Some of these feathers are weak and soft, with slender shafts and loose threads growing all round them, and these are the downy feathers which lie close to the body and keep the bird warm. Others, which cover the outside and form the wings and tail are flat, with strong quills and shafts, and a double set of barbs growing upon each shaft; and if you look at these wing feathers under a strong microscope you will see that they have a special arrangement for helping them to resist the air. For not only have all the little featherlets or _barbs_ rows of other featherlets or _barbules_ growing upon them, but these again are covered with fine horny threads, often hooked at the tip, which cling to the next barb, so that the feather is woven together as it were, in a close web, and if you strike it against the air you will find that it resists it strongly. Now in a bird’s wing the feathers are so arranged that they lap one under the other from the outside of the wing to the body, so that when the bird strikes downwards they are firmly pressed together, and the whole wing, which is hollow like the bowl of a spoon, encloses a wingful of air, and as this is forced out behind, where the tips of the feathers are yielding and elastic, he is driven upwards and forwards. When, however, he lifts his wing again, the feathers turn edgeways and are separated, so that the air passes through them, and he still rises while preparing for the next stroke. All this goes on so rapidly that even the heron makes 300 strokes in a minute, and the wild duck 500, while in most birds they are so rapid that it is impossible to count them; yet all the while the little creature can direct his flight where he will, can pause and direct his wings to the breeze so as to soar, can swoop or hover, wheel or strike, guiding himself by the outspread tail and a thousand delicate turns of the wing. All this complicated machinery, however, would not have served the bird much if his body had been as heavy, and his blood as cold, as those of the lizard and the crocodile. But here he has made a great step forward. In the first place, he has a heart with four chambers, two on the right side and two on the left; and while one of those on the right side receives the worn-out blood from the body and pumps it _to_ the lungs to be refreshed, one of those on the left side receives it _from_ the lungs when it is refreshed, and the other pumps it into the arteries to feed the body. So here we see for the first time among our backboned animals a creature whose good and bad blood are never mixed in the heart (compare pp. 23 and 76), but it gets all the benefit possible from its breathing, and the blood is kept fresh and pure. Moreover, a bird’s lungs are large, and are continued into several large air-sacs, which in their turn open out into tubes which carry air actually _into the bones_, many of which are hollow instead of containing marrow like those of other animals.[94]