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 suffocation, 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 increases also the ventilation of the shady place.
The fan acts on the same principle. The pankha in the bungalow, by increased conduction and evaporation, keeps the blood of the European at its normal temperature of 992°. 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 absolutely dry and an absolutely saturated air at 32° and 86° Fahr. is highly instructive. We lose:
|at 32°||and dry||1,172||caloric units.|
|at 32°||and saturated||1,060|
|difference as much as||640||caloric units.|