The goose, duck (fig. [107], p. 204), pigeon (fig. [106], p. 203) and crow, are intermediate both as regards the form of the wing and the rapidity with which it is moved.
The heron (fig. [60], p. 126) and humming-bird furnish extreme examples in another direction,—the heron having a large wing with a leisurely movement, the humming-bird a comparatively large wing with a greatly accelerated one.
But I need not multiply examples; suffice it to say that flight may be attained within certain limits by every size and form of wing, if the number of its oscillations be increased in proportion to the weight to be raised.
Reasons why the effective Stroke should be delivered downwards and forwards.—The wings of all birds, whatever their form, act by alternately presenting oblique and comparatively non-oblique surfaces to the air,—the mere extension of the pinion, as has been shown, causing the primary, secondary, and tertiary feathers to roll down till they make an angle of 30° or so with the horizon, in order to prepare it for giving the effective stroke, which is delivered, with great rapidity and energy, in a downward and forward direction. I repeat, “downwards and forwards;” for a careful examination of the relations of the wing in the dead bird, and a close observation of its action in the living one, supplemented by a large number of experiments with natural and artificial wings, have fully convinced me that the stroke is invariably delivered in this direction.[94] If the wing did not strike downwards and forwards, it would act at a manifest disadvantage:—
1st. Because it would present the back or convex surface of the wing to the air—a convex surface dispersing or dissipating the air, while a concave surface gathers it together or focuses it.
2d. In order to strike backwards effectually, the concavity of the wing would also require to be turned backwards; and this would involve the depression of the anterior or thick margin of the pinion, and the elevation of the posterior or thin one, during the down stroke, which never happens.
3d. The strain to which the pinion is subjected in flight would, if the wing struck backwards, fall, not on the anterior or strong margin of the pinion formed by the bones and muscles, but on the posterior or weak margin formed by the tips of the primary, secondary, and tertiary feathers—which is not in accordance with the structure of the parts.
4th. The feathers of the wing, instead of being closed, as they necessarily are, by a downward and forward movement, would be inevitably opened, and the integrity of the wing impaired by a downward and backward movement.
5th. The disposition of the articular surfaces of the wing (particularly that of the shoulder-joint) is such as to facilitate the downward and forward movement, while it in a great measure prevents the downward and backward one.
6th and lastly. If the wing did in reality strike downwards and backwards, a result the converse of that desired would most assuredly be produced, as an oblique surface which smites the air in a downward and backward direction (if left to itself) tends to depress the body bearing it. This is proved by the action upon the air of free inclined planes, arranged in the form of a screw.