358 PRINCIPLES OF ANIMAL NUTRITION. 
Relative Humidity—The relative humidity of the air affects the 
emission of heat in two principal ways. At Jow temperatures, 
where the evaporation of water plays a subordinate réle, it increases 
the rate of emission by increasing the conductivity and specific 
heat of the air, and also the conductivity of the skin and the body 
covering (hair, fleece, clothing), these effects outweighing its in- 
fluence in diminishing the relatively small amount of evaporation 
Moist cold is, therefore, more trying than dry cold. 
At high temperatures, on the other hand, where a large pro- 
portion of the heat is removed by evaporation, a high relative 
humidity, by checking this evaporation, hinders the emission of 
heat, this effect overbalancing any slight increase in conductivity. 
Moist heat is accordingly more oppressive than dry heat. 
An increase in the relative humidity, then, abbreviates the 
thermal range at both ends, while at moderate temperatures it 
appears to have but little effect, a diminution of the loss by evap- 
oration being compensated for by an increase in radiation and 
conduction. 
CriticAL THERMAL ENVIRONMENT.—From the above it is 
obvious that the so-called critical temperature is not a constant, 
even for the same species or the same individual, but that other 
factors than the temperature of the air materially affect it. 
What is constant (relatively at least) is the rate at which heat 
is produced in the body by the metabolism necessary to sustain its 
various physiological activities, that is, by its internal work. In 
order to maintain the normal body temperature, the total outflow 
of heat through its various channels must, at its minimum, be equal 
to the amount thus liberated in the organism. The outflow of 
heat, as we have seen, is affected directly or indirectly by the 
external conditions, and Jargely by the three just mentioned. In- 
numerable combinations of these conditions are possible, and any 
one of them whose combined effect upon the animal is to make 
the outflow of heat equal to the rate of evolution due to the 
internal work will constitute a critical point in the above sense. 
Any change in such a set of conditions which tends to increase the 
outflow of heat will, like a fall in temperature, be met chiefly by an 
increased heat production.~ Any change tending in the opposite 
direction will be compensated for by the effects upon the organ- 
