The kite, the sparrow and the buzzard practice the kind of flight called the sailing flight, but the true kings of the air are the eagles, the vultures and the condors which travel through space without a single beating of the wings like living aeroplanes, capable even of remaining immovable, as though fastened in the sky.

Even superficial observation enables one to ascertain that the same bird may have different modes of flight and that the different species fly in different ways.

Two kinds of flying are in fact admitted; that obtained exclusively by the beating of the wings, the flight by rowing, as it may be called, and the soaring flight, during which the bird keeps its wings extended in the course of its progress through the air. There are two different modes of soaring flight; soaring flight properly so called, which is only

the wind for their movement. Simple observation permits us to establish among birds some clearly defined classes:

First, birds exclusively rowers; second, birds practicing the rowing flight and soaring flight; third, birds practicing rowing and sailing; fourth, birds practicing only sailing flight.

Are these distinctions, which are based only on the different variety of methods for accomplishing the action of flying, dependent upon the construction of the wing, and are there found in the wings of a bird such modifications that we may without hesitation attribute to them the special character of the flight? Observers have always replied in the affirmative, after having proved that the form of the wings is essentially variable and adapted to the kind of flying; but it is modern research which has scientifically established this dependence.

There exist two well defined types of wings among which, evidently, all the gradations are observed, but the distinction is easy. It is sufficient indeed, to glance at the two figures five and six in order to discover that the hawk's wing is stretched to a point, while the eagle's is rounded. This is due to the length of the quili feathers which diminishes from the first one to the last in

temporary and accessory to rowing flight, and sailing flight, which is the normal mode of locomotion for large birds capable of remaining entire days in the air by

the hawk's wing, while in the eagle's wing the longest feather is the sixth. Besides, the rowing wing is homogeneous on its posterior side, whilst the sailing wing is jagged. This last peculiarity is due to the special conformation of the primary feathers which, instead of presenting the form of a knife as do the rowing feathers, become narrow at the middle of their length while the quill loses some of its rigidity. There results a certain suppleness of the edge of the sailing wing which, during flight, becomes convex and appears in

have a more ample flight, possess narrower wings which, in addition, facilitate their veerings. The wing stretches in proportion as the bird, from being a rower, becomes a sailer. But here again one finds some broad and some narrow wings. The latter, provided for work in violent winds, belong to sea birds, such as the petrel and frigate birds; the others have been given to the great birds of prey in order that they may take advantage of the least breath of wind and may cover enough space find their prey.

dented by reason of the divergence and the bending of the feathers. This attitude is striking in figure ten, and it is precisely this lack of rigidity which renders sailing birds unfit for rowing flight.

Besides these characteristics, based for the most part on the structure of the wing, one may again examine the relation of the two dimensions, length and breadth. According to the French Mouillard, who was one of the most careful observers, the rowers all have short

FIG. 5. THE ROWING WING OF A FALCON.

wings but the width of these wings varies according to the necessities of the bird due to peculiar character of its life. The sparrow, the partridge and the quail have no long distances to traverse by one flight but they must have powerful wings in order to rise from the ground and escape danger as swiftly as possible: SO their wings are short and broad. The duck and the pigeon, which

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Now, how does the bird use its wings in order to keep itself up and to proceed through the air? Flying always com prises three phases or periods; the departure, the flight, properly so called, and lastly the alighting. We are going to study these three phases in each one of the groups of birds that we have described: rowers, half sailers, and sail

ers.

The average species of small size, belonging to the rowing group, take their start after a hop

made by the relaxation of their legs. accompanied by a vigorous beating of the wings which raises them from the ground at an angle of about forty-five degrees. The sparrow, the quail, the partridge, gallinaceous birds, and pigeons rise in this way. Certain aquatic species with short wings do not need to hop in order to release their wings; for some ducks it is sufficient to straighten their bodies vertically so

FIG. 6. THE SAILING WING OF AN EAGLE.

as to permit at the first a flapping of wings. When these birds rest on an elevated support, it is sufficient for them to let themselves fall in order to acquire the start or indispensable spring for the action of the wings.

The necessary effort for flight is considerable, but it diminishes rapidly in proportion as the speed of the bird approaches the normal condition. One gains an understanding of this by appreciating the speed and amplitude of the flapping of the wings; in the sea gull the amplitude attains one hundred to one hundred and ten degrees at the departure and lowers from thirty degrees to forty degrees in normal flying; in the partridge the expenditure of force is so depressing that the little cries that this bird makes at the moment of its flight are attributed to fatigue.

During the flight the raising of the wing is obtained only by the action of the middle pectoral muscle which has no other function and acts intermittently; but this would be insufficient if nature did not aid by an artifice to reduce the resistance of the air to its lowest value. It has been discovered, in short, that, during the first flappings, the wings are like the slats of open Venetian blinds. The feathers meet the air edgewise. This arrangement, which offers a minimum of resistance to the passage through the air, is caused by an automatic pivoting of the feathers, due to a very complicated disposition of their elastic ligaments. In proportion as the bird gains speed the raising of the wing is caused by an increasingly weaker action of the middle pectoral muscle, and it becomes entirely passive when the motion is normal. It is indeed, the relative wind produced by the speed of the bird which acts on the convex face of the wing. Here again we find two

components-the vertical and the horizontal; the first serving to raise the wing and the other, directed with a contrary motion, consequently slackens the speed of the bird. When to the relative wind there is added the absolute wind, that is, when the bird flies against the wind, -the result is still more defined, and this explains why many rowing birds try always to take their flight with the beak toward the wind.

All authors are agreed in defining soaring as a word which signifies that kind of flight which a bird executes without flapping the wings, and with the wings more or less extended. Soaring thus understood comprises two different. forms; soaring flight properly so called, for which the bird utilizes the speed acquired during a course of beating the wings or by a fall from an elevated point, and sailing flight, in which the bird has essential need of the assistance of the wind.

The bird that soars may be compared to a kite that one draws behind him while running and which rises and keeps itself up if the air is calm. All the rowers of medium size, herons, storks, buzzards, sea gulls and hawks, practice this kind of flying, and it is always easy to observe the periods during which they suspend the flapping of their wings, and continue their course, hold

ing their wings extended almost without losing any speed; then a few vigorous strokes of the wings will quickly start them again on their way. Such flying is a gliding over the air, and the great force acquired during the period of the beating of the wings is utilized by the bird for taking support on the air and for continuing its course either while rising, remaining at the same height, or descending. According as its passage follows the one or the other of these directions the speed diminishes rapidly, slowly, or increases. The bird which presents most frequently these different modes of flying is the falcon in hunting. When from a great height it perceives its prey, it lets itself fall almost vertically in such a manner as to return and attack its victim from beneath; if it fails, it sets its wings and its body with the purpose of utilizing the enormous force acquired during the fall to mount again to a height when it commences again this manoeuvre and continues these duckings, without interruption, and consequently without fatigue, until the prey is captured.

The pigeon also offers us frequent examples of soaring. When, perched on a roof, it wishes to descend to the ground, it lets itself fall vertically then, reducing its speed by flapping its wings or, if it has some space before it, it lets itself glide following a parabolic curve which places it gently on the ground.

By observation of different soaring birds and by experiments performed with soaring machines it has been shown that, for the bird wishing to land as far as

possible from an elevated. starting point, it is necessary to fall one yard in order to traverse eight yards. A bird which would soar to the height of 1,000 yards would thus be able to land without fatigue at a distance of 8,000 yards.

Among soaring birds the sustaining qualities qualities commence to gain on the propelling qualities; the concavity of the wing becoming no longer necessary except at the moment of the flappings, is assured only by the elasticity of the last quill feathers opposed to the rigidity of the first; besides, the surface of the wing is reduced and the tip gains a considerable development. Thus there is produced a narrow wing with a convex edge which is at once an excellent organ of propulsion and a very good soarer, qualities indispensable for assuring to these birds their busy existence in the pursuit of very fugutive prey.

Clearly then this condition, pushed to the extreme, will lead to flight by sailing, practiced only by birds which are no longed rowers and which borrow the necessary energy for their movement from another source than their muscles. Among the large sailing birds only the sustaining qualities exist; no more concavity of the wing, no more convexity or predominance of the tip, but large flat wings provided with extremely supple feathers with slight spread of the tip. The form of the wing itself is modified; the enlargement of the surface is obtained not by increasing the span, which would be inconvenient for the start, but by filling up the angle at the end of the wing which from being pointed becomes rectangular.

These are actual observations, and although denied by authors who have never witnessed this kind of flying, flight by sailing, are today definitely admitted, and no longer does any one deny that, in the large bird, the propelling and sustaining powers are obtained, simply by the reaction of the air in motion, by the wind alone. The agreement ceases when there is a question of explaining the

mechanism of the flight by sailing and in particular this paradoxical fact that the bird can rise and make headway against the wind.

Many explanations have been proposed, some fantastic, others the discussion of which leads to such absurd deductions as the realization of perpetual motion; others again, true perhaps in certain particular cases, could not be embraced in a general theory. To this last class belong the hypotheses based on the utilization of the ascendant currents and on the variations of the velocity of the wind. It is certain that the bird gains in the ascending currents of the air; but there have been noted also many sailing flights with the wind horizontal or even descending; so that if the theory of the ascendant wind suits very well certain cases, as also that which utilizes the variations of velocity and direction of the wind, there remains to be found a general theory which may be applied to all these different cases.

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There results in these birds a sort of balancing very balancing very clearly noticeable observers. This balancing is very much diminshed in large sailing birds with supple wings; the shifting of the wings comes into play and as the total mass of the body and of the wings requires much greater energy in order to be displaced from its equilibrium, it is the wings alone that feel the effect of the changes of the direction of the wind. And when the variations of the wind are very slight, the feathers alone, the respective independence of which makes them like so

The sailing bird, having only the aid of the wind to support it in the air, must be built for utilizing the slightest current of air from whatever direction it comes. It is necessary also that its organs be delicate enough to adapt it without delay to the changes of the direction of the wind which are almost always very sudden. These results are assured by the suppleness of the wings themselves and by that of the feathers, since the position of the plane to the air has to be modified. In fact each change of direction of the wind requires, if the wings are not sufficiently supple, an oscillation of the body and of the wings all together. This is what is produced in the semi-sailing birds with the wings relatively rigid, the tips of which are convex and predominant as in the case of the sea gull and swallow.

many small wings for independent shifting, receive the puff of air and absorb its energy.

The large sailing birds having to count only upon the wind are necessarily constructed SO as to utilize the lightest breath of air; everything with them tends to this result, from the sensitiveness of the feathers and the suppleness of the wing, to the ability of spreading the wing like a fan when, if they wish. to rise, they have need to increase their sail. This explains the different positions reproduced in our figures 4 and 8, the first corresponding to a light wind. for the utilization of which it is necessary to set full sail, the other becoming necessary when the wind freshens and there is need of taking a reef. Between these two extreme cases, there is an infinity of intermediate situations that the shifting of the feathers, or that of the wings, if it is necessary, suffice to regulate. The quill-feathers, in fact, constitute an automatic shifting device which assures the longitudinal and lateral equilibrium in normal conditions; the assistance of the wing, and of the entire body, are used only in extremely violent strokes. This automatic shifting is indispensable to birds that practice flight by sailing, and those which are not provided with it, the