79 o THE POPULAR SCIENCE MONTHLY. 



per cent of the increase of heat which it produces under a sustained 

 effort. The problem we have to solve is not to seek for a way of 

 producing less heat, but to find a means of getting rid of that which 

 we do produce. 



Water is a much more effective refrigerant than air, because of its 

 much greater conductibility ; at the same temperature, a bath of 

 water will refresh one more than a bath of air ; but baths are neces- 

 sarily of limited use. The important matter should be to diminish the 

 temperature of the air that comes in contact with the body. 



We have next to consider the effect of evaporation through the 

 lungs and the skin. When the thermometer indicates more than 98 

 in the shade, the body can no longer be cooled by contact or by radia- 

 tion, and only a single way is left by which the surplus heat can be 

 dissipated. It can only expend itself in vaporizing the water which 

 transpiration carries to the skin and to the mucous membrane of the 

 respiratory apparatus. The lungs, as a rule, exhale about half as much 

 water as is excreted by the skin. Both together remove about a kilo- 

 gramme of water every twenty-four hours, disposing of as much heat 

 as would boil five quarts of water ; but the quantity of water and of 

 heat removed in this way may be doubled and even tripled when all 

 the channels of transpiration are fully opened under the pressure of an 

 excess of internal heat. The vapor disengaged by these operations is 

 absorbed by the surrounding air with a facility proportioned to the 

 dryness of the atmosphere, or to the degree in which it is removed 

 from the point of saturation. There is a limit at every degree of tem- 

 perature to the proportion of vapor which the air can contain ; and 

 the interval between the points of dryness and of saturation increases 

 with the temperature. An atmosphere at the same time very moist 

 and very hot seems heavy to us because it hinders the evaporation of 

 the water that transpiration brings to the surface of the body. This is 

 why hot and moist climates are so much more unhealthy than hot and 

 dry ones. 



When the internal calorification is increased in consequence of vio- 

 lent exercise, the excess of sensible heat is eliminated by a more in- 

 tense radiation, by ascending air-currents, and by a more abundant 

 transpiration ; it thus happens that after several hours of sustained 

 effort we sometimes observe a slight cooling of the body, an effect 

 which is the result of a too rapid using up of disposable materials. 

 Hence, to cite the illustrations given of this fact by M. Bouchardat, 

 dogs, which have run long at the hunt, and the overworked and ex- 

 hausted children in the Belgian coal-mines, returning to the lodge or 

 to their home, first of all things, before even satisfying their hunger, 

 stretch themselves before the bright fire for warmth. 



Thus the means of refrigeration at the disposal of Nature are quite 

 varied ; they complement and replace each other according to circum- 

 stances. But it is necessary to avoid the too abrupt changes which 



