Assiduous labor exacts a large quantity of food, while an excess of surplus heat simultaneously results from it; for the organism can convert into mechanical labor only about twenty-five