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Page:Popular Science Monthly Volume 4.djvu/551

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ments; and, in human locomotion, the same phenomenon is observed. The diagram, Fig. 5, shows the curved track made by man walking. The accuracy of this is easily verified by observation. As the limbs swing forward, they move in the arc of an ellipse; that is, in a slight curve outward, and with the arms form the double curves, as shown in the figure. In the movement of the horse, walking or trotting, the same phenomenon appears, as the figure shows.

Fig. 6.
PSM V04 D551 Horse in the act of trotting.jpg

Horse in the Act of trotting.—In this, as in all the other paces, the body of the horse is levered forward by a diagonal twisting of the trunk and extremities, the extremities describing a figure-of-8 track (s, u, r, t).

The wings of birds, bats, and insects, describe similar curves. They are produced by the rotation of the wing, as it rises and falls, so that it twists, screw-like, on its long axis, one-half of the figure being formed in the ascent, the other in the descent of the wing.

Fig. 7.
PSM V04 D551 Imitation of wing movement.jpg
Imitation of Wing Movement by a Reed with Flexible Sail moving on a Ball-and-Socket Joint, showing the Double Curves.

The double curves or figure-of-8 lines which thus occur are not mere coincidences, nor in any sense accidental, but the expression of a