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Page:Popular Science Monthly Volume 4.djvu/555

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fishes are astonishingly great. A blow from the head of a sperm-whale may endanger a strong ship, and the sword of a sword-fish has been driven through the oak-planks of a vessel more than twelve inches in thickness.

Fig. 10.
PSM V04 D555 The porpoise.jpg
The Porpoise.—Here the tail is principally engaged in swimming, the anterior extremities being rudimentary, and resembling the pectoral fins of fishes.

When the flying-fish rises in the air, it is by the momentum it attains in the water by the lashing of its fins and tail. Fairly in the air, its wings give it support, and, in the opinion of Dr. Pettigrew, act as true pinions within certain limits, but are too small to sustain the creature indefinitely.

The transition from swimming to progression in the air is natural and easy. The method by which the flying-fish rises from the water is similar to that of the albatross, that prince of flying-birds, and, indeed, to perhaps all other birds, when in the act of taking flight upon the water. Momentum is obtained by rushing forward with both feet and wings. The albatross frequently goes in this way many rods before it is fairly launched upon the air. Then, with powerful strokes, it rises above the waves. Its expenditure of force is chiefly in rising, when, without further effort, except to screw and unscrew its pinions upon the wind, it floats facing the gale. For more than an hour it will sail with wings apparently motionless, and it seems most at rest when the winds are highest. In this case it is sustained by the momentum it attained, and the wonderful kite-like position and adjustment of its wings. But, it manifestly could not maintain its position in this way, if moving before the wind, or in a perfectly calm atmosphere. The wings must then be called into play to afford lifting as well as propelling power. The momentum must be supplied.

Birds rise from the ground most readily facing the wind, but usually run or leap, and the wings, by vigorous strokes, continue the impulse secured. With the first down-stroke of the wings the body is lifted, and some velocity attained; when the wings rise, the body falls somewhat, but is at the same time advancing. This rise and fall of the body in flying continue, and the body, in progressing, undulates above and below a given line. In the flight of birds with large wings and slow stroke, it is easily observed. The illustration, Fig. 11, shows