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A TEXTBOOK OF PHYSIOLOGY 



while marching with a load of 31 kilogrammes, produces 7-73 calories 

 per minute, or enough heat to raise his body by 1 C. in 8-7 minutes. 

 This shows the importance, then, of properly clothing the soldier for 

 marching in hot weather. 



The thermometer gives the average temperature of the surround- 

 ings; it tells nothing as to the rate of cooling the matter of greatest 

 importance to the body. The katathermometer has been introduced 

 to measure this. This is a large-bulbed spirit thermometer which is 

 warmed above body temperature. The rate of cooling is determined 

 as the meniscus falls from 100 to' 95 F. with a stop- 

 f| watch (Fig. 237). A factor is determined for each 

 instrument, by means of which is reckoned the rate 

 of cooling per sq. cm. of surface per second at body 

 temperature. The dry-bulb katathermometer indi- 

 cates the rate of heat loss by radiation and convec- 

 tion, the wet-bulb by radiation, convection, and evap- 

 oration. The difference between the two gives the 

 rate of cooling by evaporation. The instrument 

 shows the great cooling effect of wind, and is of 

 value in investigating the conditions of the open air 

 and of ventilation in buildings (see p. 316). 



Clothes. The cutaneous fat is the natural garment 

 of the body. The warming value of fat may be 

 gathered from the fact that with a temperature 

 difference of 18-2 C. a layer of skin, 2 millimetres 



4 thick, lets through 0-00248 calory per minute, whereas 



| 2 millimetres of skin plus 2 millimetres of fat lets 

 J % through only 0-00123 calory. Anointing the skin 

 rott with grease wards off " frost-bite " in those exposed 

 to cold and wet. The grease prevents the macera- 

 tion of the skin by water. If water soaks into the 

 V^ffl skin its non-conducting power is greatly reduced. 

 Exposure to sea water can be borne much better 

 than fresh water because salt water, owing to its 

 THE isotonic properties, does not macerate the skin. 

 Channel swimmers thickly cover their bodies with 

 and are noted for the thickness of their 

 cutaneous fat. The vernix caseosa of the new-born, 

 washed off by the midwife, is designed to protect it 

 from wet and cold after birth. 



Clothes increase evaporation and lessen loss of heat by convection 

 and radiation, in still air lo an extent of 47 per cent. At 18 C. 

 a clothed man of 74 kilos lost 79 calories per hour clothed, and 

 124 calories per hour naked. The clothes entangle and render 

 .stationary the air within their cellular structure and between their 

 layers. The more garments we put on over one another, the 

 more laj^ers of entangled air. To keep us warm, the clothes must 

 prevent the wind sweeping away the entangled air; to keep cool, 

 the wind must have free play. When dry, cellular, wool, or cotton 



FIG. 237. 



KATA THERMO- 

 METER. (Re- 

 duced from PM. grease, 

 Trans. Hoy. 

 .S'oc.) 



