FIG. 4.-The King Penguin in the positions assumed by a bird in (a) swimming, (b) diving, and (c) flying. (Pettigrew.) propel a balloon against even a moderate breeze would re- | Bult in its destruction. The balloon cannot be transferred with any degree of certainty from one point of the earth's surface to another, and hence the chief danger in its em Montgolfier brothers in 1782. It was first inflated by heated air obtained by burning trusses of straw under it, then by hydrogen gas, and lastly by coal gas. It is in no sense to be regarded a flying machine. It resembles the Fre. 5.-The Red-throated Dragon (Draco hematopogon). (Pettigrew, 1867.) FIG. 6.-The Flying Colugo (Galeopithecus volans); also called flying lemur and flying squirrel. (Pettigrew, 1867.) FIG. 7.-The Flying Fish (Exocoetus exiliens). (Pettigrew, 1867.) flying creature only in this, that it is immersed in the ocean of air in which it sustains itself. The mode of suspension is wholly different. The balloon floats because it is lighter than the air; the flying creature floats because it extracts from the air, by the vigorous downward action of its wings, a certain amount of upward recoil. The balloon is passive; the flying creature is active. The balloon is controlled by the wind; the flying creature controls the wind. The balloon in the absence of wind can only rise and fall in a vertical line; the flying creature can fly in a horizontal plane in any given direction. The balloon is inefficient because of its levity; the flying creature is efficient because of its weight. Weight, however paradoxical it may appear, is necessary to flight. Everything which flies is vastly heavier than the air. The inertia of the mass of the flying creature enables it to control and direct its movements in the air. Many are of opinion that flight is a mere matter of levity and power. This is quite a mistake. No machine, however light and powerful, will ever fly whose travelling surfaces are not properly fashioned and properly applied to the air. It was supposed at one time that the air sacs of birds contributed in some mysterious way to flight, but this is now known to be erroneous. The bats and some of the best flying birds have no air sacs. Similar remarks are to be made of the heated air imprisoned within the bones of certain birds. Feathers even are not necessary to flight. Insects and bats have no feathers, and yet fly well. The only facts in natural history which appear even indirectly to countenance the flotation theory are the presence of a swimming bladder in some fishes, and the existence of membranous expansions or pseudowings in certain animals, such as the flying fish, flying dragon, and flying squirrel. As, however, the animals referred to do not actually fly, but merely dart into the air and there sustain themselves for brief intervals, they afford no real support to the theory. ployment. It may, quite as likely as not, carry its occu- | The so-called floating animals are well depicted at figs. 5, 6, pants out to sea. The balloon is a mere lifting machine. and 7. It has no analogue in nature, and, as its history sufficiently shows, is incapable of improvement. The balloon, as is well known, was introduced by the According to Dr. Crisp, the swallow, martin, snipe, and many part xxv. 1857, p. 13. birds of passage have o air in their bones.-Proc. Zool. Soc., Lond. It has been asserted, and with some degree of plausibility, that a fish lighter than the water might swim, and that a bird lighter than the air might fly; it ought, however, to be borne in mind that, in point of fact, a fish lighter than the water could not hold its own if the water were in the least perturbed, and that a bird lighter than the air would be swept into space by even a moderate breeze without hope of return. Weight and power are always associated in living animals, and the fact that living animals are made heavier than the medium they are to navigate may be regarded as a conclusive argument in favor of weight being necessary alike to the swimming of the fish and the flying of the bird. It may be stated once for all that flying creatures are for the most part as heavy, bulk for bulk, as other animals, and that flight in every instance is the product, not of superior levity, but of weight and power directed upon properly constructed flying organs. This fact is important as bearing on the construction of flying machines. It shows that a flying machine need not necessarily be a light, airy structure exposing an immoderate amount of surface. On the contrary, it favors the belief that it should be a compact and moderately heavy and powerful structure, which trusts for elevation and propulsion entirely to its flying appliances-whether actively moving wings, or screws, or aëro-planes wedged forward by screws. and never give the air an opportunity of attacking or subIt should attack and subdue the air, dning it. It should smite the air intelligently and as a master, and its vigorous well-directed thrusts should in every instance elicit an upward and forward recoil. The flying machine of the future, there is reason to believe, will be a veritable example of "multum in parvo." It will launch itself in the ocean of air, and will extract from that air, by means of its travelling surfaces-however fashioned and however applied-the recoil or resistance necessary to elevate and carry it forward. indeed are contra-indicated in a flying machine, as they Extensive inert surfaces approximate it to the balloon, which, as has been shown, cannot maintain its position in the air if there are air currents. A flying machine which could not face air currents would necessarily be a failure. To obviate this difficulty we are forced to fall back upon weight, or rather the structures and appliances which weight represents. These appliances as indicated should not be unnecessarily expanded, but when expanded they should, wherever practicable, be converted into actively moving flying surfaces, in preference to fixed or inert dead surfaces. The question of surface is a very important one in aërostation; it naturally resolves itself into one of active and passive surface. As there are active and passive surfaces in the flying animal, so there are, or should be, active and passive surfaces in the flying machine. Art should follow nature in this matter. creatures are always greatly in excess of the passive ones, The active surfaces in flying from the fact that the former virtually increase in proportion to the spaces through which they are made to travel. Nature not only distinguishes between active and passive Fig. 8. Fig. 9. 271 faces. While, however, diminishing the surfaces of the flying animal as a whole, she increases as occasion demands the active or wing surfaces by wing movements, and the pas sive or dead surfaces by the forward motion of the body in progressive flight. She knows that if the wings are driven with sufficient rapidity they practically convert the spaces through which they move into solid bases of support; she also knows that the body in rapid flight derives support from all the air over which it passes. The manner in which the wing surfaces are increased by the wing movements will be readily understood from the accompanying illustrations of the blow fly with its wings at rest and in motion (figs. 8 and 9). In fig. 8 the surfaces exposed by the body of the insect and the wings are, as compared with those of fig. 9, trifling. The wind would have much less purchase on fig. 8 than on fig. 9, provided the surfaces they are not dead surfaces; they represent the spaces occupied by the rapidly vibrating wings, which are actively exposed by the latter were passive or dead surfaces. But moving flying organs. at a much higher speed than any wind that blows, they are superior to and control the wind; they enable the As, moreover, the wings travel insect to dart through the wind in whatever direction it pleases. in a flying machine would be a mistake. It is found to be paper to realize that extensive, inert, horizontal aëro-planes1 The reader has only to imagine figs. 8 and 9 cut out in out would be heavier than fig. 8, and if both were exposed to a current of air, fig. 9 would be more blown about than so practically, as will be shown by and by. Fig. 9 so cut fig. 8. are the elytra or wing cases-thin, light, horny structures It is true that in beetles and certain other insects there which in the act of flight are extended horizontally and act front of the wings, of which they may be regarded as formatively long narrow structures which occupy a position in as sustainers or gliders. The elytra, however, are comparing the anterior parts. The elytra are to the delicate wings stronger wings. The elytra, moreover, are not wholly passive structures. They can be moved, and the angles made of some insects what the thick anterior margins are to by their under surfaces with the horizon adjusted. Finally, they are not essential to flight, as flight in the great majority of instances is performed without them. The elytra serve as protectors to the wings when the wings are folded upon the back of the insect, and as they are extended horindirectly, in virtue of their being carried forward by the body in motion. izontally when the insect is flying they contribute to flight air, so as practically to increase the basis of support, raises the whole subject of natural flight. It is necessary, thereThe manner in which the wings of the insect traverse the what fully to the subject of flight, as witnessed in the insect, fore, at this stage to direct the attention of the reader somebat, and bird,-a knowledge of natural flight preceding, and being in some senses indispensable to, a knowledge of artificial flight. The bodies of flying creatures are, as a rule, very strong, comparatively light, and of an elongated form, -the bodies of birds being specially adapted for cleaving the air. Flying creatures, however, are less remarkable for their strength, shape, and comparative levity than for the size and extraordinarily rapid and complicated movements of their wings. To Professor J. Bell Pettigrew is due the merit of having first satisfactorily analyzed those movements, and of having reproduced them by the aid of artificial wings. This physiologist in 1867 showed that all natural wings, whether of the insect, bat, or bird, are screws structurally, and that they act as screws when they are made to vibrate, from the fact that they twist in opposite directions during the down and up strokes. He also to the air, through which they pass much in the explained that all wings act upon a common principle, and that they present oblique, kite-like surintended to float or rest upon the air, and calculated to afford a cer-same way that an oar passes through water in sculling. He tain amount of support to any body attached to it. 1 By the term aero-plane is meant a thin, light, expanded structure Fie. 8.-Blow Fly (Musca vomitoria) with its wings at rest. (Pettigrew.) FIG. 9.-Blow Fly with its wings in motion as in flight. (Pettigrew.) surfaces in flying animals, but she strikes a just balance | faces between them, and utilizes both. She regulates the surfaces to the strength and weight of the flying creature and the air currents to which the surfaces are to be exposed and upon which they are to operate. In her calculations she never forgets that her flying subjects are to control and not to be controlled by the air. As a rule she reduces the passive surfaces of the body to a minimum; she likewise reduces as far as possible the actively moving or flying sur J. Bell Pettigrew, M.D., F.R.S., etc. (Proceedings of the Royal Institution further pointed out that the wings of flying creatures (con-est towards its root and anterior margin, where they supply the trary to received opinions, and as has been already indicated) strike downwards and forwards during the down strokes, and upwards and forwards during the up strokes. Lastly, and most important of all, he demonstrated that the wings of flying creatures, when the bodies of said creatures are fixed, describe figure-of-8 tracks in space, the figure-of-8 tracks, when the bodies are released and advancing as in rapid flight, being opened out and converted into waved tracks. Professor Pettigrew's discovery of the figure-of-8 and waved movements, concerning which so much has been said and written, was confirmed some two years after it was made by Professor E. J. Marey1 by the aid of the "sphygmograph." The movements in question are now regarded as fundamental, from the fact that they are alike essential to natural and artificial flight. The following is Professor Pettigrew's description of wings and wing movements published in 1867: "The wings of insects and birds are, as a rule, more or less triangular in shape, the base of the triangle being directed towards the body, its sides anteriorly and posteriorly. They are also conical on section from within outwards and from before backwards, this shape converting the pinions into delicatelygraduated instruments balanced with the utmost nicety to satisfy the requirements of the muscular system on the one hand and the resistance and resiliency of the air on the other. While all wings are graduated as explained, innumerable varieties occur as to their general contour, some being falcated or scythe-like, others oblong, others rounded or circular, some lanceolate, and some linear. The wings of insects may consist either of one or two pairs, the anterior or upper pair, when two are present, being in some instances greatly modified and presenting a corneous condition. They are then known as elytra, from the Greek iλurpov, a sheath. Both pairs are composed of a duplica Fig. 10. Fig. 11. FIG. 10.-Right wing of the Beetle (Goliathus micans) when at rest; ture of the integument or investing membrane, and are strength- place of the arm in bats and birds. The neuræ are arranged at the axis of the wing after the manner of a fan or spiral stairthe anterior one occupying a higher position than that farther back, and so of the others. As this arrangement extends also and present a certain degree of convexity on their superior or to the margins, the wings are more or less twisted upon themselves, upper surface, and a corresponding concavity on their inferior or under surface-their free edges supplying those fine curves which act with such efficacy upon the air in obtaining the maximum of resistance and the minimum of displacement. A illustrative examples of the form of wings alluded to, those of the beetle, bee, and fly may be cited-the pinions in those insects acting as helices, or twisted levers, and elevating weights much greater than the area of the wings would seem to war rant" (figs. 10 and 11). . "To confer on the wings the multiplicity of movements which they require, they are supplied with double hinge or compound joints, which enable them to move not only in an upward, downward, forward, and backward direction, but also at various intermediate degrees of obliquity. An insect with wings thus hinged may, as far as steadiness of body is concerned, be not inaptly compared to a compass set upon gimbals, where the universality of motion in one direction ensures comparative fixedness in another." ... "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 when the sudden or effective stroke is being given, and gradually decreasing in an opposite direction during flexion, or when the wing is being more slowly recovered preparatory to making a second stroke. The effective stroke in insects, and this holds true also of birds, is therefore delivered downwards and forwards, and not, as the majority of writers believe, vertically, --t ... FIG. 12 shows the figure-of-8 made by the margins of the wing in ex- FIG. 18.-Wave track made by the wing in progressive flight. a, b, crests of the wave; c, d, e, up strokes; z, z, down strokes; f. point cor responding to the anterior margin of the wing, and forming a centre for the downward rotation of the wing (a, g); g, point corresponding to the posterior margin of the wing, and forming a centre for the upward rotation of the wing (d, f). (Pettigrew, 1867.) spectator, represents the movements in question. The down and up strokes, as will be seen from this account, cross each other • Revue des Cours Scientifiques de la France et de l'Étranger, 1869. The sphygmograph, as its name indicates, is a recording instrument. It consists of a smoked cylinder revolving by means of clock work at a known speed, and a style or pen which inscribes its surface by scratching or brushing away the lampblack. The movements to be registered are transferred to the style or pen by one or more levers, and the pen in turn transfers them to the cylinder, where they appear as legible tracings. In registering the movements of the wings, the tips and margins of the pinions were, by an ingenious modification, employed as the styles or pens. By this arrangement the different parts of the wings were made actually to record their own move the wing smiting the air during its descent from above, as in the bird and bat, and during its ascent from below as in the flying fish and boy's kite" (fig. 12). "The figure-of-8 action of the wing explains how an insect or bird may fix itself in the air, the backward and forward reciprocating action of the pinion affording support, but no propulsion. In these instances the backward and forward strokes are made to counterbalance each other. Although the figureof-8 represents with considerable fidelity the twisting of the wing ments. As will be seen from this account, the figure-of-8 or wave theory of stationary and progressive flight has been made the subject of a rigorous experimentum crucis. upon its axis during extension and flexion, when the insect is playing its wings before an object, or still better when it is artificially fixed, it is otherwise when the down stroke is added and the insect is fairly on the wing and progressing rapidly. In this case the wing, in virtue of its being carried forward by the body in motion, describes an undulating or spiral course. as shown in fig. 13." "The down and up strokes are compound movements the termination of the down stroke embracing the beginning of that the wing, and the curtain or fringe of the wing which the primary and secondary feathers form, shall be screwed into and down upon the wind in extension, and unscrewed or withdrawn from the wind during flexion. The wing of the bird may therefore be compared to a huge gimlet or auger, the axis of the gimlet representing the bones of the wing, the flanges or spiral thread of the gimlet the primary and secondary feathers" (figs. 15 and 16). ... "From this description it will be evident that by the mere rotation of the bones of the forearm and hand the maximum and minimum of resistance is secured much in the same way that this object is attained by the alternate dipping and feathering of an oar." "The wing, both when at rest and when in motion, may not inaptly be compared to the blade of an ordinary screw propeller as employed in navigation. Thus the general outline of the wing corresponds closely with the outline of the propeller (figs. 11, 16, and 18), and the track described by the wing in space is twisted upon itself propeller fashion' (figs. 12, 20, 21, 22, 23). The great velocity with which the wing is driven converts the impression or blur into what is equivalent to a solid for the time being, in the same way that the spokes of a wheel in violent motion, as is well understood, more or less completely occupy the space contained within the rim or circumference of the wheel" (figs. 20, and 21). 9, FIG. 14.-a, b, line along which the wing travels during extension and flexion. The arrows indicate the direction in which the wing is spread out in extension and closed or folded in flexion. (Pettigrew, 1867.) the up stroke, and the termination of the up stroke including | the beginning of the down stroke. This is necessary in order that the down and up strokes may glide into each other in such a manner as to prevent jerking and unnecessary retardation."... "The wing of the bird, like that of the insect, is concavoconvex, and more or less twisted upon itself when extended, so FIG. 15.-Right wing of the Red-legged Partridge (Perdiz rubra). Dorsal aspect as seen from above. (Pettigrew, 1867.) that the anterior or thick margin of the pinion presents a different degree of curvature to that of the posterior or thin margin. This twisting is in a great measure owing to the manner in which the bones of the wing are twisted upon themselves, and the spiral nature of their articular surfaces,-the long axes of the joints always intersecting each other at right angles, and the bones of the elbow and wrist making a quarter of a turn or so during extension and the same amount during flex FIG. 16.-Right wing of the Red-legged Partridge (Perdix rubra). Dorsal and ventral aspects as seen from behind; showing sugerlike conformation of wing. Compare with figs. 11 and 18. (Pettigrew, 1867.) ion. As a result of this disposition of the articular surfaces, the wing may be shot out or extended, and retracted or flexed in nearly the same plane, the bones composing the wing_rotating on their axes during either movement (fig. 14). The secondary action or the revolving of the component bones on "The wing of the bat bears considerable resemblance to that of the insect, inasmuch as it consists of a delicate, semi-transparent, continuous membrane, supported in divers directions, particularly towards its anterior margin, by a system of stays or stretchers which confer upon it the degree of rigidity requisite for flight. It is, as a rule, deeply concave on its under or ventral surface, and in this respect resembles the wing of the heavy-bodied birds. The movement of the bat's wing in extension is a spiral one, the spiral running alternately from below upwards and forwards and from above downwards and backwards. The action of the wing of the bat, and the movements of its component bones, are essentially the same as in the bird” (figs. 17 and 18). "The wing strikes the air precisely as a boy's kite would if it were jerked by its string, the only difference being that the kite is pulled forwards upon the wind by the string and the FIG. 18.-Right wing of the Bat (Phyllocina gracilis). Dorsal and ventral aspects, as seen from behind. These show the screw-like configuration of the wing, and also shows how the wing twists and untwists during its action. (Pettigrew, 1867.) hand, whereas in the insect, bat, and bird the wing is pushed forwards on the wind by the weight of the body and the power residing in the pinion itself" (fig. 19).3 The figure-of-8 and kite-like action of the wing referred to lead us to explain how it happens that the wing, which in many instances is a comparatively small and delicate organ, can yet attack the air with such vigor as to extract from it the recoil necessary to elevate and propel the flying creature. The accompanying figures from one of Professor rationale (figs. 20, 21, 22, and 23). Pettigrew's more recent memoirs will serve to explain the As will be seen from these figures, the wing during its vibration sweeps through a comparatively very large space. "The importance of the twisted configuration or screw-like form 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 na ture; in fact, one entire rowing feather is equivalent-morphologic ally 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 wing may consequently be removed without FIG. 17. Bight wing of the Bat (Phyllocina gracilis). Dorsal aspect essentially altering either its form or its functions. This is proved as seen from above. (Pettigrew, 1867.) their own axes, is of the greatest importance in the movements of the wing, as it communicates to the hand and forearm, and consequently to the primary and secondary feathers which they bear, the precise angles necessary for flight. It in fact ensures 1 This continuity of the down into the up stroke and the converse greatly facilitated by the elastic ligaments at the root or in the substance of the wing. These assist in elevating, and, when necessary, in flexing and elevating it. They counteract in some measure what may be regarded as the dead weight of the wing, and are especially useful in giving it continuous play. VOL. IX.-392 by making sections in various directions, and by finding that in some instances as much as two-thirds of the wing may be lopped off without materially impairing the power of flight" (Trans. Roy. Soc. Elin., vol. xxvi. pp. 325, 326). 8 "On the various modes of Flight in relation to Aeronautics" (Proceedings of the Royal Institution of Great Britain, March 22d, 1867); "On the Mechanical Appliances by which Flight is attained in the Animal Kingdom" (Transactions of the Linnean Society, vol. xxvi, read June 6th and 20th, 1867), by J. Bell Pettigrew, M.D., F.R.8, Professor of Medicine and Anatomy, University of St. Andrews. "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. Edin., vol. xxvi.). 1 This space, as already explained, is practically a solid basis of support for the wing and for the flying animal. The wing attacks the air in such a manner as virtually to have no slip, this for two reasons. The wing reverses instantly and acts as a kite during nearly the entire down The compound rotation of the wing is greatly facilitated by the wing being elastic and flexible. It is this which causes the wing to twist and untwist diagonally on its long axis when it is made to vibrate. The twisting referred to is partly a vital and partly a mechanical act;-that is, it is occasioned in part by the action of the muscles and in part by the greater resistance experienced from the air by the tip and posterior margin of the wing as compared with the root and anterior margin,the resistance experienced by the tip and posterior margin causing them to reverse always subsequently to the root and anterior margin, which has the effect of throwing the anterior and posterior margins of the wing into figure-of-8 curves, as shown at figs. 9, 11, 12, 16, 18, 20, and 21. FIG. 19.-The Cape Barn-owl (Striz capensis), showing the kite-like surfaces presented by the ventral aspect of the wings and body in flight. (Petti grew, 1867.) and up strokes. The angles, moreover, made by the wing with the horizon during the down and up strokes are at no two intervals the same, but (and this is a remarkable circumstance) they are always adapted to the speed at which the wing is travelling for the time being. The increase and The compound rotation of the wing, as seen in the bird, is represented in fig. 24. Not the least curious feature of the wing movements is the remarkable power which the wing possesses of making and utilizing its own currents. Thus, when the wing descends it draws after it a strong current, which, being met by the wing during its ascent, greatly increases the efficacy of the up stroke. Similarly and conversely when the wing ascends, it creates an upward current, which, being met by the wing when it descends, pow erfully contributes to the efficiency of the down stroke. This statement can be readily verified by experiment both with natural and artificial wings. Neither the up nor the down strokes are complete in themselves. The wing to act efficiently must be driven at a certain speed, and in such a manner that the down and up strokes shall glide into each other. It is only in this way that the air can be made to pulsate, and that the rhythm of the wing and the air waves can be made to correspond. The air must be seized and let go in a certain order and at a certain speed to extract a a maximum recoil. The rapidity of the wing movements is regulated by the size of the wing, small wings being driven at a very much higher speed than larger ones. The different parts of the wing, moreover, are made to travel at different degrees of velocity-the tip and posterior margin of the wing always rushing through a much greater space in a given time than the root and anterior margin. FIGS. 20, 21, 22, and 23 show the area mapped out by the left wing of the Wasp The rapidity of travel of the insect wing is in some cases enormous. The wasp, for instance, is said to ply its wings at the rate of 110, and the common housefly at the rate of 330 beats per second. Quick as are the vibrations of natural wings, the speed of certain parts of the wing is amazingly increased. Wings as a rule are 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 f h FIG. 24.-Wing of bird with its root (a, b) cranked forwards. a, b, short axis the air. wing is receded from. This is explained on a principle well understood in mechanics, viz., that when a wing or rod hinged at one end is made to move in a circle, the tip or free end of the wing or rod describes a much wider circle in a given time than portion of the wing or rod nearer the hinge (fig. 25). One naturally inquires why the high speed of wings and why the progressive increase of speed at their tips and posterior margins. The answer is not far to seek. If the wings were not driven at a high speed, and if they were not eccentrics made to revolve upon two separate axes, they would of necessity be large cumbrous structures; but large heavy wings would be difficult to work, and what is worse, they would (if too large), instead of controlling the air, be controlled by it, and so cease to be flying organs. be made to vibrate at high speeds. The air, as ex There is, however, another reason why wings should plained, is a very light thin, elastic medium, which yields on the slightest pressure, and unless the wing attacked it with great violence the necessary recoil or resistance could not be obtained. The atmosphere, because of its great tenuity, mobility, and comparative imponderability, presents little resistance to bodies pass ing through it at low velocities. If, however, the speed be greatly accelerated, the action of even an ordinary cane is sufficient to elicit a recoil. This comes of the action and reaction of matter, the resistance experienced varying according to the density of the atmosphere and |