EVAPORATION OF MOISTURE 443 



(iii.) Force of Wind. 



Ill still air the loss by conduction is very sliojit. Air is not a 

 very good thermal conductor. A layer of warmed air soon 

 forms an envelope round the cooling body and prevents rapid 

 conduction. A very slight movement of the air may produce a 

 very appreciable effect by driving off the warm particles and 

 bringing cool ones into contact with the body. This may arise 

 naturally by the formation of convection currents. The heated air 

 expands and becomes lighter and so rises and allows the colder air 

 to flow in. 



The cooling effect of an air current is appreciable when the air 

 moves at the rate of about 0-4 to 0-5 metre per second. A non- 

 perceptible draught with a velocity of 0-2 metre per second playing 

 on the naked arm may increase the heat loss over that experienced 

 in still air by 19 to 75 per cent, depending on the temperature of the 

 air. A moderate breeze at 8 metres per second (15 miles per hour) 

 with a temperature of 20° C. causes more rapid chilling of a naked 

 man than exposure to still air at 2° C. (see also p. 445). 



(iv.) Humidity of Atmosphere. 



Water is a better conductor of heat than is air. A moment's 

 consideration of the feeling of cold in cold air before entering 

 water at the same temperature and during the process will 

 convince one of this. We have already mentioned the difference 

 in the cold sensation caused by a dry cold wind compared with 

 that produced by a wind of even a much higher temperature 

 but laden with moisture. 



III. Evaporation of Moisture 



As the temperature of the air and of the body approach the same 

 value, the heat lost by the body by radiation and conduction will 

 be exactly balanced by the heat absorbed from the environment 

 by the body. If the atmosphere attain a higher temperature 

 than the body, then these means of heat elimination wdll become 

 inadequate, and the body temperature would increase synchron- 

 ously with that of its surroundings. That normally this is not 

 so is common knowledge. Many interesting instances have been 

 known of persons submitted to high temperatures, and an examina- 

 tion of some of these cases may lead us to a knowdedge of the 

 mechanism called into play to prevent untoward results. On one 

 point, preliminary emphasis must be laid, viz. the times during 

 which these extreme temperatures were endured were of very 

 short duration. 



