Some of the families of the Swimming Birds are valuable additions to the poultry yards. Ducks and Geese furnish delicate and nourishing food; the Swan is gracefully ornamental on our lakes and ponds. The down of all the aquatic Birds as an article of commerce is of great value in northern countries. Their eggs constitute good food, and in many countries the inhabitants consume them in great quantities. But their usefulness does not end here. Guano, so eagerly sought for by the farmer, is the excrement of aquatic Fowls which has accumulated for ages, until in the South Pacific Ocean it is said to have formed whole islands; some of them being covered with this valuable agricultural assistant to the depth of ninety or a hundred yards. This does not seem so marvellous when it is considered that twenty-five or thirty thousand Sea-birds sleep on these islands night after night, and that each of them will yield half a pound of guano daily, which owes its unrivalled fertilizing power to the ammoniacal salts, phosphate of lime, and fragments of feathers of which it is composed. Although the numerous Swimming Birds are alike in having webbed feet and oily plumage that cannot be saturated with water, they have also many points of difference which make it necessary to divide them into various families. For instance, some of the Swimmers are feeble and slow in their flight, and others cannot even rise from the water as their wings are so small. On the other hand, there are species which possess wonderful power of traversing the air, their well-developed wings enabling them to pass through space with marvellous rapidity. The Petrels seem to delight in storms and tempests, mingling their cries with the roar of the waves; and the dread which is experienced by the mariner at the approach of a gale is unknown to the Sea Gull and Albatros, for they appear to delight in the warring elements. Because of these differences in their characteristics, Naturalists have divided the Swimming Birds in various ways, but the best and the simplest is the division into four great families. First, the Divers, or the Sea Birds with thin, short wings; second, the large family to which the Swan and Ducks and Geese belong; third, the Pelican family; fourth, the Swimming Birds with long wings. THE FAMILY OF DIVERS. The most important birds found in this family are the Great Northern Diver, the Arctic Diver, Penguins, Auks, Grebes, and Guillemots. All these Birds are distinguished by wings so thin and short as to be almost useless for flying. They are all habitual divers and tireless swimmers, using their wings as Fish do their fins. To raise their wings after taking a down stroke requires much greater effort than a Bird of flight makes in raising its wings in the air; for this reason the muscle in the wings of the Diving Birds has an unusually large development to give them greater strength.