228 THE HUMAN MOTOR 



The effect of this great pressure is nil because it acts equally 

 both inside and outside the body. 



The influence of the various factors in the external environment 

 will now be considered in detail, so far as they affect the conditions 

 of industrial labour. 



165. Influence of Temperature. The influence of the tempera- 

 ture of the environment on the vital organism is of first import- 

 ance. Numerous observations have demonstrated that decrease 

 in the temperature of the environment causes a proportionate 

 increase in the expenditure of the organism. Cold stimulates 

 vital combustions ; heat relaxes them. 



Bodies cool by radiation. The lower the external temperature, 

 the more the cutaneous surface of the animal radiates and tends 

 to lose heat. The speed of cooling of hot bodies increases pro- 

 portionally to the excess of their own temperature over that of 

 the external environment. This is New- 

 ton's law. If the human body is repre- 

 sented by a circle of hot water (fig. 144) , the 

 radiation is measured by the number of 

 small calories emitted by a surface of 1 

 square centimetre. As the emission, ac- 

 cording to Newton's law, is proportional to 

 the difference of temperature between the 

 surface, and the external environment, the 

 emissive power, K, is the number of calories FIO 144 

 per second per degree difference. T/ &nd T ^ ^ in _ 



Thus for a difference of temperature teraal and external tem- 

 (T t), the loss per second per square centi- P erature of th e skin : t is 



( the 



metre of the radiating surface will be : 



q = K x (T /) small calories. 



Radiation tends to diminish the temperature of the body, 

 and therefore more heat has to be produced by the body in order 

 to maintain its temperature constant (about 37-50 C.) by pro- 

 ducing more heat (chemical regulation). The value of the emis- 

 sive power K varies according to the nature and the colour of the 

 radiating surface. 



In addition to the radiation, the air, in contact with the skin 

 and constantly changing, carries away a quantity of heat by 

 convection. The loss due to convection depends not only on the 

 difference of temperature (T t) and on the area of the surface, 

 but also on the shape of the latter. According to Pclet's classic 

 researches, this factor can be represented by a co-efficient, K 2 

 000066 c. per square centimetre per second ; whence : 



ft =K,(T-/) 

 in small calories. 



