Page:Popular Science Monthly Volume 10.djvu/678

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instinctively try to increase the movement of the air—that means, its quantity in proportion to your surface; you want to increase your loss by conduction, and, if possible, by evaporation, and take to fanning, in order to facilitate the departure of your rising heat by the two open routes.

The loss by radiation can be very considerable under certain circumstances; 50 per cent. of the whole quantity of heat generally going that way, it is obvious that radiation deserves our full consideration. Particularly, an unequal radiation may be very injurious, such as takes place when a person is sitting or lying near a cold wall which is not covered by some bad heat-conductor, or near a window, etc.

On school-forms, the exposed sides of the first and last pupils are always more cooled than the sides directed toward their neighbors. In this respect there are numbers of practical points which are far from being sufficiently taken into consideration.

Let us now consider some instances in which the abstraction of heat by evaporation is predominant, or preeminently felt. The best known is that experiment by which one tries to learn the direction of the wind when the air appears calm and the sky cloudless. The moistened forefinger feels colder on that side which looks toward the wind, because more evaporation takes place there. The experiment does not succeed so well when the air is moist, because the moisture in the air prevents further reception of moisture by it; in our case, preventing the evaporation from the moistened finger.

Our organism acts similarly in all cases where there is an increased production of heat in our body, or where less heat is sent away by the two other routes. It has the power of dilating or narrowing the small blood-vessels in our skin and internal organs. The blood-vessel nerves which govern this motion are not subject to our will, but liable to be excited by external causes. When a person blushes, he gives off heat, because more blood rushes into the dilated blood-vessels of his cheeks and periphery generally, and more heat leaves the body. Under similar circumstances the whole surface of our body becomes fuller of blood and warmer, there is more heat to radiate and to be conducted away, and to be consumed by increased evaporation of the watery part of the blood.

The great value of evaporation for the cooling of our body can be estimated by calculating that as little as fifteen drops of water requires 214 caloric units to be changed into vapor.

We have at Munich a great apparatus for studying the process of respiration. It was given by the late King of Bavaria, Maximilian II., to the hygienic department of the university. Prof. Voit and myself have, by aid of this apparatus, investigated the quantity of water evaporated by men and animals during twenty-four hours. The constant result was that, under other similar circumstances, the quantity of evaporated water always rose in proportion to an increased meta-