Part 13

It is a curious circumstance, that if portions be removed from the posterior margins of the wings of a buzzing insect, such as the wasp, bee, blue-bottle fly, etc., the note produced by the vibration of the pinions is raised in pitch. This is explained by the fact, that an insect whose wings are curtailed requires to drive them at a much higher speed in order to sustain itself in the air. That the velocity at which the wing is urged is instrumental in causing the sound, is proved by the fact, that in slow-flying insects and birds no note is produced; whereas in those which urge the wing at a high speed, a note is elicited which corresponds within certain limits to the number of vibrations and the form of the wing. It is the posterior or thin flexible margin of the wing which is more especially engaged in producing the sound; and if this be removed, or if this portion of the wing, as is the case in the bat and owl, be constructed of very soft materials, the character of the note is altered. An artificial wing, if properly constructed and impelled at a sufficiently high speed, emits a drumming noise which closely resembles the note produced by the vibration of short-winged, heavy-bodied birds, all which goes to prove that sound is a concomitant of rapidly vibrating wings.

_The Wing area Variable and in Excess._--The travelling-surfaces 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 comparatively wide range. Thus there are light-bodied and large-winged 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 muscular development 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,[70] an unvarying relation between the weight of 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, 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 than the air. It must tread and rise upon the air as a swimmer 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 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.

[70] 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.

That no fixed relation exists between the area of the wings and the size and weight of the body, is evident on comparing the dimensions of the wings and bodies of the several orders of insects, bats, and birds. If such comparison be made, it will be found that the pinions in some instances diminish while the bodies increase, and the converse. No practical good can therefore accrue to aërostation from elaborate measurements of the wings and trunks of any flying thing; neither can any rule be laid down as to the extent of surface required for sustaining a given weight in the air. The wing area is, as a rule, considerably in excess of what is actually required for the purposes of flight. This is proved in two ways. First, by the fact that bats can carry their young without inconvenience, and birds elevate surprising quantities of fish, game, carrion, etc. I had in my possession at one time a tame barn-door owl which could lift a piece of meat a quarter of its own weight, after fasting four-and-twenty hours; and an eagle, as is well known, can carry a moderate-sized lamb with facility.

The excess of wing area is proved, secondly, by the fact that a large proportion of the wings of most volant animals may be removed without destroying the power of flight. I instituted a series of experiments on the wings of the fly, dragon-fly, butterfly, sparrow, etc., with a view to determining this point in 1867. The following are the results obtained:--

_Blue-bottle Fly._--_Experiment 1._ Detached posterior or thin half of each wing in its long axis. Flight perfect.

_Exp. 2._ Detached posterior _two-thirds_ of either wing in its long axis. Flight still perfect. I confess I was not prepared for this result.

_Exp. 3._ Detached one-third of anterior or thick margin of either pinion obliquely. Flight imperfect.

_Exp. 4._ Detached one-half of anterior or thick margin of either pinion obliquely. The power of flight completely destroyed. From experiments 3 and 4 it would seem that the anterior margin of the wing, which contains the principal nervures, and which is the most rigid portion of the pinion, cannot be mutilated with impunity.

_Exp. 5._ Removed one-third from the extremity of either wing transversely, _i.e._ in the direction of the short axis of the pinion. Flight perfect.

_Exp. 6._ Removed _one-half_ from either wing transversely, as in experiment 5. Flight very slightly (if at all) impaired.

_Exp. 7._ Divided either pinion in the direction of its long axis into three equal parts, the anterior nervures being contained in the anterior portion. Flight perfect.

_Exp. 8._ Notched two-thirds of either pinion obliquely from behind. Flight perfect.

_Exp. 9._ Notched anterior third of either pinion transversely. The power of flight destroyed. Here, as in experiment 4, the mutilation of the anterior margin was followed by loss of function.

_Exp. 10._ Detached posterior two-thirds of right wing in its long axis, the left wing being untouched. Flight perfect. I expected that this experiment would result in loss of balancing-power; but this was not the case.

_Exp. 11._ Detached half of right wing transversely, the left one being normal. The insect flew irregularly, and came to the ground about a yard from where I stood. I seized it and detached the corresponding half of the left wing, after which it flew away, as in experiment 6.

_Dragon-Fly._--_Exp. 12._ In the dragon-fly either the first or second pair of wings may be removed without destroying the power of flight. The insect generally flies most steadily when the posterior pair of wings are detached, as it can balance better; but in either case flight is perfect, and in no degree laboured.

_Exp. 13._ Removed one-third from the posterior margin of the first and second pairs of wings. Flight in no wise impaired.

If more than a third of each wing is cut away from the posterior or thin margin, the insect can still fly, but with effort.

Experiment 13 shows that the posterior or thin flexible margins of the wings may be dispensed with in flight. They are more especially engaged in propelling. Compare with experiments 1 and 2.

_Exp. 14._ The extremities or tips of the first and second pair of wings may be detached to the extent of one-third, without diminishing the power of flight. Compare with experiments 5 and 6.

If the mutilation be carried further, flight is laboured, and in some cases destroyed.

_Exp. 15._ When the front edges of the first and second pairs of wings are notched or when they are removed, flight is completely destroyed. Compare with experiments 3, 4, and 9.

This shows that a certain degree of stiffness is required for the front edges of the wings, the front edges indirectly supporting the back edges. It is, moreover, on the front edges of the wings that the pressure falls in flight, and by these edges the major portions of the wings are attached to the body. The principal movements of the wings are communicated to these edges.

_Butterfly._--_Exp. 16._ Removed posterior halves of the first pair of wings of white butterfly. Flight perfect.

_Exp. 17._ Removed posterior halves of first and second pairs of wings. Flight not strong but still perfect. If additional portions of the posterior wings were removed, the insect could still fly, but with great effort, and came to the ground at no great distance.

_Exp. 18._ When the tips (outer sixth) of the first and second pairs of wings were cut away, flight was in no wise impaired. When more was detached the insect could not fly.

_Exp. 19._ Removed the posterior wings of the brown butterfly. Flight unimpaired.

_Exp. 20._ Removed in addition a small portion (one-sixth) from the tips of the anterior wings. Flight still perfect, as the insect flew upwards of ten yards.

_Exp. 21._ Removed in addition a portion (one-eighth) of the posterior margins of anterior wings. The insect flew imperfectly, and came to the ground about a yard from the point where it commenced its flight.

_House Sparrow._--The sparrow is a heavy small-winged bird, requiring, one would imagine, all its wing area. This, however, is not the case, as the annexed experiments show.

_Exp. 22._ Detached the half of the secondary feathers of either pinion in the direction of the long axis of the wing, the primaries being left intact. Flight as perfect as before the mutilation took place. In this experiment, one wing was operated upon before the other, in order to test the balancing-power. The bird flew perfectly, either with one or with both wings cut.

_Exp. 23._ Detached the half of the secondary feathers and a fourth of the primary ones of either pinion in the long axis of the wing. Flight in no wise impaired. The bird, in this instance, flew upwards of 30 yards, and, having risen a considerable height, dropped into a neighbouring tree.

_Exp. 24._ Detached nearly the half of the primary feathers in the long axis of either pinion, the secondaries being left intact. When one wing only was operated upon, flight was perfect; when both were tampered with, it was still perfect, but slightly laboured.

_Exp. 25._ Detached rather more than a third of both primary and secondary feathers of either pinion in the long axis of the wing. In this case the bird flew with evident exertion, but was able, notwithstanding, to attain a very considerable altitude.

From experiments 1, 2, 7, 8, 10, 13, 16, 22, 23, 24, and 25, it would appear that great liberties may be taken with the posterior or thin margin of the wing, and the dimensions of the wing in this direction materially reduced, without destroying, or even vitiating in a marked degree, the powers of flight. This is no doubt owing to the fact indicated by Sir George Cayley, and fully explained by Mr. Wenham, that in all wings, particularly long narrow ones, the elevating power is transferred to the anterior or front margin. These experiments prove that the upward bending of the posterior margins of the wings during the down stroke is not necessary to flight.

_Exp. 26._ Removed alternate primary and secondary feathers from either wing, beginning with the first primary. The bird flew upwards of fifty yards with very slight effort, rose above an adjoining fence, and wheeled over it a second time to settle on a tree in the vicinity. When one wing only was operated upon, it flew irregularly and in a lopsided manner.

_Exp. 27._ Removed alternate primary and secondary feathers from either wing, beginning with the _second primary_. Flight, from all I could determine, perfect. When one wing only was cut, flight was irregular or lopsided, as in experiment 26.

From experiments 26 and 27, as well as experiments 7 and 8, it would seem that the wing does not of necessity require to present an unbroken or continuous surface to the air, such as is witnessed in the pinion of the bat, and that the feathers, when present, may be separated from each other without destroying the utility of the pinion. In the raven and many other birds the extremities of the first four or five primaries divaricate in a marked manner. A similar condition is met with in the _Alucita hexadactyla_, where the delicate feathery-looking processes composing the wing are widely removed from each other. The wing, however, _ceteris paribus_, is strongest when the feathers are not separated from each other, and when they _overlap_, as then they are arranged so as mutually to support each other.

_Exp. 28._ Removed half of the primary feathers from either wing transversely, _i.e._ in the direction of the short axis of the wing. Flight very slightly, if at all, impaired when only one wing was operated upon. When both were cut, the bird flew heavily, and came to the ground at no very great distance. This mutilation was not followed by the same result in experiments 6 and 11. On the whole, I am inclined to believe that the area of the wing can be curtailed with least injury in the direction of its long axis, by removing successive portions from its posterior margin.

_Exp. 29._ The carpal or wrist-joint of either pinion rendered immobile by lashing the wings to slender reeds, the elbow-joints being left free. The bird, on leaving the hand, fluttered its wings vigorously, but after a brief flight came heavily to the ground, thus showing that a certain degree of twisting and folding, or flexing of the wings, is necessary to the flight of the bird, and that, however the superficies and shape of the pinions may be altered, the movements thereof must not be interfered with. I tied up the wings of a pigeon in the same manner, with a precisely similar result.

The birds operated upon were, I may observe, caught in a net, and the experiments made within a few minutes from the time of capture.

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,[71] 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.

[71] “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.

_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 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:[72]--

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

+------------------------------------------------------+ | INSECTS. | +------------------------------+-----------------------+ | | Referred to the | | | kilogramme | | NAMES. |= 2lbs. 8oz. 3dwt. 2gr.| | | Avoird. | | | = 2lbs. 3oz. 4·428dr. | +------------------------------+-----------------------+ | | sq. ft. in. | | | yds. | |Gnat, | 11 8 92 | |Dragon-fly (small), | 7 2 56 | |Coccinella (Lady-bird), | 5 13 87 | |Dragon-fly (common), | 5 2 89 | |Tipula, or Daddy-long-legs, | 3 5 11 | |Bee, | 1 2 74-1/2 | |Meat-fly, | 1 3 54-1/2 | |Drone (blue), | 1 2 20 | |Cockchafer, | 1 2 50 | |Lucanus} Stag beetle (female),| 1 1 39-1/2 | | cervus} Stag-beetle (male), | 0 8 33 | |Rhinoceros-beetle, | 0 6 122-1/2 | +------------------------------+-----------------------+ +------------------------------------------------------+ | BIRDS. | +------------------------------+-----------------------+ | | Referred | | NAMES. | to the | | | kilogramme. | +------------------------------+-----------------------+ | | sq. | | | yds. ft. in. | | Swallow, | 1 1 104-1/2 | | Sparrow, | 0 5 142-1/2 | | Turtle-dove, | 0 4 100-1/2 | | Pigeon, | 0 2 113 | | Stork, | 0 2 20 | | Vulture, | 0 1 116 | | Crane of Australia, | 0 0 139 | +------------------------------+-----------------------+