100 THE STRUCTURE AND LIFE OF BIRDS  cHapP. 
be poison. And yet a bird does not perspire at all, 
and is perfectly at ease when flying under a hot sun. 
A man, if he is to endure heat; must resemble one of 
the porous earthenware pots used in India for cooling 
water. Put them in wind which, however hot, is dry, 
and, the evaporation increasing, the water cools all the 
more rapidly, and the sahib’s bath will be ready all 
the sooner. A man’s skin is highly porous, being 
covered with sweat glands, little tubes leading into the 
skin and communicating with the capillary blood 
vessels, from which the moisture permeates to the tube 
and so to the surface. Even when perspiration is 
imperceptible, a great deal is given off in the course of 
the day. A bird, on the contrary, has no sweat glands; 
it must therefore, have some other means of keeping 
down its temperature. True, some evaporation will go 
on though there are no pores, for if a bladder full of 
water be hung up in the air, the water will ooze 
through. Still, the process is a very slow one, and such 
evaporation cannot be of much service to a bird. 
It is now time to investigate more exactly the various 
means by which the body rids itself of superfluous 
heat. The processes at work are evaporation, con- 
duction, radiation. The chilling effect of evapora- 
tion, every one is familar with. We are conscious of 
loss of heat by conduction when we touch cold 
iron. But the same cause is always at work. By con- 
duction, the air that is next to the body is warmed 
—ie, the body gives off heat to the air. Clothes, 
according to the material of which they are made, 
vary very much in their power to lessen conduction ; 
they can never arrest it altogether. Radiation takes 
