Page:Popular Science Monthly Volume 91.djvu/20

 Popular Science Monthly

��in the surrounding plumage, so that the grown bird shows only traces, the fact that the young of the species pass rapidly through the same evolution that is repre- sented in the succession of innumerable generations of their ancestry, almost clinches the conclusion that birds are descended from a type equipped with wings on all four limbs, as the Tetrapteryx, and that Nature has learned gradually to replace four small and imperfect wings, weakly mus- cled, by two larger and stronger wings under perfect control.

Frederic A. Lucas, Director of the American Museum of Natural History, called attention last year in the American Museum Journal to the great force of the evidence which has thus been collected to prove how Nature learned to ac- complish flight, the in- terest centering in birds, on account of their con- siderable weight, rather than in bats and insects

The ancestor-bird, faithfully re- produced from the record, and the ancestor-airplane are presented in illustration herewith, side by side. The dimensions of the bird have been relatively exaggerated to fa- cilitate the comparison, and the re- semblance in structure is striking. Langley's "aerodrome" repeatedly flew over the Potomac in 1895, sustaining its own weight in the air for more than one minute at a time by the action of its two pairs of planes or wings and two rotary propellers, of five to six feetdiameter, driven from a diminutive steam engine developing one to one and one-half horsepower. The necessity for placing the power equipment and the propellers amidships called for an elongated body for the machine as a whole, so that the weight might be evenly supported by planes at the rear as well as in front. This con- structive difficulty has been over- come in modern airplanes, but it was decisive for Langley's machine, with its small power, in the same degree and almost for the same reasons as for the

���As the wings in- creased in power the rear wing decreased in size

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��Gradually the tail shortened and the feath- ers lengthened

���The fan-shaped tail of the bird of to-day is a kind of a rudder

��original, four-winged bird. The latter came from a race whose fore and hind limbs were spaced well apart, whose legs were relatively heavy and whose arm mus- cles were weak. Its struc- ture had to be modified by hereditary influences before it would balance at all in the air, hung from the arm sockets alone, as birds do. Mean- while Nature did practically as Langley did. She adopted the compromise solution of upholding the rear weight by large feathery extensions from the legs and tail, and it may be noticed that the fantastic feather tail of the Tet- rapteryx was built up around a tail-like appendage and was not all feathers under muscular con- trol like that of the modern bird. Nature evidently found it impos- sible to change the bony structure in less than millions of S||y years, working from the basis of a reptile with only growth and heredity as the tools at command, but she could make feathers grow in the place of horny scales, which are made of almost the same material, by a comparatively brief evolution.

To transform Langley's ma- chine into the modern airplane was a task much simplified by the advent of the compact and powerful gasoline engine, small enough to be moved forward in line with the support from planes arranged on the biplane or mono- plane principle and ' strong enough to pull the machine safely, in most cases, through dis- turbing eddies of the atmosphere.

This decided change in the machine took the place of all the transformations in bone dimen- sions, balance and muscle strength by which the Tetrap- teryx became a bird after start- ing out in the world with an ana- tomical construction very much like an animated parachute or gliding machine. But the mechanical flyer is still an infant compared with Nature's eon-old product.

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