346 REGULATION OF TEMPERATURE 



(3) The gradient of temperature has much to do with the 

 amount of heat lost by evaporation. This is an indirect effect. 

 It has been suggested that the pigmented skin of tropical races, 

 by absorbing radiant energy and so becoming unduly warm, stimu- 

 lates epidermal nerve-endings and produces vasodilatation and 

 profuse sweating. The evaporation of this water then cools the 

 body in a similar way to the Indian chatti or water cooler. Accord- 

 ing to G. F. Hearne, exhaustion of the sweat mechanism always 

 preceded heat-stroke in Mesopotamia. 



(4) The presence of a current of dry air removes the gaseous 

 water from the surface, and so brings dry air in contact with the 

 body. In other words, the partial pressure of the gaseous water 

 in the layer next to the body is kept at a minimum. This drying 

 effect of wind is operative irrespective of the temperature of the 

 wind, as witness the drying of shallow pools of water and of wet 

 clothes by cold and by warm breezes. 



(5) Humidity of Air. The presence of moisture in the air affects 

 its cooling powers in two ways, (a) Obviously, if the air is 

 already saturated with moisture it is unable to absorb more, and 

 evaporation from the body even when played upon by a draught 

 will be at a minimum. (/3) On the other hand, as we have already 

 mentioned, moist air is a much better conductor of heat than dry 

 air, and therefore, the heat lost by conduction tends towards a 

 maximum in a humid atmosphere. The former of these factors 

 plays a major part when the surroundings are warm, the latter 

 operates maximally in cold climates. 



In the assessment of the conditions of the atmosphere making for ideal 

 heat-loss, two instruments are in common use, viz., the wet and dry 

 bulb thermometer and Hill's " Kata " thermometer (or its popular modifi- 

 cation the Comfimeter). The first of these instruments consists of two 

 similar mercury thermometers fastened side by side on a stand. The 

 reservoir of one is covered by a close fitting muslin bag which is kept 

 moist by connection to a wick dipping in water. The water evaporates 

 from the bulb at a rate depending principally on the degree of saturation 

 of the atmosphere with aqueous vapour. It is obvious that, if the air is 

 already saturated with moisture and no evaporation is taking place from 

 the wet bulb, both thermometers will register the same temperature. On 

 the other hand, if the difference in the temperatures recorded is great, one 

 may consider that the air is dry. 



Example. The degree of humidity of the air may be calculated from the 

 formulae, 



degree of humidity = i- and /= F - AH (t - 6), 



