_The Wing a Lever of the Third Order._--To meet the peculiarities stated above, the insect, bat, and bird are furnished with extensive surfaces in the shape of pinions or wings, which they can apply with singular velocity and power, as levers of the third order (fig. 3, p. 20),[61] at various angles, or by alternate slow and sudden movements, to obtain the necessary degree of resistance and non-resistance. Although the third order of lever is particularly inefficient when the fulcrum is _rigid_ and _immobile_, it possesses singular advantages when these conditions are reversed, _i.e._ when the fulcrum, as happens with the air, is _elastic_ and _yielding_. In this case a very slight movement at the root of the pinion, or that end of the lever directed towards the body, is succeeded by an immense sweep of the extremity of the wing, where its elevating and propelling power is greatest. This arrangement insures that the large quantity of air necessary for propulsion and support shall be compressed under the most favourable conditions. [61] In this form of lever the power is applied between the fulcrum and the weight to be raised. The mass to be elevated is the body of the insect, bat, or bird,--the force which resides in the living pinion (aided by the inertia of the trunk) representing the power, and the air the fulcrum. It follows from this that those insects and birds are endowed with the greatest powers of flight whose wings are the longest. The dragon-fly and albatross furnish examples. The former on some occasions dashes along with amazing velocity and wheels with incredible rapidity; at other times it suddenly checks its headlong career and hovers or fixes itself in the air after the manner of the kestrel and humming-birds. The flight of the albatross is also remarkable. This magnificent bird, I am informed on reliable authority, sails about with apparent unconcern for hours together, and rarely deigns to flap its enormous pinions, which stream from its body like ribbons to the extent, in some cases, of seven feet on either side. The manner in which the wing levers the body upwards and forwards in flight is shown at fig. 52. [Illustration: FIG. 52.]