Chapter VI: Part 6

[67] According to Dr. Crisp the swallow, martin, snipe, and many
birds of passage have no air in their bones (Proc. Zool. Soc., Lond.
part xxv. 1857, p. 13). The same author, in a second communication
(pp. 215 and 216), adds that the glossy starling, spotted flycatcher,
whin-chat, wood-wren, willow-wren, black-headed bunting, and canary,
five of which are birds of passage, have likewise no air in their
bones. The following is Dr. Crisp’s summary:--Out of ninety-two birds
examined he found “air in many of the bones, five (_Falconidæ_);
air in the humeri and not in the inferior extremities, thirty-nine;
no air in the extremities and probably none in the other bones,
forty-eight.”

[68] Nearly allied to this is the great gular pouch of the bustard.
Specimens of the air-sac in the orang, emu, and bustard, and likewise
of the air-sacs of the swan and goose, as prepared by me, may be seen
in the Museum of the Royal College of Surgeons of England.

The same may be said of the hollow bones,--some really admirable fliers, as the swifts, martins, and snipes, having their bones filled with marrow, while those of the wingless running birds alluded to have air. Furthermore and finally, a living bird weighing 10 lbs. weighs the same when dead, plus a very few grains; and all know what effect a few grains of heated air would have in raising a weight of 10 lbs. from the ground.

_How Balancing is effected in Flight, the Sound produced by the Wing, etc._--The manner in which insects, bats, and birds balance themselves in the air has hitherto, and with reason, been regarded a mystery, for it is difficult to understand how they maintain their equilibrium when the wings are beneath their bodies. Figs. 67 and 68, p. 141, throw considerable light on the subject in the case of the insect. In those figures the space (_a_, _g_) mapped out by the wing during its vibrations is entirely occupied by it; _i.e._ the wing (such is its speed) is in every portion of the space at nearly the same instant, the space representing what is practically a solid basis of support. As, moreover, the wing is jointed to the upper part of the body (thorax) by a universal joint, which admits of every variety of motion, the insect is always suspended (very much as a compass set upon gimbals is suspended); the wings, when on a level with the body, vibrating in such a manner as to occupy a circular area (_vide_ _r d b f_ of fig. 56, p. 120), in the centre of which the body (_a e c_) is placed. The wings, when vibrating above and beneath the body occupy a conical area; the apex of the cone being directed upwards when the wings are below the body, and downwards when they are above the body. Those points are well seen in the bird at figs. 82 and 83, p. 158. In fig. 82 the inverted cone formed by the wings when above the body is represented, and in fig. 83 that formed by the wings when below the body is given. In these figures it will be observed that the body, from the insertion of the roots of the wings into its upper portion, is always suspended, and this, of course, is equivalent to suspending the centre of gravity. In the bird and bat, where the stroke is delivered more vertically than in the insect, the _basis of support_ is increased by the tip of the wing folding inwards and backwards in a more or less horizontal direction at the end of the down stroke; and outwards and forwards at the end of the up stroke. This is accompanied by the rotation of the outer portion of the wing upon the wrist as a centre, the tip of the wing, because of the ever varying position of the wrist, describing an ellipse. In insects whose wings are broad and large (butterfly), and which are driven at a comparatively low speed, the balancing power is diminished. In insects whose wings, on the contrary, are long and narrow (blow-fly), and which are driven at a high speed, the balancing power is increased. It is the same with short and long winged birds, so that the function of balancing is in some measure due to the form of the wing, and the speed with which it is driven; the long wing and the wing vibrated with great energy increasing the capacity for balancing. When the body is light and the wings very ample (butterfly and heron), the reaction elicited by the ascent and descent of the wing displaces the body to a marked extent. When, on the other hand, the wings are small and the body large, the reaction produced by the vibration of the wing is scarcely perceptible. Apart, however, from the shape and dimensions of the wing, and the rapidity with which it is urged, it must never be overlooked that all wings (as has been pointed out) are attached to the bodies of the animals bearing them by some form of universal joint, and in such a manner that the bodies, whatever the position of the wings, are accurately balanced, and swim about in a more or less horizontal position, like a compass set upon gimbals. To such an extent is this true, that the position of the wing is a matter of indifference. Thus the pinion may be above, beneath, or on a level with the body; or it may be directed forwards, backwards, or at right angles to the body. In either case the body is balanced mechanically and without effort. To prove this point I made an artificial wing and body, and united the one to the other by a universal joint. I found, as I had anticipated, that in whatever position the wing was placed, whether above, beneath, or on a level with the body, or on either side of it, the body almost instantly attained a position of rest. The body was, in fact, equally suspended and balanced from all points.

]

[69] In this diagram I have purposely represented the right wing
by a straight _rigid_ rod. The natural wing, however, is curved,
_flexible_, and _elastic_. It likewise _moves in curves_, the curves
being most marked towards the end of the up and down strokes, as
shown at _m n_, _o p_. The curves, which are double figure-of-8
curves, are obliterated towards the middle of the strokes (_a r_).
This remark holds true of all natural wings, and of all artificial
wings properly constructed. The curves and the reversal thereof
are necessary to give continuity of motion to the wing during its
vibrations, and what is not less important, to enable the wing
alternately to seize and dismiss the air.

_Rapidity of Wing Movements partly accounted for._--Much surprise has been expressed at the enormous rapidity with which some wings are made to vibrate. The wing of the insect is, as a rule, very long and narrow. As a consequence, a comparatively slow and very limited movement at the root confers great range and immense speed at the tip; the speed of each portion of the wing increasing as the root of the wing is receded from. This is explained on a principle well understood in mechanics, viz. that when a rod hinged at one end is made to move in a circle, the tip or free end of the rod describes a much wider circle _in a given time_ than a portion of the rod nearer the hinge. This principle is illustrated at fig. 56. Thus if _a b_ of fig. 56 be made to represent the rod hinged at _x_, it travels through the space _d b f_ in the same time it travels through _j k l_; and through _j k l_ in the same time it travels through _g h i_; and through _g h i_ in the same time it travels through _e a c_, which is the area occupied by the thorax of the insect. If, however, the part of the rod _b_ travels through the space _d b f_ in the same time that the part _a_ travels through the space _e a c_, it follows of necessity that the portion of the rod marked _a_ moves very much slower than that marked _b_. The muscles of the insect are applied at the point _a_, as short levers (the point referred to corresponding to the thorax of the insect), so that a comparatively slow and limited movement at the root of the wing produces the marvellous speed observed at the tip; the tip and body of the wing being those portions which occasion the blur or impression produced on the eye by the rapidly oscillating pinion (figs. 64, 65, and 66, p. 139), But for this mode of augmenting the speed originally inaugurated by the muscular system, it is difficult to comprehend how the wings could be driven at the velocity attributed to them. The wing of the blow-fly is said to make 300 strokes per second, _i.e._ 18,000 per minute. Now it appears to me that muscles to contract at the rate of 18,000 times in the minute would be exhausted in a very few seconds, a state of matters which would render the continuous flight of insects impossible. (The heart contracts only between sixty and seventy times in a minute.) I am, therefore, disposed to believe that the number of contractions made by the thoracic muscles of insects has been greatly overstated; the high speed at which the wing is made to vibrate being due less to the separate and sudden contractions of the muscles at its root than to the fact that the speed of the different parts of the wing is increased in a direct ratio as the several parts are removed from the driving point, as already explained. Speed is certainly a matter of great importance in wing movements, as the elevating and propelling power of the pinion depends to a great extent upon the rapidity with which it is urged. Speed, however, may be produced in two ways--either by a series of separate and opposite movements, such as is witnessed in the action of a piston, or by a series of separate and opposite movements acting upon an instrument so designed, that a movement applied at one part increases in rapidity as the point of contact is receded from, as happens in the wing. In the piston movement the motion is uniform, or nearly so; all parts of the piston travelling at very much the same speed. In the wing movements, on the contrary, the motion is gradually accelerated towards the tip of the pinion, where the pinion is most effective as an elevator, and decreased towards the root, where it is least effective--an arrangement calculated to reduce the number of muscular contractions, while it contributes to the actual power of the wing. This hypothesis, it will be observed, guarantees to the wing a very high speed, with comparatively few reversals and comparatively few muscular contractions.

In the bat and bird the wings do not vibrate with the same rapidity as in the insect, and this is accounted for by the circumstance, that in them the muscles do not act exclusively at the root of the wing. In the bat and bird the muscles run along the wing towards the tip for the purpose of flexing or folding the wing prior to the up stroke, and for opening out and expanding it prior to the down stroke.

As the wing must be folded or flexed and opened out or expanded every time the wing rises and falls, and as the muscles producing flexion and extension are long muscles with long tendons, which act at long distances as long levers, and comparatively slowly, it follows that the great short muscles (pectorals, etc.) situated at the root of the wing must act slowly likewise, as the muscles of the thorax and wing of necessity act together to produce one pulsation or vibration of the wing. What the wing of the bat and bird loses in speed it gains in power, the muscles of the bat and bird’s wing acting directly upon the points to be moved, and under the most favourable conditions. In the insect, on the contrary, the muscles act indirectly, and consequently at a disadvantage. If the pectorals only moved, they would act as short levers, and confer on the wing of the bat and bird the rapidity peculiar to the wing of the insect.

The tones emitted by the bird’s wing would in this case be heightened. The swan in flying produces a loud whistling sound, and the pheasant, partridge, and grouse a sharp whirring noise like the stone of a knife-grinder.

It is a mistake to suppose, as many do, that the tone or note produced by the wing during its vibrations is a true indication of the number of beats made by it in any given time. This will be at once understood when I state, that a long wing will produce a higher note than a shorter one driven at the same speed and having the same superficial area, from the fact that the tip and body of the long wing will move through a greater space in a given time than the tip and body of the shorter wing. This is occasioned by all wings being jointed at their roots, the sweep made by the different parts of the wing in a given time being longer or shorter in proportion to the length of the pinion. It ought, moreover, not to be overlooked, that in insects the notes produced are not always referable to the action of the wings, these, in many cases, being traceable to movements induced in the legs and other parts of the body.

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.

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. _b_ Posterior wing. _e_ Anterior margin of wing. _f_ Ditto
posterior margin. _g_ Ditto outer margin. Compare with beetle, fig.
58.--_Original._]

FIG. 58.--Under-surface of large beetle (_Goliathus micans_), with
deeply concave 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 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.

FIG. 59.--The Red-legged Partridge (_Perdix rubra_) with wings fully
extended as in rapid flight, 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._]

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 partridge, fig. 59.--_Original._]

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 |
+------------------------------+-----------------------+

“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 much 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 less than the pigeon, and has forty times more surface; it weighs 3,000,000 times less than the crane of Australia, and possesses 149 times more of surface than this latter, the weight of which is about 9 kilogrammes 500 grammes (25 lbs. 5 oz. 9 dwt. troy, 20 lbs. 15 oz. 2-1/4 dr. avoirdupois).

“The Australian crane is the heaviest bird that I have weighed. It is that which has the smallest amount of surface, for, referred to the kilogramme, it does not give us a surface of more than 899 square centimetres (139 square inches), that is to say about an eleventh part of a square metre. But every one knows that these grallatorial animals are excellent birds of flight. Of all travelling birds they undertake the longest and most remote journeys. They are, in addition, the eagle excepted, the birds which elevate themselves the highest, and the flight of which is the longest maintained.”[73]

[73] M. de Lucy, _op. cit._

Strictly in accordance with the foregoing, are my own measurements of the gannet and heron. The following details of weight, measurement, etc., of the gannet were supplied by an adult specimen which I dissected during the winter of 1869. Entire weight, 7 lbs. (minus 3 ounces); length of body from tip of bill to tip of tail, three feet four inches; head and neck, one foot three inches; tail, twelve inches; trunk, thirteen inches; girth of trunk, eighteen inches; expanse of wing from tip to tip across body, six feet; widest portion of wing across primary feathers, six inches; across secondaries, seven inches; across tertiaries, eight inches. Each wing, when carefully measured and squared, gave an area of 19-1/2 square inches. The wings of the gannet, therefore, furnish a supporting area of three feet three inches square. As the bird weighs close upon 7 lbs., this gives something like thirteen square inches of wing for every 36-1/3 ounces of body, _i.e._ one foot one square inch of wing for every 2 lbs. 4-1/3 oz. of body.

The heron, a specimen of which I dissected at the same time, gave a very different result, as the subjoined particulars will show. Weight of body, 3 lbs. 3 ounces; length of body from tip of bill to tip of tail, three feet four inches; head and neck, two feet; tail, seven inches; trunk, nine inches; girth of body, twelve inches; expanse of wing from tip to tip across the body, five feet nine inches; widest portion of wing across primary and tertiary feathers, eleven inches; across secondary feathers, twelve inches.

Each wing, when carefully measured and squared, gave an area of twenty-six square inches. The wings of the heron, consequently, furnish a supporting area of four feet four inches square. As the bird only weighs 3 lbs. 3 ounces, this gives something like twenty-six square inches of wing for every 25-1/2 ounces of bird, or one foot 5-1/4 inches square for every 1 lb. 1 ounce of body.

In the gannet there is only one foot one square inch of wing for every 2 lbs. 4-1/3 ounces of body. The gannet has, consequently, less than half of the wing area of the heron. The gannet’s wings are, however, long narrow wings (those of the heron are broad), which extend transversely across the body; and these are found to be the most powerful--the wings of the albatross--which measure fourteen feet from tip to tip (and only one foot across), elevating 18 lbs. without difficulty. If the wings of the gannet, which have a superficial area of three feet three inches square, are capable of elevating 7 lbs., while the wings of the heron, which have a superficial area of four feet four inches, can only elevate 3 lbs., it is evident (seeing the wings of both are twisted levers, and formed upon a common type) that the gannet’s wings must be vibrated with greater energy than the heron’s wings; and this is actually the case. The heron’s wings, as I have ascertained from observation, make 60 down and 60 up strokes every minute; whereas the wings of the gannet, when the bird is flying in a straight line to or from its fishing-ground, make close upon 150 up and 150 down strokes during the same period. The wings of the divers, and other short-winged, heavy-bodied birds, are urged at a much higher speed, so that comparatively small wings can be made to elevate a comparatively heavy body, if the speed only be increased sufficiently.[74] Flight, therefore, as already indicated, is a question of power, speed, and small surfaces _versus_ weight. Elaborate measurements of wing, area, and minute calculations of speed, can consequently only determine the minimum of wing for elevating the maximum of weight--flight being attainable within a comparatively wide range.

[74] The grebes among birds, and the beetles among insects, furnish
examples where small wings, made to vibrate at high speeds, are
capable of elevating great weights.

_Wings, their Form, etc.; all Wings Screws, structurally and functionally._--Wings vary considerably as to their general contour; some being falcated or scythe-like, some oblong, some rounded or circular, some lanceolate, and some linear.[75]

[75] “The wing is short, broad, convex, and rounded in grouse,
partridges, and other rasores; long, broad, straight, and pointed
in most pigeons. In the peregrine falcon it is acuminate, the
second quill being longest, and the first little shorter; and in
the swallows this is still more the case, the first quill being the
longest, the rest rapidly diminishing in length.”--Macgillivray,
Hist. Brit. Birds, vol. i. p. 82. “The hawks have been classed as
noble or ignoble, according to the length and sharpness of their
wings; and the falcons, or long-winged hawks, are distinguished from
the short-winged ones by the second feather of the wing being either
the longest or equal in length to the third, and by the nature of the
stoop made in pursuit of their prey.”--Falconry in the British Isles,
by F. H. Salvin and W. Brodrick. Lond. 1855, p. 28.

FIG. 61.--Right wing of the Kestrel, drawn from the specimen, while
being held against the light. Shows how the primary (_b_), secondary
(_a_), and tertiary (_c_) feathers overlap and buttress or support
each other in every direction. Each set of feathers has its coverts
and subcoverts, the wing being conical from within outwards, and from
before backwards. _d_, _e_, _f_ Anterior or thick margin of wing.
_b_, _a_, _c_ Posterior or thin margin. The wing of the kestrel is
intermediate as regards form, it being neither rounded as in the
partridge (fig. 96, p. 176), nor ribbon-shaped as in the albatross
(fig. 62), nor pointed as in the swallow. The feathers of the
kestrel’s wing are unusually symmetrical and strong. Compare with
figs. 92, 94, and 96, pp. 174, 175, and 176.--_Original._]

All wings are constructed upon a common type. They are in every instance carefully graduated, the wing tapering from the root towards the tip, and from the anterior margin in the direction of the posterior margin. They are of a generally triangular form, and twisted upon themselves in the direction of their length, to form a helix or screw. They are convex above and concave below, and more or less flexible and elastic throughout, the elasticity being greatest at the tip and along the posterior margin. They are also moveable in all their parts. Figs. 61, 62, 63 (p. 138), 59 and 60 (p. 126), 96 and 97 (p. 176), represent typical bird wings; figs. 17 (p. 36), 94 and 95 (p. 175), typical bat wings; and figs. 57 and 58 (p. 125), 89 and 90 (p. 171), 91 (p. 172), 92 and 93 (p. 174), typical insect wings.

In all the wings which I have examined, whether in the insect, bat, or bird, the wing is recovered, flexed, or drawn towards the body by the action of elastic ligaments, these structures, by their mere contraction, causing the wing, when fully extended and presenting its maximum of surface, to resume its position of rest and plane of least resistance. The principal effort required in flight is, therefore, made during extension, and at the beginning of the down stroke. The elastic ligaments are variously formed, and the amount of contraction which they undergo is in all cases accurately adapted to the size and form of the wing, and the rapidity with which it is worked; the contraction being greatest in the short-winged and heavy-bodied insects and birds, and least in the light-bodied and ample-winged ones, particularly such as skim or glide. The mechanical action of the elastic ligaments, I need scarcely remark, insures an additional period of repose to the wing at each stroke; and this is a point of some importance, as showing that the lengthened and laborious flights of insects and birds are not without their stated intervals of rest.

FIG. 62.--Left wing of the albatross. _d_, _e_, _f_ Anterior or
thick margin of pinion. _b_, _a_, _c_ Posterior or thin margin,
composed of the primary (_b_), secondary (_a_), and tertiary (_c_)
feathers. In this wing the first primary is the longest, the primary
coverts and subcoverts being unusually long and strong. The secondary
coverts and subcoverts occupy the body of the wing (_e_, _d_), and
are so numerous as effectually to prevent any escape of air between
them during the return or up stroke. This wing, which I have in my
possession, measures over six feet in length.--_Original._]

All wings are furnished at their roots with some form of universal joint which enables them to move not only in an upward, downward, forward, or backward direction, but also at various intermediate degrees of obliquity. All wings obtain their leverage by presenting oblique surfaces to the air, the degree of obliquity gradually increasing in a direction from behind forwards and downwards during extension and the down stroke, and gradually decreasing in an opposite direction during flexion and the up stroke.

FIG. 63.--The Lapwing, or Green Plover (_Vanellus cristatus_, Meyer),
with one wing (_c b_, _d´ e´ f´_) fully extended, and forming a
long lever; the other (_d e f_, _c b_) being in a flexed condition
and forming a short lever. In the extended wing the anterior or
thick margin (_d´ e´ f´_) is directed _upwards_ and _forwards_
(_vide_ arrow), the posterior or thin margin (_c_, _b_) _downwards_
and _backwards_. The reverse of this happens during flexion, the
anterior or thick margin (_d_, _e_, _f_) being directed _downwards_
and _forwards_ (_vide_ arrow), the posterior or thin margin (_c b_)
bearing the rowing-feathers _upwards_ and _backwards_. The wings
therefore twist in opposite directions during extension and flexion;
and this is a point of the utmost importance in the action of all
wings, as it enables the volant animal to rotate the wings on and
off the air, and to present at one time (in extension) resisting,
kite-like surfaces, and at another (in flexion) knife-like and
comparatively non-resisting surfaces. It rarely happens in flight
that the wing (_d e f_, _c b_) is so fully flexed as in the figure.
As a consequence, the under surface of the wing is, as a rule,
inclined upwards and forwards, even in flexion, so that it acts
as a kite in extension and flexion, and during the up and down
strokes.--_Original._]

In the insect the oblique surfaces are due to the conformation of the shoulder-joint, this being furnished with a system of check-ligaments, and with horny prominences or stops, set, as nearly as may be, at right angles to each other. The check-ligaments and horny prominences are so arranged that when the wing is made to vibrate, it is also made to rotate in the direction of its length, in the manner explained.

In the bat and bird the oblique surfaces are produced by the spiral configuration of the articular surfaces of the bones of the wing, and by the rotation of the bones of the arm, forearm, and hand, upon their long axes. The reaction of the air also assists in the production of the oblique surfaces.

FIG. 64. FIG. 64 shows left wing (_a_, _b_) of wasp in the act of
twisting upon itself, the tip of the wing describing a figure-of-8
track (_a_, _c_, _b_). From nature.--_Original._]

FIGS. 65 and 66 show right wing of blue-bottle fly rotating
on its anterior margin, and twisting to form double or figure-of-8
curves (_a b_, _c d_). From nature.--_Original._]

That the wing twists upon itself structurally, not only in the insect, but also in the bat and bird, any one may readily satisfy himself by a careful examination; and that it twists upon itself during its action I have had the most convincing and repeated proofs (figs. 64, 65, and 66). The twisting in question is most marked in the posterior or thin margin of the wing, the anterior and thicker margin performing more the part of an axis. As a result of this arrangement, the anterior or thick margin cuts into the air quietly, and as it were by stealth, the posterior one producing on all occasions a violent commotion, especially perceptible if a flame be exposed behind the vibrating wing. Indeed, it is a matter for surprise that the spiral conformation of the pinion, and its spiral mode of action, should have eluded observation so long; and I shall be pardoned for dilating upon the subject when I state my conviction that it forms the fundamental and distinguishing feature in flight, and must be taken into account by all who seek to solve this most involved and interesting problem by artificial means. The importance of the twisted configuration or screw-like form of the wing cannot be over-estimated. That this shape is intimately associated with flight is apparent from the fact that the rowing feathers of the wing of the bird are every one of them distinctly spiral in their nature; in fact, one entire rowing feather is equivalent--morphologically and physiologically--to one entire insect wing. In the wing of the martin, where the bones of the pinion are short and in some respects rudimentary, the primary and secondary feathers are greatly developed, and banked up in such a manner that the wing as a whole presents the same curves as those displayed by the insect’s wing, or by the wing of the eagle where the bones, muscles, and feathers have attained a maximum development. The conformation of the wing is such that it presents a waved appearance in every direction--the waves running longitudinally, transversely, and obliquely. The greater portion of the pinion may consequently be removed without materially affecting either its form or its functions. This is proved by making sections in various directions, and by finding, as has been already shown, that in some instances as much as two-thirds of the wing may be lopped off without visibly impairing the power of flight. The spiral nature of the pinion is most readily recognised when the wing is seen from behind and from beneath, and when it is foreshortened. It is also well marked in some of the long-winged oceanic birds when viewed from before (figs. 82 and 83, p. 158), and cannot escape detection under any circumstances, if sought for,--the wing being essentially composed of a congeries of curves, remarkable alike for their apparent simplicity and the subtlety of their detail.

_The Wing during its action reverses its Planes, and describes a Figure-of-8 track in space._--The twisting or rotating of the wing on its long axis is particularly observable during extension and flexion in the bat and bird, and likewise in the insect, especially the beetle, cockroach, and such as fold their wings during repose. In these in extreme flexion the anterior or thick margin of the wing is directed downwards, and the posterior or thin one upwards. In the act of extension, the margins, in virtue of the wing rotating upon its long axis, reverse their positions, the anterior or thick margins describing a spiral course from below upwards, the posterior or thin margin describing a similar but opposite course from above downwards. These conditions, I need scarcely observe, are reversed during flexion. The movements of the margins during flexion and extension may be represented with a considerable degree of accuracy by a figure-of-8 laid horizontally.

FIGS. 67, 68, 69, and 70 show the area mapped out by the left wing of
the wasp when the insect is fixed and the wing made to vibrate. These
figures illustrate the various angles made by the wing as it hastens
to and fro, how the wing reverses and reciprocates, and how it twists
upon itself and describes a figure-of-8 track in space. Figs. 67 and
69 represent the forward or down stroke; figs. 68 and 70 the backward
or up stroke. The terms forward and back stroke are here employed
with reference to the head of the insect.--_Original._]