the action of even an ordinary cane is sufficient to elicit a recoil.
This comes of the action and reaction of matter, the resistance
experienced varying according to the density of the atmosphere
and the shape, extent and velocity of the body acting upon it.
While, therefore, scarcely any impediment is offered to the progress
of an animal in motion in the air, it is often exceedingly
difficult to compress the air with sufficient rapidity and energy
to convert it into a suitable fulcrum for securing the necessary
support and forward impetus. This arises from the fact that
bodies moving in air experience a minimum of resistance and
occasion a maximum of displacement. Another and very obvious
difficulty is traceable to the great disparity in the weight of air
as compared with any known solid, and the consequent want of
buoying or sustaining power which that disparity involves. If
we compare air with water we find it is nearly 1000 times lighter.
To meet these peculiarities the insect, bird and bat are furnished
with extensive flying surfaces in the shape of wings, which they
apply with singular velocity and power to the air, as levers of
the third order. In this form of lever the power is applied
between the fulcrum and the weight to be raised. The power
is represented by the wing, the fulcrum by the air, and the
weight by the body of the flying animal. 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, that is, when the fulcrum, as happens
with the air, is elastic and yielding. In this instance a very slight
movement at the root of the pinion, or that end of the lever
directed towards the body,
is followed by an immense
sweep of the extremity of
the wing, where its elevating
and propelling power
is greatest—this arrangement
ensuring that the
large quantity of air
necessary for support and
propulsion shall be compressed
under the most
favourable conditions.
In this process the weight of the body performs an important part, by acting upon the inclined planes formed by the wings in the plane of progression. The power and the weight may thus be said to reciprocate, the two sitting as it were side by side and blending their peculiar influences to produce a common result, as indicated at fig. 26.
When the wings descend they elevate the body, the wings being active and the body passive; when the body descends it contributes to the elevation of the wings,[1] the body being active and the wings more or less passive. It is in this way that weight forms a factor in flight, the wings and the weight of the body reciprocating and mutually assisting and relieving each other. This is an argument for employing four wings in artificial flight,—the wings being so arranged that the two which are up shall always by their fall mechanically elevate the two which are down. Such an arrangement is calculated greatly to conserve the driving power, and as a consequence, to reduce the weight.
 |
| Fig. 27.—a, b, quill feathers; c, cork; d, e, f, g, downward and forward curved trajectory made by the feathers and cork before reaching the ground (h, i). |
That the weight of the body plays an important part in the production of flight may be proved by a very simple experiment. If two quill feathers are fixed in an ordinary cork, and so arranged that they expand and arch above it (fig. 27), it is found that if the apparatus be dropped from a vertical height of 3 yds. it does not fall vertically downwards, but downwards and forwards in a curve, the forward travel amounting in some instances to a yard and a half. Here the cork, in falling, acts upon the feathers (which are to all intents and purposes wings), and these in turn act upon the air, in such a manner as to produce a horizontal transference.
In order to utilize the air as a means of transit, the body in motion, whether it moves in virtue of the life it possesses, or because of a force super-added, must be heavier than air. It must tread with its wings and rise upon the air as a swimmer upon the water, or as a kite upon the wind. This is necessary for the simple reason that the body must be active, the air passive. The flying body must act against gravitation, and elevate and carry itself forward at the expense of the air and of the force which resides in it, whatever that may be. If it were otherwise—if it were rescued from the law of gravitation on the one hand, and bereft of independent movement on the other, it would float about uncontrolled and uncontrollable like an ordinary balloon.
In flight one of two things is necessary. Either the wings must attack the air with great violence, or the air in rapid motion must attack the wings: either suffices. If a bird attempts to fly in a calm, the wings must be made to smite the air after the manner of a boy’s kite with great vigour and at a high speed. In this case the wings fly the bird. If, however, the bird is fairly launched in space and a stiff breeze is blowing, all that is required in many instances is to extend the wings at a slight upward angle to the horizon so that the under parts of the wings present kite-like surfaces. In these circumstances the rapidly moving air flies the bird. The flight of the albatross supplies the necessary illustration. If by any chance this magnificent bird alights upon the sea he must flap and beat the water and air with his wings with tremendous energy until he gets fairly launched. This done he extends his enormous pinions[2] and sails majestically along, seldom deigning to flap his wings, the breeze doing the work for him. A familiar illustration of the same principle may be witnessed any day when children are engaged in the pastime of kite-flying. If two boys attempt to fly a kite in a calm, the one must hold up the kite and let go when the other runs. In this case the under surface of the kite is made to strike the still air. If, however, a stiff autumn breeze be blowing, it suffices if the boy who formerly ran when the kite was let go stands still. In this case the air in rapid motion strikes the under surface of the kite and forces it up. The string and the hand are to the kite what the weight of the flying creature is to the inclined planes formed by its wings.
The area of the insect, bird and bat, when the wings are fully expanded, is greater than that of any other class of animal, their weight being proportionally less. As already stated, however, it ought never to be forgotten that even the lightest insect, bird or bat is vastly heavier than the air, and that no fixed relation exists between the weight of body and expanse of wing in any of the orders. We have thus light-bodied and
- ↑ The other forces which assist in elevating the wings are—(a) the elevator muscles of the wings, (b) the elastic properties of the wings, and (c) the reaction of the compressed air on the under surfaces of the wings.
- ↑ The wings of the albatross, when fully extended, measure across the back some 14 ft. They are exceedingly narrow, being sometimes under a foot in width.
