we should suddenly take
away the outside air in any way, such as covering a person up with an
air-pump receiver, and quickly and completely exhausting the air, the
consequences of the inside pressure would be very terrible, and if the
experiment could be tried quickly enough the body would burst like an
exploding gun, with a loud noise.

[Illustration: THE EAGLE (BIRD OF PREY).]

When people go up rapidly in a balloon or climb very high mountains,
they are troubled by a ringing noise and a feeling of great pressure in
the ears and head, and by palpitation of the heart, bleeding at the
nose, and fainting. These unpleasant and often dangerous symptoms are
caused by the expansion of the air inside their bodies. In ascending
very high mountains it is necessary to go very slowly and to stop very
often, to give time for some of the expanded air to escape, and equalize
the pressure again. Now, many birds, the condor, for example, fly over
the tops of the highest mountains, and nearly all birds, either
occasionally or habitually, ascend to very great altitudes, and, unless
there were some plan for regulating the pressure of the air inside their
bodies, they would suffer great inconvenience and even pain and danger.
But they are provided with an arrangement by which the air within them
can escape easily as it expands and thus keep the pressure within just
equal to that outside, so that they can ascend and descend as rapidly as
they wish, without feeling the least inconvenience. In the body of the
bird there are several large bags, like the lungs, called air-chambers;
many of their bones are hollow, and others are pierced with long winding
tubes called air-tubes. All these air-chambers and air-tubes are
connected with the lungs so that air can pass into and out of them at
each breath. The connection between these chambers and the lungs is so
complete that a wounded hawk can breathe through a broken wing almost as
well as through its mouth. When a bird mounts upward, the air inside its
body gradually expands, but the bird does not feel any inconvenience;
for, at each breath, part of the air passes from the air-chambers into
the lungs, so that the pressure on the inside does not become greater
than that on the outside.

[Illustrations: PENGUINS (SWIMMERS AND DIVERS).]

I could easily fill the whole of this chapter with an account of the
different ways in which the body of a bird is fitted for life in the
air, but we have room