104 ARON. 



therefore, the evaporation of water from the body lessened, the loss of 

 heat is inhibited.^" It will be recalled that in many localities in America 

 or Africa the thermometer in summer often is much higher than it is 

 in the Tropics, yet the heat by no means produces the same effect. 

 The humidity in the Tropics is always comparatively high, because 

 the air for considerable periods of time is nearly saturated with 

 water vapor. It might be stated that it is not regions of high 

 air temperatures, but those having a high relative humidity which pro- 

 duce especially untoward effects by reason of their climate.^^ However, 

 if the air is in motion, even if it is very humid and hot, increased 

 water evaporation and conduction bring about a great loss of heat. 

 This fact is of great importance in the Tropics. The fresh winds prev- 

 alent here render the climate of Manila in the months of May to 

 August much more tolerable to human beings than is the case in certain 

 parts of the Chinese coast or even on the Atlantic seaboard which lie 

 considerably farther to the north. 



Generally speaking, the majority of people living in the Tropics are 

 on the coastal or intermontane plains, where the climatic conditions 

 are nearly alike throughout the year. The high air temperatures and 

 high relative humidity are maintained so that conditions retarding the 

 loss of body heat are practically continuous, in distinction from those 

 regions where, despite the fact that at certain times the heat and relative 

 humidity are high, nevertheless the average for the year is low. How- 

 ever, the body temperature of man and probably also of animals, in 

 spite of this fact, normally does not exceed the physiologic limits. 

 This has been shown by a large number of careful measurements of 

 body temperatures of white and colored men in the Tropics, and of the 

 same people in the Tropics and in temperate climates. Variations, when 

 they have been observed, are doubtless not greater than the daily ones 

 encountered in other climates. 



Finally, the radiation from the sun is obviously an important factor. 

 Any object exposed to the sun's rays absorbs a portion of them. The 

 majority of substances, and among them is included the animal body, 

 have a much higher coefficient of absorption for heat than has the air, 

 and therefore they become hotter in the sun than does the surrounding 

 air. This effect of the heat radiated from the sun, while generally most 

 intense in the Tropics, is present in all latitudes. Eubner, Cramer,^^ 

 and Wolpert ^^ have studied the results of insolation in temperate 

 climates. According to their experience we can calculate approximately 



"Haldane, Journ. Hyg., Cambridge (1905), 5, 494. 



" Of course it must be recalled that in the Tropics, where the relative humidity 

 is high, the sun is often obscured by clouds. 



"^Arch. f. Hyg. (1894), 20, 313-344; (1892), 33, 206-228; (1902), 44, 322- 

 338. 



