ANIMAL HEAT AND FEVER. 137 



enough, with the result that the body temperature falls some- 

 what, and catarrhs, etc., are the result. There can be little doubt 

 that much of the benefit of open-air sleeping is due to the con- 

 stant stimulation of the metabolic processes which it brings 

 about. 



The importance of the evaporation of sweat in bringing about 

 loss of heat in man partly explains why climate should have so 

 important an influence on his well-being. It is not so much the 

 temperature of the air, as its relative humidity, that is of impor- 

 tance; that is, the degree, expressed in percentage, to which the 

 air is saturated with moisture at the temperature of observation. 

 Thus, a relative humidity of 75 per cent at 15 C. means that 

 the air contains 75 per cent of the total amount of moisture which 

 it would contain if it were saturated with moisture at a tempera- 

 ture of 15 C. A high relative humidity at a high temperature 

 makes it impossible for much sweat to be evaporated, with the 

 result that the body cannot cool properly, and the body tempera- 

 ture is likely to rise unless muscular activity be reduced to a 

 minimum. This explains why it is impossible to do much muscu- 

 lar work in hot humid atmospheres. On the other hand, if the 

 relative humidity is low, the temperature may rise to an extraor- 

 dinary degree (even above that of the body itself) without caus- 

 ing fever, provided always that the body is not so covered with 

 clothing that evaporation of sweat is impossible. 



At low temperatures of the air, relative humidity has an effect 

 which is exactly opposite to that which it has at high tempera- 

 tures, for now it affects, not the evaporation of sweat, but the 

 heat conductivity of the air itself. Cold moist air conducts 

 away heat' much more rapidly than cold dry air. Hence, a tem- 

 perature many degrees below zero on the dry plains of the West 

 may be much more tolerable to man than a much higher tem- 

 perature along the shores of the Great Lakes. 



Fever. Any rise of temperature above the normal limits 

 constitutes fever. When of slight degree, as it is in many semi- 

 acute diseases, its detection demands l'iv<|iient observation, so 

 as to allow for the normal diurnal variation of the body tempera- 

 ture. For example, if the temperature were recorded in the 



