CLOTHING AND THE ATMOSPHERE. 789 



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 fur- 

 niture were still cold and abstracted so much caloric as to provoke 

 radiation from the body. The loss of heat becomes less and the sensa- 

 tion 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 ex- 

 posed 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 transpira- 

 tion. 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 ther- 

 mometer 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 tempera- 

 ture produced in the trees by their faculty of evaporation, and the facil- 

 ity 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 sec- 

 ond, 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 conse- 

 quence of the differences in density arising from the unequal heating. 

 Notwithstanding all the devices that have been contrived for the re- 

 duction 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 ca- 

 pacity 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 



