THE necessity of Contrivance for the accomplishment of Purpose arises out of the immutability of Natural Forces. They must be conformed to, and obeyed. Therefore, where they do not serve our purpose directly, they can only be made to serve it by ingenuity and contrivance. This necessity, then, may be said to be the index and the measure of the power of Law. And so, on the other hand, the certainty with which Purpose can be accomplished by Contrivance, is the index and the measure of mental knowledge and resource. It is by wisdom and knowledge that the Forces of Nature — even those which may seem most adverse — are yoked to service. This idea of the relation in which Law stands to Will, and [128/129] in which Will stands to Law, is familiar to us in the works of Man: but it is less familiar to us as equally holding good in the works of Nature. We feel, sometimes, as if it were an unworthy notion of the Will which works in Nature, to suppose that it should never act except through the use of means. But our notions of unworthiness are themselves often the unworthiest of all. They must be ruled and disciplined by observation of that which is, — not founded on a priori conceptions of what ought to be. Nothing is more certain than that the whole Order of Nature is one vast system of Contrivance. And what is Contrivance but that kind of arrangement by which the unchangeable demands of Law are met and satisfied? It may be that all natural Forces are resolvable into some One Force, and indeed in the modern doctrine of the Correlation of Forces, an idea which is a near approach to this, has already entered the domain of Science. It may also be that this One Force, into which all others return again, is itself but a mode of action of the Divine Will. But we have no instruments whereby to reach this last analysis. Whatever the [129/130] ultimate relation may be between mental and material Force, we can at least see clearly this,that in Nature there is the most elaborate machinery to accomplish Purpose through the instrumentality of means. It seems as if all that is done in Nature as well as all that is done in art, were done by knowing how to do it. It is curious how the language of the great Seers of the Old Testament corresponds with this idea. They uniformly ascribe all the operations of Nature — the greatest and the smallest — to the working of Divine Power. But they never revolt — as so many do in these weaker days — from the idea of this Power working by wisdom and knowledge in the use of means: nor, in this point of view, do they ever separate between the work of first Creation, and the work which is going on daily in the existing world. Exactly the same language is applied to the rarest exertions of power, and to the gentlest and most constant of all natural operations. Thus the saying that"The Lord by wisdom hath founded the Earth: by understanding hath He established the Heavens," — is coupled in the same breath with this other saying, "By His knowledge [130/131] the depths are broken up, and the clouds drop down the dew."1
Every instance of Contrivance which we can thoroughly follow and understand, has an intense interest — as casting light upon this method of the Divine government, and upon the analogy between the operations of our own minds and the operations of the Creator. Some instances will strike us more than others — and those will strike us most which stand in some near comparison with our own human efforts of ingenuity and contrivance. There is one such instance which I propose to consider in this chapter — the machinery by which a great purpose has been accomplished in Nature — a purpose which Man has never been able to accomplish in art, and that is the Navigation of the Air. No more beautiful example can be found, even in the wide and rich domain of Animal Mechanics — none in which we can trace more clearly, too, the mode and method in which laws the most rigorous and exact, are used as the supple instruments of Purpose. [131/132]"The way of an Eagle in the air" was one of the things of which Solomon said, that "he knew it not." No wonder that the Wise King reckoned it among the great mysteries of Nature! The Force of Gravitation, though its exact measure was not ascertained till the days of Newton, has been the most familiar of all Forces in all ages of Mankind. How, then, in violation of its known effects, could heavy bodies be supported upon the thin air — and be gifted with the power of sustaining and directing movements more easy, more rapid, and more certain than the movements of other animals upon the firm and solid earth? No animal motion in Nature is so striking or so beautiful as the —
"Scythe — like sweep of wings, that dare
The headlong plunge through eddying gulfs of air."2
Nor will the wonder cease when, so far as the mechanical problem is concerned, the mystery of flight is solved. If we wish to see how material laws can be bent to purpose, we shall study this problem.
In the first place it is remarkable that the Force which seems so adverse — the Force of Gravitation [132/133] drawing down all bodies to the earth, is the very Force which is the principal one concerned in flight, and without which flight would be impossible. It is curious how completely this has been forgotten in almost all human attempts to navigate the air. Birds are not lighter than the air, but immensely heavier. If they were lighter than the air they might float, but they could not fly. This is the difference between a bird and a balloon. A balloon rises because it is lighter than the air, and floats upon it. Consequently, it is incapable of being directed, because it possesses in itself no active Force enabling it to resist the currents of the air in which it is immersed, and because if it had such a force it would have no fulcrum, or resisting medium against which to exert it. It becomes, as it were, part of the atmosphere, and must go with it where it goes. No bird is ever for an instant of time lighter than the air in which it flies; but being, on the contrary, always greatly heavier, it keeps possession of a Force capable of supplying momentum, and therefore capable of overcoming any lesser Force, such as the ordinary resistance of the atmosphere, and even of heavy gales of wind. [133/134] The Law of Gravitation, therefore, is used in the flight of birds as one of the most essential of the Forces which are available for the accomplishment of the end in view.
The next law appealed to, and pressed into the service, is again a law which would seem an impediment in the way. This is the resisting force of the atmosphere in opposing any body moving through it. In this force an agent is sought and found for supplying the requisite balance to the Force of Gravity. But in order that the resisting force of air should be effectual for this purpose, it must be used under very peculiar conditions. The resisting force of fluids, and of airs or gases, is a force acting equally in all directions, unless special means are taken to give it predominant action in some special direction. If it is a force strong enough to prevent a body from falling, it is also a force strong enough to prevent it from advancing. In order, therefore, to solve the problem of flight, the resisting power of the air must be called into action as strongly as possible in the direction opposite to the Force of Gravity, and as little as possible in any other. Consequently a body [134/135] capable of flight must present its maximum of surface to the resistance of the air in the perpendicular direction, and its minimum of surface in the horizontal direction. Now, both these conditions are satisfied (1) by the great breadth or length of surface presented to the air perpendicularly in a bird's expanded wings, and by (2) the narrow lines presented in its shape horizontally, when in the act of forward motion through the air. But something more yet is required for flight. Great as the resisting force of air is, it is not strong enough to balance the Force of Gravity by its mere pressure on an expanded wing — unless that pressure is increased by an appeal to yet other laws — and other properties of its nature. Every sportsman must have seen cases in which a flying bird has been so wounded as to produce a rigid expansion of the wings. This does not prevent the bird from falling, although it breaks the fall, and makes it come more or less gently to the ground.
Yet further, therefore, to accomplish flight, another law must be appealed to, and that is the immense elasticity of the air, and the reacting force it exerts against compression. To enable an [135/136] animal heavier than the air to support itself against the Force of Gravity, it must be enabled to strike the air downwards with such force as to occasion are bound upwards of corresponding power. The wing of a flying animal must therefore do something more than barely balance Gravity. It must be able to strike the air with such violence as to call forth a reaction equally violent, and in the opposite direction. This is the function assigned to the powerful muscles by which the wings of birds are flapped with such velocity and strength. We need not follow this part of the problem further, because it does not differ in kind from the muscular action of other animals. The connexion, indeed, between the Wills of animals and the mechanism of their frame is the last and highest problem of all in the mechanics of Nature, but it is merged and hid for ever in the one great mystery of Life! But so far as this difficulty is concerned the action of an Eagle's wing is not more mysterious than the action of a Man's arm. There is a greater concentration of muscular power in the organism of birds than in most other animal frames, because it is an essential part of the [136/137] problem to be solved in flight, that the engine which works the wings should be very strong, very compact, of a special form, and that, though heavier than the air, it should not have an excessive weight. These conditions are all met in the power, in the outline, and in the bulk of the pectoral muscles which move the wings of birds. Few persons have any idea of the force expended in the action of ordinary flight. The pulsations of the Wing in most birds are so rapid that they cannot be counted. Even the Heron seldom flaps its wings at a rate of less than from 120 to 150 strokes in a minute. This is counting only the downward strokes, preparatory to each one of which there must be an upward stroke also: so that there are from 240 to 300 separate movements per minute. Yet the Heron is remarkable for its slow and heavy flight, and it is difficult to believe, until one has timed the pulsations with a watch, that they have a rapidity approaching to two in a second. But this difficulty is an index to the enormous comparative rapidity of the faster — flying birds. Let any one try to count the pulsations of the wing in ordinary flight of a Pigeon, or of a Blackcock, or of a [137/138] Partridge, or still more of any of the diving sea-fowl. He will find that though in the case of most of these birds the quickness of sight enables him to see the strokes separate from each other, it is utterly impossible to count them; whilst in some birds, especially in the Divers, as well as in the Pheasant and Partridge tribe, the velocity is so great that the eye cannot follow it at all, and the vibration of the wings leaves only a blurred impression on the eye.
Our subject here, however, is not so much the amount of vital force bestowed on birds, as the mechanical laws which are appealed to in order to make that force effective in the accomplishment of flight. The elasticity of the air is the law which offers itself for the counteraction of gravity. But in order to make it available for this purpose, there must be some great force of downward blow in order to evoke a corresponding rebound in the opposite, or upward direction. Now, what is the nature of the implement required for striking this downward blow? There are many conditions it must fulfil. First, it must be large enough in area to compress an adequate volume of air; next, it [138/139] must be light enough in substance not to add an excess of weight to the already heavy body of the bird; next, it must be strong enough in frame to withstand the pressure which its own action on the air creates. The first of these conditions is met by an exact adjustment of the size or area of the wing to the size and weight of the bird which it is to lift. The second and the third conditions are both met by the provision of a peculiar substance, feathers, which are very light, and very strong; whilst the only heavy parts of the framework, namely, the bones in which the feathers are inserted, are limited to a very small part of the area required.
But there is another difficulty to be overcome a difficulty opposed by natural laws, and which can only be met by another adjustment, if possible more ingenious and beautiful than the rest. It is obvious that if a bird is to support itself by the downward blow of its wings upon the air, it must at the end of each downward stroke lift the wing upwards again, so as to be ready for the next. But each upward stroke is in danger of neutralising the effect of the downward stroke. It must be made with equal velocity, and if it required [139/140] equal force, it must produce equal resistance, — an equal rebound from the elasticity of the air. If this difficulty were not evaded somehow, flight would be impossible. But it is evaded by two mechanical contrivances, which, as it were, triumph over the laws of aërial resistance by conforming to them. One of these contrivances is that the upper surface of the wing is made convex, whilst the under surface is concave. The enormous difference which this makes in atmospheric resistance is familiarly known to us by the difference between the effect of the wind on an umbrella which is exposed to it on the under or the upper side. The air which is struck by a concave or hollow surface, is gathered up, and prevented from escaping, whereas the air struck by a convex or bulging surface escapes readily on all sides, and comparatively little pressure or resistance is produced: And so, from the convexity of the upper surface of a bird's wing, the upward stroke may be made with comparatively trifling injury to the force gained in the downward blow.
But this is only half of the provision made against a consequence which would be so fatal to [140/141] the end in view. The other half consists in this that the feathers of a bird's wing are made to underlap each other, so that in the downward stroke the pressure of the air closes them upwards against each other, and converts the whole series of them into one connected membrane, through which there is no escape; whilst in the upward stroke the same pressure has precisely the reverse effect — it opens the feathers, separates them from each other, and converts each pair of feathers into a self — acting valve, through which the air rushes at every point. Thus the same implement is changed in the fraction of a second from a close and continuous membrane which is impervious to the air, into a series of disconnected joints through which the air passes without the least resistance — the machine being so adjusted that when pressure is required the maximum of pressure is produced, and when pressure is to be avoided, it is avoided in spite of rapid and violent action.
This, however, exhausts but a small part of the means by which Law is made to do the work of Will in the machinery of flight? It might easily be that violent and rapid blows struck downwards [141/142] against the elastic air, might enable animals possessed of such power to lift themselves from the ground and nothing more. There is a common toy which lifts itself in this manner from the force exerted by the air in resisting, and reacting upon little vanes which are set spinning by the hand. But the toy mounts straight up, and is incapable of horizontal motion. So, there are many structures of wing which might enable animals to mount into the air, but which would not enable them to advance or to direct their flight. How, then, is this essential purpose gained? Again we find an appeal made to natural laws, and advantage taken of their certainty and unchangeableness.
The power of forward motion is given to birds, first by the direction in which the whole wing feathers are set, and next by the structure given to each feather in itself. The wing feathers are all set backwards, that is, in the direction opposite to that in which the bird moves, whilst each feather is at the same time so constructed as to be strong and rigid toward its base, and extremely flexible and elastic towards its end. On the other hand, the front of the wing, along the greater part of its [142/143] length, is a stiff hard edge, wholly unelastic and unyielding to the air. The anterior and posterior webs of each feather are adjusted on the same principle. The consequence of this disposition of the parts as a whole, and of this construction of each of the parts, is, that the air which is struck and compressed in the hollow of the wing, being unable to escape through the wing, owing to the closing upwards of the feathers against each other, and being also unable to escape forwards owing to the rigidity of the bones and of the quills in that direction, finds its easiest escape backwards. In passing backwards it lifts by its force the elastic ends of the feathers; and thus whilst effecting this escape, in obedience to the law of action and reaction, it communicates, in its passage along the whole line of both wings, a corresponding push forwards to the body of the bird. By this elaborate mechanical contrivance the same volume of air is made to perform the double duty of yielding pressure enough to sustain the bird's weight against the Force of Gravity, and also of communicating to it a forward impulse. The bird, therefore, has nothing to do but to repeat with the requisite [143/144] velocity and strength its perpendicular blows upon the air, and by virtue of the structure of its wings the same blow both sustains and propels it.3
The truth of this explanation of the mechanical theory of flight may be tested in various ways: In the first place it is quite visible to the eye. In many birds flying straight to us, or straight from us, the effect of aërial resistance in bending upwards the ends of the quill feathers is very conspicuous. The flight of the common Rook affords an excellent example where the bird is seen foreshortened. In Eagles the same effect is very marked — the wing tips forming a sharp upward curve. I have seen it equally obvious in that splendid bird the Gannet, or Solan Goose; and when we recollect the great weight which those few quill feathers are thus seen sustaining, we begin to appreciate the degree in which lightness, strength, and imperviousness to the passage of [144/145] air are combined in this wonderful implement of flight.
But perhaps the simplest test of the action and reaction of the air and the wing feathers in producing forward motion is an actual experiment. If we take in the hand the stretched wing of a Heron, which has been dried in that position, and strike it quickly downwards in the air, we shall find that it is very difficult indeed to maintain the perpendicular direction of the stroke, requiring, in fact, much force to do so; and that if we do not apply this force, the hand is carried irresistibly forward, from the impetus in that direction which the air communicates to the wing in its escape backwards from the blow. Another test is one of reasoning and observation. If the explanation now given be correct, it must follow that since no bird can flap its wings in any other direction than the vertical — i.e., perpendicular to its own axis, (which is ordinarily horizontal,) and, as this motion has been shown to produce necessarily a forward motion, no bird can ever fly backwards. Accordingly no bird ever does so — no man ever saw a bird, even for an instant, fly tail [145/146] foremost. A bird can, of course, allow itself to fall backwards by merely slowing the action of its wings so as to allow its weight to overcome their sustaining power; and this motion may sometimes give the appearance of flying backwards, — as when a Swift drops backwards from the eaves of a house, or when a Humming Bird allows itself to drop in like manner from out of the large tubular petals of a flower. But this backward motion is due to the action of gravity, and not to the action of the bird's wing. In short, it is falling downwards, not flying backwards. Nay, more, if the theory of flight here given be correct, it must equally follow that even standing still, which is the easiest of all things to other animals, must be very difficult, if not altogether impossible, to a bird when flying. This also is true in fact. To stand still in the air is not indeed impossible to a flying bird, for reasons to be presently explained, but it is one of the most difficult feats of flying [inanship??], a feat which many birds, not otherwise clumsy, can never perform at all, and which is performed only by special exertion, and generally for a very short time, by those [146/147] birds whose structure enables them to be adepts in their glorious art.
It cannot be too often repeated — because misconception on this point has been the cardinal error in human attempts to navigate the air — that in all the beautiful evolutions of birds upon the wing, it is weight, and not buoyancy, which makes those evolutions possible. It supplies them, so to speak, with a store of Force which is constant, inexhaustible, inherent in the very substance of themselves, and entirely independent of any muscular exertion. All they have to do is to give direction to that internal Force, by acting on the external Force of aërial currents, through the contraction and expansion of the implements which have been given them for that purpose. Those who have watched the flight of birds with any care, must have observed that when once they have attained a certain initial velocity and a certain elevation, by rapid and repeated strokes upon the air, they are then able to fly with comparatively little exertion, and very often to pursue their course for long distances without any flapping whatever of the wings. [147/148] The contrast between the violent efforts required for the first acquisition of the initial velocity, and the perfect ease with which flight is performed after it has been acquired, is a contrast described by Virgil in lines of incomparable beauty: —
"Qualis speluncá subito commota columba
Cui domus et dulces latebroso in pumice nidi,
Fertur in arva volans, plausumque exterrita pennis
Dat tecto ingentem; mox, aëre lapsa quieto,
Radit iter liquidum, celeres neque commovet alas."
— AEn. lib. v. 213 — 17.
Still more remarkable, as showing the power and the value of weight in flight, is the fact that birds are able to resume rapid and easy motion not only as the result of a previously — acquired momentum, but after"soaring" in an almost perfectly stationary position. Nothing, for example, is more common than to see Sea Gulls, and some large species of Hawks,"soaring" one moment, (that is, all the forces bearing on the bird brought to an equilibrium, and all motion brought consequently to nearly a perfect standstill,) and the next moment sailing onwards in rapid and apparently effortless progression. Now, how is this effect produced? If we only think of [148/149] it, the question ought rather to be, How is it ever prevented? The soaring is a much more difficult thing to do than the going onwards. It cannot be done at all in a perfectly still atmosphere. It can only be done when there is a breeze of sufficient strength. Gravity is ceaselessly acting on the bird to pull it downwards: and downwards it must go, unless there is a countervailing Force to keep it up. This force is the force of the breeze striking against the vanes of the wings. But in order to bring these two forces to nearly a perfect balance, and so to"soar," the bird must expand or contract its wings exactly to the right size, and hold them exactly at the right angle. The slightest alteration in either of these adjustments produces instantly an upsetting of the balance, and of course a resulting motion. The exact direction of that motion will depend on the degree in which the wing is contracted, and the degree in which its angle to the wind is changed. If the wing is very much contracted, and at the same time held off from the wind, that motion will be steeply downwards. Accordingly this is the action of a Hawk when it swoops upon its prey [149/150] from a great height above it. I have seen a Merlin dash down from a great distance with its wings so closed as to seem almost wholly folded. The Gannet in diving for fish does not close its wings at all, but turning them and the whole axis of its body into the perpendicular, and thus allowing its great weight to act without any counteraction, dashes itself into the sea with foam, But every variety of forward motion is attained by different degrees of contraction and exposure, according to the strength of the breeze with which the bird has to deal. The limit of its velocity is the limit of its momentum, and the limit of its momentum is the limit of its weight. The lightness of a bird is therefore a limit to its velocity. The heavier a bird is, the greater is its possible velocity of flight — because the greater is the store of force — or, to use the language of modern physics, the greater is the quantity of"potential energy" — which, with proper implements to act upon aerial resistance, it can always convert into upward, or horizontal, or downward motion, according to its own management and desires.
It will be at once seen from this view of the [150/151] forces concerned in flight, that the common explanation of Birds being assisted by air — cells for the inhalation and storage of heated air, must not only be erroneous, but founded on wholly false conceptions of the fundamental mechanical principles on which flight depends. If a Bird could inhale enough warm air to make it buoyant, its power of flight would be effectually destroyed. It would become as light as a Balloon, and consequently as helpless. If, on the other hand, it were merely to inflate itself with a small quantity of hot air insufficient to produce buoyancy, but sufficient to increase its bulk, the only effect would be to expose it to increased resistance in cleaving the air. It is true, indeed, that the bones of Birds are made more hollow and lighter than the bones of Mammals, because Birds, though requiring weight, must not have too much of it. It is true, also, that the air must have access to these hollows, else they would be unable to resist atmospheric pressure. But it is no part whatever of the plan or intention of the structure of Birds, or of any part of that structure, to afford balloon — space for heated air with a view to buoyancy. [151/152] And here, indeed, we open up a new branch of the same inquiry, showing, in new aspects, how the universality and unchangeableness of all natural laws are essential to the use of them as the instruments of Will; and how by being played off against each other they are made to express every shade of thought, and the nicest change of purpose. The movement of all flying animals in the air is governed and determined by Forces of muscular power, and of aërial resistance and elasticity being brought to bear upon the Force of Gravity, whereby, according to. the universal laws of motion, a direction is given to the animal which is the resultant, or compromise between all the Forces so employed. Weight, as we have seen, is one of these Forces — absolutely essential to that result, and no flying animal can ever for a moment of time be buoyant, or lighter than the air in which it is designed to move. But it is obvious that, within certain limits, the proportion in which these different Forces are balanced against each other, admits of immense variety. The limits of variation can easily be specified. Every flying animal must have muscular power great enough to work its [152/153] own size of wing: that size of wing must be large enough to act upon a volume of air sufficient to lift the animal's whole weight: lastly, and consequently, the weight must not be too great, or dispersed over too large a bulk. . But within these limits there is room for great varieties of adjustment, having reference to corresponding varieties of purpose. To some birds the air is almost their perpetual home — the only region in which they find their food — a region which they never leave, whether in storm or sunshine, except during the hours of darkness and the yearly days which are devoted to their nests. Other birds are mainly terrestrial, and never betake themselves to flight except to escape an enemy, or to follow the seasons and the sun. Between these extremes there is every possible variety of habit. And all these have corresponding varieties of structure. The birds which seek their food in the air have long and powerful wings, and so nice an adjustment of their weight to that power and to that length, that the faculty of self — command in them is perfect, and their power of direction so accurate that they can pick up a flying gnat whilst passing through [153/154] the air at the rate of more than a hundred miles an hour. Such especially are the powers of some species of the Swallow tribe, one of which, the common Swift, is a creature whose wonderful and unceasing evolutions seem part of the happiness of summer and of serene and lofty skies.4
There are other birds in which the wing has to be adapted to the double purpose of swimming, or rather of diving, and of flight. In this case, a large area of wing must be dispensed with, because it would be incapable of being worked under water. Consequently in all diving birds the wings are reduced to the smallest possible size which is consistent with retaining the power of flight at all; and in a few extreme Forms, the power of flight is sacrificed altogether, and the wing is reduced to the size, and adapted to the function, of a powerful fin. This is the condition of the Penguins. But in most genera of swimming Birds, both purposes are combined, and the wing is just so far reduced in size and stiffened in texture as to make it workable as a fin under water, whilst it is still just large [154/155] enough to sustain the weight of the bird in flight.
THE SWIFT [Click on thumbnail for larger image.]
And here again we have a wonderful example of the skill with which inexorable mechanical laws are subordinated to special purpose. It is a necessary consequence of the area of the wing being so reduced, in proportion to the size of the bird, that great muscular power must be used in working it, otherwise the Force of Gravity could not be overcome at all. It is a farther consequence of this proportion of weight to working power, that there must be great momentum and therefore great velocity of flight. Accordingly this is the fact with all the oceanic diving Birds. They have vast distances to go, following shoals of fish, and moving from their summer to their winter haunts. They all fly with immense velocity., and the wing-strokes are extremely rapid. But there is one quality which their flight does not possess — because it is incompatible with their structure, and because it is not required by their habits — they have no facility in evolutions, no delicate power of steering; they cannot stop with ease, nor can they resume their onward motion in a moment. They do not want it: the trackless fields of ocean over which [155/156] they roam are broad, and there are no obstructions in the way. They fly in straight lines, changing their direction only in long curves, and lighting in the sea almost with a tumble and a splash. Their rising again is a work of great effort, and generally they have to eke out the resisting power of their small wings, not only by the most violent exertion, but by rising against the wind, so as to collect its force as a help and addition to their own.
And now, again, we may see all these conditions changed where there is a change in the purpose to be served. There is another large class of oceanic Birds whose feeding ground is not under water, but on the surface of the sea. In this class all those powers of flight which would be useless to the Divers, are absolutely required, and are given in the highest perfection, by the enlistment of the same mechanical laws under different conditions. In the Gulls, the Terns, the Petrels, and in the Fulmars, with the Albatross as their typical Form, the mechanism of flight is carried through an ascending scale, to the highest degrees of power, both as respects endurance and facility of evolution. [156/157] The mechanical laws which are appealed to in all these modifications of structure require adjustments of the finest kind, and some of them are so curious and so beautiful that it is well worth following them a little further in detail.
There are two facts observable in all Birds of great and long — sustained powers of flight: — the first is, that they are always provided with wings which are rather long than broad, sometimes extremely narrow in proportion to their length; the, second is, that the wings are always sharply pointed at the ends. Let us look at the mechanical laws which absolutely require this structure for the purpose of powerful flight, and to meet which it has accordingly been devised and provided.
One law appealed to in making wings rather long than broad is simply the law of leverage. But this law has to be applied under conditions of difficulty and complexity, which are not apparent at first sight. The body to be lifted is the very body that must exert the lifting power. The force of gravity which has to be resisted may be said to be sitting side by side, occupying the same particles of matter, with the vital force which is to give [157/158] it battle. Nay, more, the one is connected with the other in some mysterious manner which we cannot trace or understand. A dead bird weighs as much as a living one. Nothing which our scales can measure is lost when the"vital force" is gone. It is The Great Imponderable. Nevertheless, vital forces of unusual power are always coupled with unusual mass and volume — in the matter through which they work. And so it is that a powerful bird must always also be comparatively a heavy bird. And then it is to be remembered that the action of gravity is constant and untiring. The vital force, on the contrary, however intense it may be, is intermitting and capable of exhaustion. If, then, this force is to be set against the force of gravity, it has much need of some implement through which it may exert itself with mechanical advantage as regards the particular purpose to be attained. Such an implement is the lever — and a long wing is nothing but a long lever. The mechanical principle, or law, as is well known, is this, — that a very small amount of motion, or motion through a very small space, at the short end of a lever produces a [158/159] great amount of motion, or motion through a long space, at the opposite or longer end. This action requires indeed a very intense force to be applied at the shorter end, but it applies that force with immense advantage for the purpose in view: because the motion which is transmitted to the end of along wing is a motion acting at that point through along space, and is therefore equivalent to a very heavy weight lifted through a short space at the end which is attached to the body of the bird. Now this is precisely what is required for the purpose of flight. The body of a bird does not require to be much lifted by each stroke of the wing. It only requires to be sustained; and when more than this is needed — as when a bird first rises from the ground, or from the sea, or when it ascends rapidly in the air — greatly increased exertion — in many cases, very violent exertion — is required 5 — And then it is to be remembered that [159/160] long wings economise the vital force in another way. When a strong current of air strikes against the wings of a bird, the same sustaining effect is produced as when the wing strikes against the air. Consequently birds with very long wings have this great advantage, that with pre-acquired momentum, they can often for a long time fly without flapping their wings at all. Under these circumstances, a bird is sustained very much as a boy's kite is sustained in the air. The string which the boy holds, and by which he pulls the kite downwards with a certain force, performs for the kite the same offices which its own weight and balance and momentum perform for the bird. The great long — winged oceanic birds often appear to float rather than to fly. The stronger is the gale, their flight, though less rapid, is all the more easy — so easy indeed as to appear buoyant ; because the blasts which strike against their wings are enough to sustain the bird with comparatively little exertion of its own, except that of holding described between the violent exertion required in first rising, and the perfect ease of flight after this first momentum has been acquired, is a striking illustration of the true mechanical principles of flight. [160/161] the wing vanes stretched and exposed at proper angles to the wind. And whenever the onward force previously acquired by flapping, becomes at length exhausted, and the ceaseless inexorable Force of Gravity is beginning to overcome it, the, bird again rises by a few easy and gentle halfstrokes of the wing. Very often the same effect is produced by allowing the force of gravity to act, and when the downward momentum has brought the bird close to the ground or to the sea, that force is again converted into an ascending impetus by a change in the angle at which the wing is exposed to the wind. This is a constant action with all the oceanic birds. Those who have seen the Albatross have described themselves as never tired of watching its glorious and triumphant motion: —
Tranquil Tranquil its spirit seemed, and floated slow;
Even in its very motion there was rest."6
Rest — where there is nothing else at rest in the tremendous turmoil of its own
stormy seas! Sometimes for a whole hour together this splendid bird will sail
or wheel round a ship in every [161/162] possible variety of direction without requiring to give a single the Albatross
has kind of wing. Its — about fourteen tip — and almost stroke to its pinions.
Now, the extreme form of this wings are immensely long or fifteen feet from
tip to as narrow in proportion as a riband:7 Our common Gannet is an excellent,
though a more modified, example of the same kind of structure. On the other
hand, birds of short wings, though their flight is sometimes very fast, are
never able to sustain it very long. The muscular exertion they require is greater,
because it does not work to the same advantage. Most of the gallinaceous birds
(such as the common Fowl, Pheasants, Partridges, &c.) have wings of this
kind; and some of them never fly except to escape an enemy, or to change their
feeding ground. [162/163]
WING OF GANNET[Click on thumbnail for larger image.]
. The second fact observable in reference to birds of easy and powerful flight — namely, that their wings are all sharply pointed at the end — will lead us still further into the niceties of adjustment which are so signally displayed in the machinery of flight.The feathers of a bird's wing have a natural threefold division, according
to the different wingbones to which they are attached. The quills which form
the end of the wing are called the 'Primaries: those which form the middle of
the vane are called the Secondaries; and those which are next the body of the
bird are called the Tertiaries. The motion of a bird's wing increases from its
minimum at the shoulder — joint to its maximum at the tip. The primary quills
which form the termination of the wing are those on which the chief burden of
flight is cast. Each feather has less and less weight to bear, .and less and
less force to exert in proportion as it lies nearer the body of the bird; and
there is nothing more beautiful in the structure of a wing than the perfect
gradation in strength and stiffness, as well as in modification of form, which
marks the series [163/164] from the first of the primary quills to the last and feeblest of the tertiaries.8
Now, the sharpness or roundness of a wing at the tip depends on the position
which is given to the longest primary quill. If the first or even the second
primary is the longest, and all that follow are considerably shorter, the wing
is necessarily a pointed wing, because the tip of a single quill forms the end;
but if the third or fourth primary quills are the longest, and the next again
on both sides are only a little shorter, the wing becomes a roundended wing.
Round — ended wings are also almost always open — ended — that is to say, the tips
of the quills do not touch each other, but leave interspaces at the end of the
wing, through which, of course, a good deal of air escapes. Since each single
quill is formed on the same principle as the whole wing — that is, with the
anterior margin stiff and the posterior margin yielding — this
WING OF A GOLDEN PLOVER. [Click on thumbnail for larger image.]
[164/165] escape is not useless for progression; but the air acts less favourably for this purpose than when struck by a more compact set of feathers. The common Rook and all the Crows are examples of this. The Peregrine Falcon, the common Swallow, and all birds of very powerful flight have been provided with the sharp — pointed structure.9The object of this structure, and the mechanical laws to which it appeals,
will be apparent when we recollect what it is on which the propelling power,
as distinct from the sustaining power, of a bird's wing depends. It depends
on the reaction of the air escaping backwards — that is, in the direction
exactly opposite to that of the intended motion of the bird. Any air which escapes
from under the wing, in any other direction, will of course react with less
advantage upon that motion But from under a round wing a good deal of air must
necessarily escape along the rounded end — that is, in a direction at
right angles — to the line of intended flight. All the reaction produced by this [165/166] escape is a reaction which is useless for propulsion. Accordingly, in all birds
to whom great velocity of flight is essential, this structure, which is common
in other birds, is carefully avoided. The Hawks have been classified as "noble"
or "ignoble," according to the length and sharpness of their wings: those
which catch their prey by velocity of flight having been uniformly provided
with the long — pointed structure. The Sparrow — Hawk and the Merlin are excellent
examples of the difference. The Sparrow — Hawk, with its comparatively short and
blunt wings,' steals along the hedgerows and pounces on its prey by surprise;
seldom chasing it, except for a short distance, and when the victim is at a
disadvantage. And well do the smaller birds know this habit, and the limit of
his powers. Many of them chase and"chaff" the Sparrow — Hawk, when he is seen
flying in the open, perfectly aware that he cannot catch them by fast flying.
But they never play these tricks with the Merlin. This beautiful little Falcon
hunts the open ground, giving fair chase to its quarry by power and speed of
flight. The Merlin delights in flying at some of the fastest birds, such as
A. KESTREL HOVERING B. MERLIN — SHARP WING. C. SPARROW HAWK — ROUND WING. [Click on thumbnail for larger image]
[166/167] the Snipe. The longest and most beautiful trial of wingmanship I have ever seen was the chase of a Merlin after a Snipe in one of the Hebrides. It lasted as far as the eye could reach, and seemed to continue far out to sea. In the Merlin, as in all the fastest Falcons, the second quill feather is the longest in the wing; the others rapidly diminish; and the point of the wing looks as sharp as a needle in the air.There is yet one other power which it is absolutely necessary to some birds that their wings should enable them to exert; and that is the power of standing still, or remaining suspended in the air without any forward motion. One familiar example of this is the common Kestrel, which, from the frequent exercise of this power, is called in some counties the "Windhover." The mechanical principles on which the machinery of flight is adapted to this purpose, are very simple. No bird can exercise this power which is not provided with wings large enough, long enough, and powerful enough to sustain its weight with ease, and without violent exertion. Large wings can always be diminished at the pleasure of the bird, by being [167/168] partially folded inwards; and this contraction of the area is constantly resorted to. But a bird which has wings so small and scanty as to compel it to strike them always at full stretch, and with great velocity in order to fly at all, is incapable of standing still in the air. No man ever saw a Diver or a Duck performing the evolution which the Kestrel may be seen performing every hour over so many English fields. The cause of this is obvious if we refer to the principles which have already been explained. We have seen that the perpendicular stroke of a bird's wing has the double effect of both propelling and sustaining. The reaction from such a stroke brings two different forces to bear upon the bird — one whose direction is upwards, and another whose direction is forwards. How can these two effects be separated from each other? How can the wing be so moved as to keep up just enough of the sustaining force without allowing the propelling force to come into play? The answer to this, although it in volves some very complicated laws connected with what mechanicians call the "parallelogram of forces," is practically a simple one. It can only [169] be done by shortening the stroke, and altering the perpendicularity of its direction. Of course, if a bird, by altering the axis of its own body, can direct its wing — stroke in some degree forwards , it will have the effect of stopping instead of promoting progression. But in order to do this, it must have a superabundance of sustaining force, because some of this force is sacrificed when the stroke is off the perpendicular. Hence it follows that birds so heavy as to require the whole action of their wings to sustain them at all, can never afford this sacrifice of the sustaining force, and except for the purpose of arresting their flight, can never strike except directly downwards, that is, directly against the opposing force of gravity. But birds with superabundant sustaining power, and long sharp wings, have nothing to do but to diminish the length of stroke, and direct it off the perpendicular at such an angle as will bring all the forces bearing upon their body to — an exact balance, and they will then remain stationary at a fixed point in the air.10 [169/170] They are greatly assisted in this beautiful evolution by an adverse current of air; and it will always be observed that the Kestrel, when hovering, turns his head to wind, and hangs his whole body at a greater or less angle to the plane of the horizon. When there is no wind, or very little, the sustaining force is kept up by a short rapid action of the pinions, and the long tail is spread out like a fan to assist in stopping any tendency to onward motion. When there is a strong breeze, no flapping is required at all — the force of the wind supplying the whole force necessary to counteract the force of gravity; and in proportion to the increasing strength of the wind, the amount of vane which must be exposed to it becomes less and less. I have seen a Kestrel stand suspended in a half gale with the wings folded close to the body, and with no visible muscular motion whatever. And so nice is the adjustment of position which is requisite to produce this exact balance of all the forces bearing on the bird, that the change in that position which again instantly results in a forward motion is very often almost insensible to the eye. It is generally a [170/171] slight expansion of the wings, and a very slight change in the axis of the body.
And here it may be observed that the tails of birds have not, as is often supposed, any function analogous to the rudder of a ship. Birds which have lost the tail are not thereby rendered incapable of turning. If the steering function had been assigned to birds' tails, the vane of the tail must have been set, not, as it is, horizontally, but perpendicularly to the line of flight. But a bird's tail has in flight no lateral motion whatever. It does, indeed, materially assist the bird in turning, because it serves to stop the way of a bird when it rises or turns in the air to take a new direction. It contributes also largely to the general balance of the body, which in itself is an important element in the facility of flight. Accordingly, almost all birds which depend on great ease of evolution in flight — or on the power of stopping suddenly, have largely developed tails. This is the case with all the birds of prey — with the Kestrel in a conspicuous degree. But there are some exceptions which show that great powers of flight are not [171/172] always dependent on the possession of a large tail — as, for example, the Swift.
Another explanation has been given of the means by which birds are able to turn in flight, which is a curious example how preconceived theories founded on false analogies will vitiate our observation of the commonest facts in nature. I do not know of any modern work which gives any account of the theory of flight, which is even tolerably correct. But in most points an admirable account is to be found in the celebrated work of Borelli," De Motu Anilrialium." On the question, however, of steerage in flight, he gives a solution which the most ordinary observation is sufficient to contradict. Borelli is quite aware that the tail in birds has no such function as that which is usually assigned to it, and he points out the true theoretical objection to the possibility of its having any guiding power — viz., its horizontal position, and its immobility in the lateral direction. But the theory which he himself propounds is equally erroneous. It is this, — that birds deflect their course to the right or to the left, as rowers turn a row — boat — by striking more quickly and [172/173] more strongly with one wing than with the other.11
To this theory there are two objections — first, that as a matter of fact birds can turn, and do turn, even to the extent of describing complete circles in the air, without any flapping either of one wing or the other; and secondly, that when birds do flap and turn at the same time, not the slightest difference in time between the two wingstrokes can ever be detected. The beats of a bird's two wings are always exactly synchronous. But the first of these two objections is of itself quite sufficient to disprove the theory. No man can have watched even for a moment the flight of the common Swallow, and especially the flight of the Swift, without seeing it perform complete gyrations in the air without any strokes of either wing. The only change which can ever be detected by the eye is a [173/174] slight elevation of one side of the whole body, and a slight depression of the other. The depression is always on that side towards which the bird is turning. On the opposite side, that from which the bird is turning, there is of course a corresponding elevation. Sometimes this is very obvious; but in general it is so slight as to require close observation to detect it. In the Albatross, when sweeping round, the wings are often pointed in a direction nearly perpendicular to the sea.12 The effect of this, of course, is to expose the two vanes at different angles to the aerial currents — and it must be remembered that in flight the balance of all the forces employed is so extremely fine that the most minute alteration in the degree in which they bear upon each other, will produce an immense change in the [174/175] result. It is not surprising, therefore, that the muscular movements which serve to turn the axis of a flying bird from one direction to another, are very often so extremely minute as generally altogether to elude the sight. But in general terms, it may be said that a bird turns in flying essentially on the same principle as that on which a man turns in walking. It is done in both cases by change in the direction of muscular pressure upon a resisting medium. By an exquisite combination of different laws, and by mechanical contrivance in the adjustment of them, it has been given to a bird to find in the thin and yielding air a medium of resistance against which its own muscular force may act, as firm and as effective as that which Man finds in the solid earth.
The Humming — Birds are perhaps the most remarkable examples in the world of the machinery of flight. rThe power of poising themselves in the air, — remaining absolutely stationary whilst they search the blossoms for insects, — is a power essential to their life.? It is a power accordingly which is enjoyed by them in the highest perfection. When they intend progressive flight, it is effected [176] with such velocity as to elude the eye. The action of the wing in all these cases is far too rapid to enable the observer to detect the exact difference between that kind of motion which keeps the bird at absolute rest in the air, and that which carries it along with such immense velocity. But there can be no doubt that the change is one from a short quick stroke delivered obliquely forward, to a full stroke, more slow, but delivered perpendicularly. This corresponds with the account given by that most accurate ornithological observer, Mr Gould. He, says:"When poised before any object, this action of the wing is so rapidly performed that it is impossible for the eye to follow each stroke, and a hazy semicircle of indistinctness on each side of the bird is all that is perceptible." There is another fact mentioned by those who have watched their movements most closely which corresponds with the explanation already given — viz., the fact that the axis of the Humming Bird's body when hovering is always highly inclined, so much so as to appear almost perpendicular in the air. In other words the wingstroke, instead of being delivered perpendicularly [176/177] downwards, which would infallibly carry the body onwards, is delivered at such an angle forwards as to bring to an exact balance the upward, the downward, and the forward forces which bear upon the body of the bird. Mr Darwin says, "when hovering by a flower, the tail is constantly shut and expanded like a fan, the body being heft in a nearly vertical position." Mr Wallace, another accurate observer, describes the Humming Birds as"balancing themselves vertically in the air."
These are a few, and a few only, of the adjustments required in order to the giving of the power of flight; — adjustments of organic growth to intensity of vital force — of external structure to external work — of shape in each separate feather to definite shape in the series as a whole — of material to resistance — of mass and form to required velocities; adjustments, in short, of law to law, of force to force, and of all to Purpose. So many are these contrivances, so various, so fine, so intricate, that a volume might be written without exhausting the beauty of the method in which this one mechanical problem has been solved. it is by knowledge of unchanging laws that these victories over them [177/178] seem to be achieved! yet not by knowledge only, except as the guide of Power. For here, as everywhere else in Nature, we see the same mysterious need of conforming to imperative conditions, side by side with absolute control over the forces through which this conformity is secured When any given purpose cannot be attained without the violation of some law, unless by some new power, and some new machinery — the requisite power and mechanism are evolved — generally out of old materials, and by modifications of pre-existing forms. There can be no better example of this than a wing — feather. It is a production wholly unlike any other animal growth — an implement specially formed to combine strength with lightness, elasticity, and imperviousness to air. Again, the bones of a bird's wing are the bones of the Mammalian arm and hand, specially modified to support the feathers. The same purpose is effected by other means in connexion with precisely the same bones in the flying Mammalia — the Bats. In these animals the finger — bones instead of being compressed or soldered together to support feathers, are separated, attenuated, and greatly [178/179] lengthened to afford attachment to a web or flying membrane which is stretched between them. In other ages of the world there were also flying Lizards. But in all these cases the mechanical principle is the same, and there has been the same ingenious adaptation of material and of force to the universal laws of motion.
On the earth and on the sea Man has attained to powers of locomotion with which, in strength, endurance, and in velocity, no animal movement can compare. But the air is an element on which he cannot travel — an ocean which he cannot navigate. The birds of heaven are still his envy, and on the paths they tread he cannot follow. As yet! for it is not certain that this exclusion is to be perpetual. His failure has resulted quite as much from his ignorance of natural laws, as from his inability to meet the conditions which they demand. All attempts to guide bodies buoyant in the air must be fruitless. Balloons are mere toys. No flying animal has ever been formed on the principle of buoyancy. Birds, and Bats, and Dragons, have been all immensely heavier than the air, and their weight is one of the forces most [179/180] essential to their flight. Yet there is a real impediment in the way of Man navigating the air — and that is the excessive weight of the only great mechanical moving powers hitherto placed at his disposal. When Science shall have discovered some moving power greatly lighter than any we yet know, in all probability the problem will be solved. But of one thing we may be sure — that if Man is ever destined to navigate the air, it will be in machines formed in strict obedience to the mechanical laws which have been employed by the Creator for the same purpose in flying animals.13
Last modified 9 December 2008