others have only one; and in a few--as the king of the vultures and the condor--vocal muscles are quite wanting. Seeing that the temperature of birds is raised so much above the usual standard by the arrangements described above, some clothing is requisite, adequate to retain the vital heat. Another indispensable provision is therefore met with in the FEATHERS with which all birds are so warmly clad. Indeed, so peculiar is the texture of these admirable fabrics, that no better distinctive appellation could be devised for the entire class than that of the "feathered tribes," by which they are frequently designated. A feather realises in its structure more qualities than imagination could have conceived it possible to combine--lightness, thickness, warmth, durability, elasticity, softness, strength, and beauty. It is one of the master-works of creation. Whoever has examined a feather under the microscope will testify to the incomparable perfection of the contrivance. Every feather is a mechanical wonder. If we look at the quill-portion, or barrel, we find it possessed of attributes not easily brought together--strength and lightness. If we cast our eye upon the upper part of the stem, we see a material made for the purpose, which is used in no other class of animals, and in no other part of birds--tough, light, pliant, elastic--the pith. This is also a substance _sui generis_; it is neither bone, flesh, membrane, nor horn. [Illustration: Fig. 2.--WING OF A BIRD PARTIALLY STRIPPED OF FEATHERS, TO SHOW THE INSERTIONS OF THE QUILLS. _a_, the Arm; _d_, the Fore-arm; _g_, the Thumb; _c_, the Secondary Quills, implanted into the Fore-arm; _f_, the Primary Quills, implanted into that portion of the Wing which represents the hand; _e_, the Spurious or Bastard Quills, derived from the Thumb.] But the most wonderfully constructed part of a feather is the _plume_, or, as it is sometimes called, the _web_. This is affixed to each side of the stem, and constitutes the broad expansion of the feather, that part which we usually strip off when making a pen. One of the first things to be remarked is that the web is much stronger when pressed in a direction perpendicular to the flat plane of the plume than when rubbed either up or down in the direction of the stem; the reason of this is that the web is composed of numerous flat, thin, and broad laminæ, arranged with their flat sides together, so that, although they easily bend towards each other, they offer great resistance in the direction in which they have to encounter the impulse and pressure of the air; and it is in this direction only that their strength is wanted and put to the test.