The covering of birds is known by the general name of _plumage_. It is composed of many individual _feathers_. The feathers are horny productions, consisting of a hollow tube or barrel, and a stem rising from it. Chemically, this covering is of the same material as the hair on Mammals and the scales on reptiles and fishes, differing only in its mechanical structure. Besides the more conspicuous feathers, most birds have an underneath covering of smaller ones known as down-feathers. A feather of the ordinary kind consists of the _tube_, or barrel, by which it is attached to the skin, varying in length according to the species; the _stem_, or _shaft_, composed internally of a soft, compact, but elastic substance of a whitish colour, and in its buoyancy not unlike cork; the _web_, which is a lateral prolongation of the external coating of the shaft, and which assumes the form of a thin linear membrane springing from it at an angle more or less acute in different species: this is the _barb_. From the upper edge of each barb two sets of minute filaments proceed at an angle similar to that of the barb itself in respect to the shaft. These smaller filaments are the _barbules_, by means of which the barbs are retained in opposition--not by the barbules of one barb interlocking with those of another in the manner of dovetailing, but by the anterior series of one barb overlapping and hooking into the recurvate formation of the barb next to it (Figs. 51, 52). The barbules themselves frequently throw out filaments in the same manner, which are called _barbicels_, whose object is apparently the same--namely, that of connecting and retaining the barbules in position. These may be observed, by the aid of a small magnifying glass, in the quills of the Golden Eagle, _Aquila chrysaëtus_. [Illustration: Fig. 51.] [Illustration: Fig. 52.] [Illustration: Fig. 53.] [Illustration: Fig. 54.] [Illustration: Fig. 55.] Feathers, then, consist of three parts--the tube, the shaft, and the webs; the webs being the barbs furnished with barbules, sometimes barbicels. They are convex above, and are thus enabled to resist flexion or fracture better from beneath than from any other direction. They are also elastic; and this property, together with their curvature, tends to keep them closer together.