The importance of this is that the evaporation of water, no matter at what temperature, requires a large amount of heat. This heat is abstracted from the nearest place, which in this case would be the body itself, so that the evaporation of sweat acts powerfully to cool off the body. In hot weather this is really our only effective means of getting rid of heat, for if the body is no warmer than its surroundings it cannot lose heat directly, but sweat can evaporate, taking heat with it, no matter how warm the surroundings may be. We said a moment ago that a high percentage of water in the air would hamper the evaporation of sweat. The practical workings of this fact are seen in connection with heat prostrations in the summertime. Careful scrutiny of the weather reports will show that these are much more numerous as well as more severe on days of high relative humidity than on days that are simply hot, but without much moisture in the air. As the sweat evaporates from the body, the resulting water vapor has a tendency to linger in the neighborhood and so to interfere with further evaporation. Under these circumstances it is desirable that this moisture-laden air be moved away and fresh air brought into its place. This is the benefit we obtain from fanning. The air that is brought upon us by a fan feels cooler than it really is; this we realize when we notice how comfortable the draft of a fan is even on the very hottest days. When one fans oneself in the old-fashioned way, the heat produced in working the fan is often almost enough to balance the loss through the improved evaporation of sweat. An electric fan, on the other hand, is a very powerful aid to comfort in hot weather. In fact it is hard to think of any mechanical device that has contributed more in recent years. An amusing illustration of the importance of moisture in the air in relation to bodily comfort is in the case of the man who thought he would obtain a cool-weather office in hot weather by installing a block of ice and causing his electric fan to play upon it. Very much to his surprise and quite contrary to his expectations the room quickly became unbearably sultry. The rapid evaporation of the ice filled the air so full of water vapor that evaporation of sweat could not take place and the persons in the room were much worse off than if they had contented themselves with merely keeping the warm and comparatively dry air of the room in rapid motion. Although the evaporation of sweat is a very effective means of getting rid of heat from the body it does not work perfectly in times of great muscular exertion in very hot weather. Careful studies of men in steel industries and other places where hard work is done in intense heat have shown that the production of heat during the course of the working day slightly out-strips its loss, so that by the end of the day it is the regular thing for the body temperature to be up two or three degrees, to 100 or 101 degrees Fahrenheit.