588 A MANUAL OF PHYSIOLOGY 



temperature at Bagdad ranged for a considerable time between 

 108 and 120 F. (42 to 49 C.), and there was great mortality. 

 A much higher temperature may be borne in dry air than in air 

 saturated with watery vapour. A shade temperature of 100 F. 

 (37' 7 C.) in the dry air of the South African plateaux is quite 

 tolerable, while a temperature of 85 F. (29*4 C.) in the moisture- 

 laden atmosphere of Bombay may be oppressive. The reason is 

 that in dry air the sweat evaporates freely and cools the skin, 

 while in moist air, although according to Rubner the loss of 

 heat by radiation and conduction is increased, the loss of heat 

 by evaporation of sweat is diminished in a still greater degree. 

 In saturated air at the body-temperature no loss of heat by 

 perspiration or by evaporation from the pulmonary surface is 

 possible ; the temperature of an animal in a saturated atmosphere 

 at 35 to 40 C. soon rises, and the animal dies. In animals 

 like the dog, which sweat little or not at all on the general sur- 

 face, the regulation of the heat-loss by respiration is relatively 

 more important than in man. 



The observations of Boycott and Haldane in a deep mine, in the 

 incubating-room of a laboratory, and in a Turkish bath illustrate 

 the important influence of the humidity of the air. In still air the 

 body-temperature rose above normal when the wet-bulb thermo- 

 meter rose above 31 C. (88 F.), and it remained normal whatever 

 the external temperature might be so long as the reading of the wet- 

 bulb thermometer did not exceed that level. The more the wet-bulb 

 thermometer rose above 31 the more rapid was the increase in the 

 body -temperature. In moving air a greater degree of humidity 

 could be "borne without increase in the body -temperature, which did 

 not occur till the temperature shown by the wet-bulb thermometer 

 exceeded 35 C. The great increase in the evaporation of sweat 

 when the temperature of the air is high is shown by the observation 

 that on a warm day (dry bulb, 79 F. ; wet bulb, 67'$ F) the average 

 loss of moisture from the body was 1,816 grammes for four soldiers 

 during a march of seven miles, while on a cold day (dry bulb, 45 F. ; 

 wet bulb, 38 F.) it was only 419 grammes during the same march by 

 the same men (Pembrey). 



The winter fur of Arctic animals is a special device of Nature 

 to meet the demands of a rigorous climate, and combat a ten- 

 dency to excessive loss of heat. The experiments of Hosslin, 

 and the experience of squatters in Australia, go to show that 

 even domesticated animals have a certain power of responding 

 to long-continued changes in external temperature by changes 

 in the radiating surfaces which affect the loss of heat. It is 

 said that in the hot plains of Queensland and New South Wales 

 the fleeces of the sheep show a tendency to a progressive decrease 

 in weight. And Hosslin found that a young dog exposed for 

 eighty-eight days to a temperature of 5 C. developed a thick 

 coat of fine woolly hairs. Another dog of the same litter, ex- 

 posed for the same length of time to a temperature of 31*5 to 



