Birds are marked off from all other vertebrate animals by the possession of feathers. How wonderful is the wing of a bird; spread wide when it is flying, and folded up like a fan when it is resting, perched upon the branch of a tree, swaying to and fro in the sunshine. But how sad it is to see such a wild, free creature as a lark, or even a thrush or a linnet, pent up in a narrow cage, where there is no room to stretch those wings so strong and light, no swinging branch to rest upon; but all the little prisoner can do is to hop from one perch to another, and beat its wings against the "wiry grate" which shuts it in so hopelessly. I suppose we don't think so much of captive birds as of other captives, because a bird in a cage is such a common sight, and when we hear it sing so sweetly it seems as if it could not be _un_happy; but when we say "as happy as a bird," I doubt if it is of birds in cages we are thinking after all. The cage may be of gilded wires, or of willow twigs; but both are alike prison bars which keep the birdie back from the liberty to which it was born. At least this was what an English sailor felt when he met a man carrying a cage full of birds. He had been a prisoner himself, away in France, and had many a time longed to be free; and now when he saw the birds in their gilded prison, he was not happy until he had made a bargain and got them, cage and all, to do what he liked with. What was the astonishment of the man from whom he had bought them, when he saw the sailor open the cage door and let them out, one by one, until all the little prisoners were free! As you have watched the birds in their flight, I daresay you have wondered how they can keep themselves up in the air. Even the little wren has some weight; much more the crows which make their nests in the topmost branches of the trees. We say "as light as a feather"; yet the downiest feather has some weight, and will find its way to the ground if not kept up by wind or breath. It is true that the "feathered fowl," as all kinds of birds are called in the Bible, are very much heavier than the air in which they float and swim, using their wings for oars, just as the fish use their fins. But do you remember that little balloon inside the fish, which enables it to rise through the water? A bird is almost a live balloon; as it flies, it breathes air into every part of its body; this air becomes heated, and is kept warm by the feathers; and as hot air becomes light, the bird is so much lighter than the air which surrounds it, that it can easily rise higher and higher, until, like the skylark, its little quivering body seems almost lost in the far blue sky, and its "waterfall of song" alone shows where it is. [Illustration: "THE WHITE SEA-GULL, THE BOLD SEA-GULL, A JOYFUL BIRD IS HE."]