RELATIONS OF THE AIR TO OUR CLOTHING. 661 



You see, therefore, that we give off more heat by conduction in 

 the open air than in a room, and in the latter proportionately more 

 by radiation and evaporation. 



The power of conduction is best appreciated when we change the 

 air for some other fluid medium, which is a better conductor than air, 

 and more capable of absorbing heat, I mean water. In air of a few 

 degrees of heat, we can feel pretty comfortable with moderately 

 warm clothes ; but, if with the same amount of clothing we were to 

 get into water of the same temperature, we should feel painfully cold, 

 and should probably be frozen to death in a few hours, although our 

 loss by evaporation would have ceased entirely, and that by radiation 

 nearly so. In hot climates, therefore, a daily bath is of great service 

 for the necessary cooling of our body, even if the water is not cooler 

 than the atmosphere. 



In the air also the loss of heat by conduction is the greater the 

 lower the temperature, and the greater the velocity of the air which 

 flows around us. This explains on the one side why it appears super- 

 fluous in a calm and cool air to make use of a fan, while this expedient 

 acts so beneficially at higher temperatures; and on the other why, as 

 a rule, a warm air in motion appears much cooler than a calm one of 

 equal temperature. Think of the sultriness before a thunder-storm, as 

 long as the air is at rest, and how differently we feel as soon as the first 

 wind rises. The air is not yet cooler, not less saturated with vapor 

 than before, and still it deprives us of so much more heat that we 

 deem it less sultry, even cool, only because it travels over us faster. 



When we fan ourselves in a hot and damp air, the same thing takes 

 place then, also, a greater amount of air passes over us in a given 

 time than if we leave the air to its own motions. The fan changes 

 nothing in the temperature and moistness of the air, it only increases 

 its velocity, and in consequence the abstraction of heat, and thus 

 affords us coolness chiefly on the uncovered or only slightly covered 

 parts of our bodies ; therefore, ladies have more reason for using it 

 than the stronger sex. 



As long as the air is our surrounding medium, an increased evap- 

 oration associates itself with the increased loss by conduction, at least 

 as long as the circulation of the blood in the skin remains active and 

 the air is not saturated with moisture. The fan scarcely ever cools by 

 increased conduction alone, but also by increased evaporation. There- 

 fore, fanning with dry air is much more cooling than fanning with a 

 moist air of equal temperature. We all know how much quicker wet 

 roads and wet clothes dry when there is a good w T ind. However 

 rapid the motion of moist air may be, it does not dry. When our body 

 is bathed in perspiration the fuller condition of the skin occasions an 

 increased transfer of heat from the dilated blood-vessels to the sur- 

 rounding air by conduction, but generally also by evaporation. 



In southern climes, at the hottest and moist time of the year, when 



