WORK UNDER PRESSURE AND IN GREAT HEAT 393 



much exposed to excessive heat, and the influence of this on 

 their working powers and health is a matter of great practical 

 importance. 



In some of the mines of Cornwall the heat is probably as 

 trying as in any place where men have to live and work ; and 

 my observations there led me to make experiments on a man's 

 power of remaining in and working in warm air. It is of 

 course well known that men can remain for a short time in 

 extremely hot air, if it is dry. A temperature of ioo° C. in a 

 Turkish bath, or even a much higher temperature, can easily 

 be borne for a few minutes ; but there is surprisingly little 

 information as to what temperature a man can continue to 

 withstand for hours at a time during his ordinary occupation. 

 My observations were made on several persons, partly in 

 mines and partly in artificially heated rooms. 1 



One main result was that in still air, during rest, and with 

 the subject of the experiment stripped to the waist, the maximum 

 temperature which could be borne for several hours without 

 marked pathological symptoms was 31° to 32° C. (88 c to 90 F.) 

 by the wet-bulb thermometer. Neither the actual temperature 

 by the dry-bulb thermometer, nor the degree of humidity, nor 

 the percentage of moisture in the air, mattered by itself. The 

 determining factor was simply the wet-bulb temperature. If 

 this exceeded 31 to 32 (the actual critical point seemed to vary 

 slightly in different individuals), the body temperature began to 

 rise steadily. The accompanying diagram shows the results of 

 several experiments made on myself. 



It was somewhat remarkable that when the air temperature 

 was just above the critical point, the rise of body temperature 

 did not slacken or cease as the difference between body tempera- 

 ture and wet-bulb air temperature began to increase. Actually 

 the rate of rise of body temperature seemed to increase rather 

 than diminish. The reason for this is the fact, clearly proved 

 in some quite recent experiments by Dr. Sutton, of Oxford 

 University, that the respiratory exchange, and consequently the 

 heat production in the body, rises very markedly with even a 

 slight increase in the rectal temperature. Hence the heat-loss 

 cannot catch up the heat-production, and the body is in a 

 "vicious circle." 



1 Details of part of these experiments will be found in the Journal of Hygiene, 

 vol. v. p. 494. 



