ioo THE STRUCTURE AND LIFE OF BIRDS chap. 



be poison. And yet a bird does not perspire at all, 

 and is perfectly at case 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 

 — i.e., 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 



