Imagens das páginas

Some of my readers will probably infer from the foregoing, that the figure-of-8 curves formed along the anterior and posterior margins of the pinions are not necessary to flight, since the tips and posterior margins of the wings may be removed without destroying it. To such I reply, that the wings are flexible, elastic, and composed of a congeries of curved surfaces, and that so long as a portion of them remains, they form, or tend to form, figure-of-8 curves in every direction.

Captain F. W. Hutton, in a recent paper “ On the Flight of Birds” (Ibis, April 1872), refers to some of the experiments detailed above, and endeavours to frame a theory of flight, which differs in some respects from my own. His remarks are singularly inappropriate, and illustrate in a forcible manner the old adage, “ A little knowledge is a dangerous thing.” If Captain Hutton had taken the trouble to look into my memoir “ On the Physiology of Wings,” communicated to the Royal Society of Edinburgh, on the 2d of August 1870,1 fifteen months before his own paper was written, there is reason to believe he would have arrived at very different conclusions. Assuredly he would not have ventured to make the rash statements he has made, the more especially as he attempts to controvert my views, which are based upon anatomical research and experiment, without making any dissections or experiments of his own.

The Wing area decreases as the Size and Weight of the Volant Animal increases.—While, as explained in the last section, no definite relation exists between the weight of a flying animal and the size of its flying surfaces, there being, as stated, heavy bodied and small-winged insects, bats, and birds, and the converse; and while, as I have shown by experiment, flight is possible within a wide range, the wings being, as a rule, in excess of what are required for the purposes of flight; still it appears, from the researches of M. de Lucy, that there is a general law, to the effect that the larger the volant animal the smaller by comparison are its flying surfaces. The existence of such a law is very encouraging as far as artificial

16 On the Physiology of Wings, being an Analysis of the Movements by which Flight is produced in the Insect, Bat, and Bird.”—Trans. Roy. Soc. of Edinburgh, vol. xxvi.

flight is concerned, for it shows that the flying surfaces of a large, heavy, powerful flying machine will be comparatively small, and consequently comparatively compact and strong. This is a point of very considerable importance, as the object desiderated in a flying machine is elevating capacity.

M. de Lucy has tabulated his results, which I subjoin :-—

[merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][ocr errors][merged small]

“It is easy, by aid of this table, to follow the order, always decreasing, of the surfaces, in proportion as the winged animal increases in size and weight. Thus, in comparing the insects with one another, we find that the gnat, which weighs 460 times less than the stag-beetle, has fourteen times more of surface. The lady-bird weighs 150 times less than the stag-beetle, and possesses five times more of surface. It is the same with the birds. The sparrow weighs about ten times less than the pigeon, and has twice as inuch surface. The pigeon weighs about eight times less than the stork, and has twice as much surface. The sparrow weighs 339 times less than the Australian crane, and possesses seven times more surface. If now we compare the insects and the birds, the gradation will become even much more striking. The gnat, for example, weighs 97,000 times

1 “On the Flight of Birds, of Bats, and of Insects, in reference to the subject of Aërial Locomotion,” by M. de Lucy, Paris.

birds, all which goes to prove that sound is a concomitant of rapidly vibrating wings.

The Wing area Variable and in Excess.—The travellingsurfaces of insects, bats, and birds greatly exceed those of fishes and swimming animals; the travelling-surfaces of swimming animals being greatly in excess of those of animals which walk and run. The wing area of insects, bats, and birds varies very considerably, flight being possible within a com


Fig.57.-Shows a butterfly with comparatively very large wings. The nervures

are seen to great advantage in this specimen ; and the enormous expanse of the pinions readily explains the irregular flight of the insect on the principle of recoil. a Anterior wing. Posterior wing. e Anterior margin of wing. f Ditto posterior margin. g Ditto outer margin. Compare with beetle, fig. 58.-Original.

paratively wide range. Thus there are light-bodied and largewinged insects and birds—as the butterfly (fig. 57) and heron (fig. 60, p. 126); and others whose bodies are comparatively heavy, while their wings are insignificantly small—as the sphinx moth and Goliath beetle (fig. 58) among insects, and the grebe, quail, and partridge (fig. 59, p. 126) among birds.

The apparent inconsistencies in the dimensions of the body and wings are readily explained by the greater musculardevelopment of the heavy-bodied short-winged insects and birds, and the increased power and rapidity with which the wings in them are made to oscillate. In large-winged animals the movements

are slow; in small-winged ones comparatively very rapid. This shows that flight may be attained by a heavy, powerful animal with comparatively small wings, as well as by a lighter one with enormously enlarged wings. While there is apparently no fixed relation between the area of the wings and the animal to be raised, there is, unless in the case of sailing birds," an unvarying relation between the weight of

FIG. 58. -Under-surface of large beetle (Goliathus micans), with deeply con

cave and comparatively small wings (compare with butterfly, fig. 57), shows that the nervures (r, d, e, f, n, n, n) of the wings of the beetle are arranged along the anterior margins and throughout the substance of the wings generally, very much as the bones of the arm, forearm, and hand, are in the wings of the bat, to which they bear a very marked resemblance, both in their shape and mode of action. The wings are folded upon themselves at the point e during repose. Compare letters of this figure with similar letters

of fig. 17, p. 36.-Original. the animal, the area of its wings, and the number of oscillations made by them in a given time. The problem of flight thus resolves itself into one of weight, power, velocity, and small surfaces; versus buoyancy, debility, diminished speed,

1 In birds which skim, sail, or glide, the pinion is greatly elongated or ribbon-shaped, and the weight of the body is made to operate upon the inclined planes formed by the wings, in such a manner that the bird when it has once got fairly under weigh, is in a measure self-supporting. This is especially the case when it is proceeding against a slight breeze-the wind and the inclined planes resulting from the upward inclination of the wings reacting upon each other, with this very remarkable result, that the mass of the bird moves steadily forwards in a more or less horizontal direction.

and extensive surfaces,—weight in either case being a sine quâ non. In order to utilize the air as a means of transit, the body in motion, whether it moves in virtue of the life it possesses, or because of a force superadded, must be heavier


FIG. 59.- The Red-legged Partridge (Perdix rubra) with wings fully extended

as in rapid fight, shows deeply concave form of the wings, how the primary and secondary feathers overlap and support each other during extension, and how the anterior or thick margins of the wings are directed upwards and forwards, and the posterior or thin ones downwards and backwards. The wings in the partridge are wielded with immense velocity and power. This is necessary because of their small size as compared with the great dimensions and weight of the body.

If a horizontal line be drawn across the feet (a, e) to represent the horizon, and another from the tip of the tail (a) to the root of the wing (d), the angle at which the wing strikes the air is given. The body and wings when taken together form a kite. The wings in the partridge are rounded and broad.

Compare with heron, fig. 60.-Original. than the air. It must tread and rise upon the air as a swim

mer upon the water, or as a kite upon the wind. It must act against gravity, and elevate and carry itself forward at the expense of the air, and by virtue of the force which

Da C

FIG. 60.-The Grey Heron (Ardea cinerea) in full flight. In the heron the wings are deeply concave, and unusually large as compared with the size of the bird. The result is that the wings are moved very leisurely, with a slow, heavy, and almost solemn beat. The heron figured weighed under 3 lbs.; and the expanse of wing was considerably greater than that of a wild goose which weighed over 9. lbs. Flight is consequently more a question of power and weight than of buoyancy and surface. d, e, f Anterior thick strong margin of right wing. c, a, b Posterior thin flexible margin, composed of primary (b), secondary (a), and tertiary (c) feathers. Compare with part

ridge, fig. 59. —Original. resides in it. If it were rescued from the law of gravity on the one hand, and bereft of independent movement on the other, it would float about uncontrolled and uncontrollable, as happens in the ordinary gas-balloon.

« AnteriorContinuar »