INTERNAL WORK. 357 
less fuel. As spring advances, the fire is reduced until it is just 
sufficient for the necessary work. If the weather still continues 
to grow warmer, since the fire cannot be further reduced the excess 
of heat is gotten rid of by opening the windows more or less, while, 
to carry out the analogy, in very hot weather we may sprinkle the 
floor or wet the walls to secure relief from heat through the evapora- 
tion of water. 
MopiricaTION OF CONCEPTION OF CRITICAL TEMPERATURE.— 
In our discussion thus far we have considered chiefly the influence 
of external temperature on metabolism and heat production. This 
is, however, by no means the only condition affecting the heat 
balance of the body. Of the other meteorological factors, three 
call for special mention, viz., wind, insolation, and in particular 
relative humidity. 
Wind.—In a perfectly still atmosphere, the layer of air next to 
the skin becomes warmed and loaded with water vapor and con- 
stitutes to a certain degree a protective envelope which is removed 
with comparative slowness by gaseous diffusion. A current of air, 
by removing this protecting layer and bringing fresh portions of air 
in contact with the body, increases the emission of heat both by 
conduction and by evaporation of water. Thisis in accord with the 
common experience that a degree of cold which can readily be 
borne when the air is still becomes intolerable in a brisk wind, while, 
on the other hand, the oppressiveness of a very hot day is sensibly 
relieved by even a slight breeze. The effect of wind, then, is to 
transpose the thermic range of the animal to a higher place in the 
thermometric scale, and to correspondingly raise the critical tem- 
perature. 
Insolation.—The direct rays of the sun impart a considerable 
amount of heat to the body. The effect of insolation, therefore, is 
the reverse of that of wind, viz., to transpose the thermal range 
and the critical temperature downward. A similar effect is pro- 
duced, of course, by the sun’s heat when reflected from surrounding 
objects, or by the radiant heat from hot objects, the earth, for ex- 
ample. On the other hand, the radiation from the body into space 
during the night, especially at high altitudes and through a dry, 
clear atmosphere, may have a very considerable effect in the con- 
trary direction. 
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