“When a strong current of air strikes against the wings of a bird, the same sustaining effect is produced as when the wing strikes against the air. Consequently birds with very long wings have this great advantage, that, with pre-acquired momentum, they can often for a long time fly without flapping their wings at all. Under these circumstances a bird is sustained very much as a boy’s kite is sustained in the air. The string which the boy holds, and by which he pulls the kite downwards with a certain force, performs for the kite the same offices which its own weight and balance and momentum perform for the bird. The great long-winged oceanic birds often appear to float rather than to fly. The stronger is the gale, their flight, though less rapid, is all the more easy, so easy indeed as to appear buoyant; because the blasts which strike against their wings are enough to sustain the bird with comparatively little exertion of its own, except that of holding the wing vanes stretched and exposed at proper angles to the wind. And whenever the onward force previously acquired by flapping becomes at length exhausted, and the ceaseless, inexorable force of gravity is beginning to overcome it, the bird again rises by a few easy and gentle half-strokes of the wing. Very often the same effect is produced by allowing the force of gravity to act, and when the downward momentum has brought the bird close to the ground or to the sea, that force is again converted into an ascending impetus by a change in the angle at which the wing is exposed to the wind.” (p. 152.)
It is to be regretted that the limits of this article prevent more extended quotations from this remarkable book.
Now let us recall what we have seen at sea.
When one stands on the after-deck of a steamer in crossing the ocean, he may watch the soaring gulls to his heart’s content. When the ship struggles painfully to force her way into the teeth of a gale, the birds make sport for themselves—they rise and dip, thus conquering the wind. How? Simply by tacking; in one sense, just as a yacht tacks to windward. Neither bird nor yacht can sail into the eye of the wind by the wind’s power, but either can, by use of that power, reach an objective point lying to windward.
But here the reader may say that the parallelism between the bird and the sailing craft is not correctly drawn, because the yacht has a keel immersed in a dense medium which resists and prevents the making of leeway.
Yet the soaring bird has something which, at necessary times, holds it against the wind just as effectually as the keel holds the yacht: that something is momentum, which, while it lasts, holds the bird against the wind as firmly as the kite-string holds the boy’s kite.
In [Fig. 1], let S represent a steamship going eastward at the rate of twenty miles per hour; W the wind blowing westward at the rate of twenty miles per hour; A a gull near the water’s surface, with momentum which for the instant gives him an eastward velocity of twenty miles per hour. While the bird’s momentum lasts it holds him firmly against the wind. At the point A the bird inclines his wings so that the wind strikes them on the under side, and he is lifted and lifted until, at the point B, his momentum is so reduced that he must tack; then he gives to the wind the thin edge of his wings and slides down to the point C, and then, with velocity regained, he repeats the manœuvre. Altitude sacrificed becomes velocity or momentum, and momentum sacrificed becomes altitude. In this description of the gull’s soaring to windward, the movement is reduced to its simplest elements, and it leaves out of account the graceful sinuosity of the bird’s airy travels, just as the teacher of dancing leaves grace out of account when she teaches the beginner the elements of the steps.
