534 THE POPULAR SCIENCE MONTHLY. 



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 pen- 

 guin, 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 jigure-of-8 track in space when the fly- 

 ing 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 re- 

 spectively 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 ad- 

 mits 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 possi- 

 ble. By his applications of natural laws he has " outraced the quad- 

 ruped 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. aerial and land loco- 

 motion. 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 noth- 

 ing. 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 



