66z THE POPULAR SCIENCE MONTHLY. 



the body cannot lose much heat by radiation toward colder objects, 

 when the temperature of the air approaches and even surpasses at 

 times that of our blood, the European often feels the heat to suffoca- 

 tion, and besides the use of the bath he has no other practical remedy 

 than the fan and the shade. 



In the shade the air is not only cooler, but also more in motion. 

 The difference of temperature between a place sheltered from the rays 

 of the sun and a neighboring one exposed to them, produces a motion, 

 a current, because bodies of air of unequal temperature are also of 

 unequal weight. They are not in equilibrium, and seek to reestablish 

 it by motion. Any one may easily convince himself thereof who, on 

 a hot day with calm air, walks alternately over places exposed to the 

 sun and sheltered from it. As soon as he comes into the shade of a 

 cloud, a house, or a tree, he feels at once a soft wind rising. The 

 shade not only protects us against the direct solar rays, but it in- 

 creases also the ventilation of the shady place. 



The fan acts on the same principle. Thepankha in the bungalow, 

 by increased conduction and evaporation, keeps the blood of the Eu- 

 ropean at its normal temperature of 99|. When the temperature of 

 the air rises to 140, when the walls of the house or bungalow are no 

 longer cool enough to provoke radiation from the heated human body, 

 man is reduced to cooling by evaporation. It greatly depends upon 

 the state of dryness of the air how far he succeeds. The drier the 

 hot air is, the better is it able to withdraw water from the skin, from 

 the respiratory organs, from the wetted floors, and consequently the 

 more heat from the human body. The moister it is the less it is able 

 to act thus. 



In order to give you an idea of the quantitative differences in play, 

 we will consider the losses of heat by respiration as they take place 

 at different temperatures and different conditions of moisture of the 

 air we draw in. In twenty-four hours the quantity of this air is on an 

 average 2,000 gallons. It has been calculated that by the process of 

 respiration a person loses 1,1 72 caloric units when the air is at 32 and 

 quite dry, 1,116 when it is half saturated by water, 1,060 when it is 

 completely so. The difference between the two extremes is only a 

 small percentage of the whole loss. But, when the temperature is 86, 

 the above numbers would be respectively 1,096, 760, and 420. 



A comparison of the losses of heat by the respiration of an abso- 

 lutely dry and an absolutely saturated air at 32 and 86 Fahr. is 

 highly instructive. We lose : 



at 32 and dry _ 1,172 caloric units. 



"86 " ' "" ' 1,096 



difference only 76 



at 32 and saturated 1,060 



"86 " 420 



difference as much as 640 caloric units. 



