law of movement of vertebrated animals, and, from a most extended series of observations, Dr. Pettigrew concludes:
"That quadrupeds walk, and fishes swim, and insects, bats, and birds fly, by figure-of-8 movements.
"That the flipper of the sea-bear, the swimming wing of the penguin, and the wing of the insect, bat, and bird, are screws structurally, and resemble the blade of an ordinary screw-propeller.
"That those organs are screws functionally, from their twisting and untwisting, and from their rotating in the direction of their length, when they are made to oscillate.
"That they have a reciprocating action, and reverse their planes more or less completely at every stroke.
"That the wing describes a figure-of-8 track in space when the flying animal is artificially fixed.
"That the wing, when the flying animal is progressing at a high rate of speed in an horizontal direction, describes a looped and then a wave track, from the fact that the figure-of-8 is gradually opened out and unraveled as the animal advances."
He constructed artificial fish-tails, fins, flippers, and wings—flexible and elastic—slightly twisted upon themselves, and applied them respectively to the water and air by a sculling or figure-of-8 motion. The curved surfaces and movements peculiar to the living organs were reproduced. The purely mechanical movement shown in this application of traveling structures to their environment scarcely admits of doubt.
Man is enabled to travel in two of the three great highways of Nature. He can progress upon the land, swim in the water, but fly he cannot; nor has he yet invented a means by which flying is possible. By his applications of natural laws he has "outraced the quadruped on the land and the fish in the sea," and the conclusion from the analogy and nature of things is, that the "tramways of the air will yet be traversed by man's ingenuity."
A balloon floats, it does not fly. It floats because it is lighter than the air; a bird is enabled to fly because it is heavier than the air, and weight is an important element in all, but especially in aërial and land locomotion. It is that by which the extremities of animals seize and hold their position in the media in which they move. If a man were no heavier than the air, the movement of his limbs would avail him nothing. The earth is his fulcrum, as the air is that of the bird, and water that of the fish. Progression, therefore, implies gravity and the power of resistance, which gravity affords. A body which floats is carried along with the media in which it is; having lost its weight, it has lost its power of self-control. A man who cannot swim is at the mercy of the slightest current or wave, if in depth at which the lifting power of the water makes his foothold insecure.
A man standing still commences to progress by throwing his body