Chapter IX: Part 9

_Consideration of the Forces which propel the Wings of Bats and Birds._--The muscular system of birds has been so frequently and faithfully described, that I need not refer to it further than to say that there are muscles which by their action are capable of elevating and depressing the wings, and of causing them to move in a forward and backward direction, and obliquely. They can also extend or straighten and bend, or flex the wings, and cause them to rotate in the direction of their length during the down and up strokes. The muscles principally concerned in the elevation of the wings are the smaller pectoral or breast muscles (_pectorales minor_); those chiefly engaged in depressing the wings are the larger pectorals (_pectorales major_). The pectoral muscles correspond to the fleshy mass found on the breast-bone or sternum, which in flying birds is boat-shaped, and furnished with a keel. These muscles are sometimes so powerful and heavy that they outweigh all the other muscles of the body. The power of the bird is thus concentrated for the purpose of moving the wings and conferring steadiness upon the volant mass. In birds of strong flight the keel is very large, in order to afford ample attachments for the muscles delegated to move the wings. In birds which cannot fly, as the members of the ostrich family, the breast-bone or sternum has no keel.[90]

[90] One of the best descriptions of the bones and muscles of the
bird is that given by Mr. Macgillivray in his very admirable,
voluminous, and entertaining work, entitled History of British Birds.
Lond. 1837.

The remarks made regarding the muscles of birds, apply with very slight modifications to the muscles of bats. The muscles of bats and birds, particularly those of the wings, are geared to, and act in concert with, elastic ligaments or membranes, to be described presently.

_Lax condition of the Shoulder-Joint in Bats, Birds, etc._--The great laxity of the shoulder-joint in bats and birds, readily admits of their bodies falling downwards and forwards during the up stroke. This joint, as has been already stated, admits of movement in every direction, so that the body of the bat or bird is like a compass set upon gimbals, _i.e._ it swings and oscillates, and is equally balanced, whatever the position of the wings. The movements of the shoulder-joint in the bird, bat, and insect are restrained within certain limits by a system of check ligaments and prominences; but in each case the range of motion is very great, the wings being permitted to swing forwards, backwards, upwards, downwards, or at any degree of obliquity. They are also permitted to rotate along their anterior margin, or to twist in the direction of their length to the extent of nearly a quarter of a turn. This great freedom of movement at the shoulder-joint enables the insect, bat, and bird to rotate and balance upon two centres--the one running in the direction of the length of the body, the other at right angles or across the body, _i.e._ in the direction of the length of the wings.

In the bird the head of the humerus is convex and somewhat oval (not round), the long axis of the oval being directed from above downwards, _i.e._ from the dorsal towards the ventral aspect of the bird. The humerus can, therefore, _glide up and down_ in the _facettes_ occurring on the articular ends of the coracoid and scapular bones with great facility, much in the same way that the head of the radius glides upon the distal end of the humerus. But the humerus has another motion; it moves _like a hinge from before backwards, and_ vice versâ. The axis of the latter movement is almost at right angles to that of the former. As, however, the shoulder-joint is connected by long ligaments to the body, and can be drawn away from it to the extent of one-eighth of an inch or more, it follows that _a third and twisting movement can be performed_, the twisting admitting of rotation to the extent of something like a quarter of a turn. In raising and extending the wing preparatory to the downward stroke two opposite movements are required, viz. one from before backwards, and another from below upwards. As, however, the axes of these movements are at nearly right angles to each other, a spiral or twisting movement is necessary to run the one into the other--to turn the corner, in fact.

From what has been stated it will be evident that the movements of the wing, particularly at the root, are remarkably free, and very varied. A directing and restraining, as well as a propelling force, is therefore necessary.

The guiding force is to be found in the voluntary muscles which connect the wing with the body in the insect, and which in the bat and bird, in addition to connecting the wing with the body, extend along the pinion even to its tip. It is also to be found in the musculo-elastic and other ligaments, seen to advantage in the bird.

_The Wing flexed and partly elevated by the Action of Elastic Ligaments--the Nature and Position of such Ligaments in the Pheasant, Snipe, Crested Crane, Swan, etc._--When the wing is drawn away from the body of the bird by the hand the posterior margin of the pinion formed by the primary, secondary, and tertiary feathers rolls down to make a variety of inclined surfaces with the horizon (_c b_, of fig. 63, p. 138). When, however, the hand is withdrawn, even in the dead bird, the wing instantly folds up; and in doing so reduces the amount of inclination in the several surfaces referred to (_c b_, _d e f_ of the same figure). The wing is folded by the action of certain elastic ligaments, which are put upon the stretch in extension, and which recover their original form and position in flexion (fig. 98, _c_, p. 181). This simple experiment shows that the various inclined surfaces requisite for flight are produced by the mere acts of extension and flexion in the dead bird. It is not, however, to be inferred from this circumstance that flight can be produced without voluntary movements any more than ordinary walking. The muscles, bones, ligaments, feathers, etc., are so adjusted with reference to each other that if the wing is moved at all, it must move in the proper direction--an arrangement which enables the bird to fly without thinking, just as we can walk without thinking. There cannot, however, be a doubt that the bird has the power of controlling its wings both during the down and up strokes; for how otherwise could it steer and direct its course with such precision in obtaining its food? how fix its wings on a level with or above its body for skimming purposes? how fly in a curve? how fly with, against, or across a breeze? how project itself from a rock directly into space, or how elevate itself from a level surface by the laboured action of its wings?

The wing of the bird is elevated to a certain extent in flight by the reaction of the air upon its under surface; but it is also elevated by muscular action--by the contraction of the elastic ligaments, and by the body falling downwards and forwards in a curve.

That muscular action is necessary is proved by the fact that the pinion is supplied with distinct elevator muscles.[91] It is further proved by this, that the bird can, and always does, elevate its wings prior to flight, quite independently of the air. When the bird is fairly launched in space the elevator muscles are assisted by the tendency which the body has to fall downwards and forwards: by the reaction of the air; and by the contraction of the elastic ligaments. The air and the elastic ligaments contribute to the elevation of the wing, but both are obviously under control--they, in fact, form links in a chain of motion which at once begins and terminates in the muscular system.

[91] Mr. Macgillivray and C. J. L. Krarup, a Danish author, state
that the wing is elevated by a vital force, viz. by the contraction
of the _pectoralis minor_. This muscle, according to Krarup, acts
with one-eighth the intensity of the _pectoralis major_ (the
depressor of the wing). He bases his statement upon the fact that
in the pigeon the pectoralis minor or elevator of the wing weighs
one-eighth of an ounce, whereas the pectoralis major or depressor
of the wing weighs seven-eighths of an ounce. It ought, however, to
be borne in mind that the volume of a muscle does not necessarily
determine the precise influence exerted by its action; for the
tendon of the muscle may be made to act upon a long lever, and,
under favourable conditions, for developing its powers, while that
of another muscle may be made to act upon a short lever, and,
consequently, under unfavourable conditions.--On the Flight of Birds,
p. 30. Copenhagen, 1869.

That the elastic ligaments are subsidiary and to a certain extent under the control of the muscular system in the same sense that the air is, is evident from the fact that voluntary muscular fibres run into the ligaments in question at various points (_a_, _b_ of fig. 98, p. 181). The ligaments and muscular fibres act in conjunction, and fold or flex the forearm on the arm. There are others which flex the hand upon the forearm. Others draw the wing towards the body.

The elastic ligaments, while occupying a similar position in the wings of all birds, are variously constructed and variously combined with voluntary muscles in the several species.

_The Elastic Ligaments more highly differentiated in Wings which vibrate rapidly._--The elastic ligaments of the swan are more complicated and more liberally supplied with voluntary muscle than those of the crane, and this is no doubt owing to the fact that the wings of the swan are driven at a much higher speed than those of the crane. In the snipe the wings are made to vibrate very much more rapidly than in the swan, and, as a consequence, we find that the fibro-elastic bands are not only greatly increased, but they are also geared to a much greater number of voluntary muscles, all which seems to prove that the musculo-elastic apparatus employed for recovering or flexing the wing towards the end of the down stroke, becomes more and more highly differentiated in proportion to the rapidity with which the wing is moved.[92] The reason for this is obvious. If the wing is to be worked at a higher speed, it must, as a consequence, be more rapidly flexed and extended. The rapidity with which the wing of the bird is extended and flexed is in some instances exceedingly great; so great, in fact, that it escapes the eye of the ordinary observer. The speed with which the wing darts in and out in flexion and extension would be quite inexplicable, but for a knowledge of the fact that the different portions of the pinion form angles with each other, these angles being instantly increased or diminished by the slightest quiver of the muscular and fibro-elastic systems. If we take into account the fact that the wing of the bird is recovered or flexed by the combined action of voluntary muscles and elastic ligaments; that it is elevated to a considerable extent by voluntary muscular effort; and that it is extended and depressed entirely by muscular exertion, we shall have difficulty in avoiding the conclusion that the wing is thoroughly under the control of the muscular system, not only in flexion and extension, but also throughout the entire down and up strokes.

[92] A careful account of the musculo-elastic structures occurring in
the wing of the pigeon is given by Mr. Macgillivray in his History of
British Birds, pp. 37, 38.

An arrangement in every respect analogous to that described in the bird is found in the wing of the bat, the covering or web of the wing in this instance forming the principal elastic ligament (fig. 17, p. 36).

_Power of the Wing--to what owing._--The shape and power of the pinion depend upon one of three circumstances--to wit, the length of the humerus,[93] the length of the cubitus or forearm, and the length of the primary feathers. In the swallow the humerus, and in the humming-bird the cubitus, is very short, the primaries being very long; whereas in the albatross the humerus or arm-bone is long and the primaries short. When one of these conditions is fulfilled, the pinion is usually greatly elongated and scythe-like (fig. 62, p. 137)--an arrangement which enables the bird to keep on the wing for immense periods with comparatively little exertion, and to wheel, turn, and glide about with exceeding ease and grace. When the wing is truncated and rounded (fig. 96, p. 176), a form of pinion usually associated with a heavy body, as in the grouse, quail, diver, and grebe, the muscular exertion required, and the rapidity with which the wing moves are very great; those birds, from a want of facility in turning, flying either in a straight line or making large curves. They, moreover, rise with difficulty, and alight clumsily and somewhat suddenly. Their flight, however, is perfect while it lasts.

[93] “The humerus varies extremely in length, being very short in
the swallow, of moderate length in the gallinaceous birds, longer
in the crows, very long in the gannets, and unusually elongated in
the albatross. In the golden eagle it is also seen to be of great
length.”--Macgillivray’s British Birds, vol. i. p. 30.

The goose, duck (fig. 107, p. 204), pigeon (fig. 106, p. 203) and crow, are intermediate both as regards the form of the wing and the rapidity with which it is moved.

The heron (fig. 60, p. 126) and humming-bird furnish extreme examples in another direction,--the heron having a large wing with a leisurely movement, the humming-bird a comparatively large wing with a greatly accelerated one.

But I need not multiply examples; suffice it to say that flight may be attained within certain limits by every size and form of wing, if the number of its oscillations be increased in proportion to the weight to be raised.

_Reasons why the effective Stroke should be delivered downwards and forwards._--The wings of all birds, whatever their form, act by alternately presenting oblique and comparatively non-oblique surfaces to the air,--the mere extension of the pinion, as has been shown, causing the primary, secondary, and tertiary feathers to roll down till they make an angle of 30° or so with the horizon, in order to prepare it for giving the effective stroke, which is delivered, with great rapidity and energy, in a _downward_ and _forward_ direction. I repeat, “downwards and forwards;” for a careful examination of the relations of the wing in the dead bird, and a close observation of its action in the living one, supplemented by a large number of experiments with natural and artificial wings, have fully convinced me that the stroke is invariably delivered in this direction.[94] If the wing did not strike downwards and _forwards_, it would act at a manifest disadvantage:--

[94] _Prevailing Opinions as to the Direction of the Down
Stroke._--Mr. Macgillivray, in his History of British Birds,
published in 1837, states (p. 34) that in flexion the wing is drawn
upwards, forwards, and inwards, but that during extension, when the
effective stroke is given, it is made to strike outwards, downwards,
and _backwards_. The Duke of Argyll holds a similar opinion. In
speaking of the hovering of birds, he asserts that “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;” and that, “Except for the purpose of
_arresting_ their flight, birds can never strike except _directly
downwards_--that is, directly against the opposing force of
gravity.”--Good Words, Feb. 1865, p. 132.

Mr. Bishop, in the Cyc. of Anat. and Phys., vol. iii. p. 425, says,
“In consequence of the planes of the wings being disposed either
_perpendicularly_ or _obliquely backwards_ to the direction of
their motion, a corresponding impulse is given to their centre of
gravity.” Professor Owen, in like manner, avers that “a downward
stroke would only tend to raise the bird in the air; to carry it
forwards, the wings require to be moved in an oblique plane, so as to
_strike backwards_ as well as downwards.”--Comp. Anat. and Phys. of
Vertebrates, vol. ii. p. 115.

The following is the account given by M. E. Liais:--“When a bird is
about to depress its wing, this is a little inclined from before
backwards. When the descending movement commences, the wing does not
descend parallel to itself in a direction from before backwards;
but the movement is accompanied by a rotation of several degrees
round the anterior edge, so that the wing becomes more in front
than behind, and the _descending movement is transferred more and
more backwards_.... When the wing has completely descended, it
is both _further back_ and lower than at the commencement of the
movement.”--“On the Flight of Birds and Insects.” Annals of Nat.
Hist. vol. xv. 3d series, p. 156.

_1st._ Because it would present the back or convex surface of the wing to the air--a convex surface dispersing or dissipating the air, while a concave surface gathers it together or focuses it.

_2d._ In order to strike backwards effectually, the concavity of the wing would also require to be turned backwards; and this would involve the depression of the anterior or thick margin of the pinion, and the elevation of the posterior or thin one, during the down stroke, which never happens.

_3d._ The strain to which the pinion is subjected in flight would, if the wing struck _backwards_, fall, not on the anterior or strong margin of the pinion formed by the bones and muscles, but on the posterior or weak margin formed by the tips of the primary, secondary, and tertiary feathers--which is not in accordance with the structure of the parts.

_4th._ The feathers of the wing, instead of being closed, as they necessarily are, by a downward and _forward_ movement, would be inevitably opened, and the integrity of the wing impaired by a downward and _backward_ movement.

_5th._ The disposition of the articular surfaces of the wing (particularly that of the shoulder-joint) is such as to facilitate the downward and _forward_ movement, while it in a great measure prevents the downward and _backward_ one.

_6th and lastly._ If the wing did in reality strike downwards and _backwards_, a result the converse of that desired would most assuredly be produced, as an oblique surface which smites the air in a downward and _backward_ direction (if left to itself) tends to depress the body bearing it. This is proved by the action upon the air of free inclined planes, arranged in the form of a screw.

_The Wing acts as an Elevator, Propeller, and Sustainer, both during extension and flexion._--The wing, as has been explained, is recovered or drawn off the wind principally by the contraction of the elastic ligaments extending between the joints, so that the pinion during flexion enjoys a certain degree of repose. The time occupied in recovering is not lost so long as the wing makes an angle with the horizon and the bird is in motion, it being a matter of indifference whether the wing acts on the air, or the air on the wing, so long as the body bearing the latter is under weigh; and this is the chief reason why the albatross, which is a very heavy bird,[95] can sail about for such incredible periods without flapping the wings at all. Captain Hutton thus graphically describes the sailing of this magnificent bird:--“The flight of the albatross is truly majestic, as with outstretched motionless wings he sails over the surface of the sea--now rising high in air, now with a bold sweep, and wings inclined at an angle with the horizon, descending until the tip of the lower one all but touches the crest of the waves as he skims over them.”[96]

[95] The average weight of the albatross, as given by Gould, is 17
lbs.--Ibis, 2d series, vol. i. 1865, p. 295.

[96] “On some of the Birds inhabiting the Southern Ocean,” by Capt.
F. W. Hutton.--Ibis, 2d series, vol. i. 1865, p. 282.

_Birds of Flight divisible into four kinds:--_

_1st._ Such as have heavy bodies and short wings with a rapid movement (fig. 59, p. 126).

_2d._ Such as have light bodies and large wings with a leisurely movement (fig. 60, p. 126; fig. 103, p. 186).

_3d._ Such as have heavy bodies and long narrow wings with a decidedly slow movement (fig. 105, p. 200).

_4th._ Such as are intermediate with regard to the size of body, the dimensions of the wing, and the energy with which it is driven (fig. 102, p. 183; fig. 106, p. 203; fig. 107, p. 204).

They may be subdivided into those which float, skim, or glide, and those which fly in a straight line and irregularly.

The pheasant, partridge (fig. 59, p. 126), grouse, and quail, furnish good examples of the heavy-bodied, short-winged birds. In these the wing is rounded and deeply concave. It is, moreover, wielded with immense velocity and power.

The heron (fig. 60, p. 126), sea-mew (fig. 103, p. 186), lapwing (fig. 63, p. 138), and owl (fig. 104), supply examples of the second class, where the wing, as compared with the body, is very ample, and where consequently it is moved more leisurely and less energetically.

FIG. 104.--The Cape Barn-Owl (_Strix capensis_, Smith), as seen
in full flight, hunting. The under surface of the wings and body
are inclined slightly upwards, and act upon the air after the
manner of a kite. (Compare with fig. 59, p. 126, and fig. 102,
p. 183.)--_Original._]

The albatross (fig. 105, p. 200) and pelican afford instances of the third class, embracing the heavy-bodied, long-winged birds.

The duck (fig. 107, p. 204), pigeon (fig. 106, p. 203), crow and thrush, are intermediate, both as regards the size of the wing and the rapidity with which it is made to oscillate. These constitute the fourth class.

The albatross (fig. 105, p. 200), swallow, eagle, and hawk, provide instances of sailing or gliding birds, where the wing is ample, elongated, and more or less pointed, and where advantage is taken of the weight of the body and the shape of the pinion to utilize the air as a supporting medium. In these the pinion acts as a long lever,[97] and is wielded with great precision and power, particularly at the shoulder.

[97] _Advantages possessed by long Pinions._--The long narrow wings
are most effective as elevators and propellers, from the fact
(pointed out by Mr. Wenham) that at high speeds, with very oblique
incidences, the supporting effect becomes transferred to the _front
edge_ of the pinion. It is in this way “that the effective propelling
area of the two-bladed screw is tantamount to its entire circle
of revolution.” A similar principle was announced by Sir George
Cayley upwards of fifty years ago. “_In very acute angles with the
current_, it appears that the centre of resistance in the sail does
not coincide with the centre of its surface, _but is considerably
in front of it_. As the obliquity of the current decreases, these
centres approach, and coincide when the current becomes perpendicular
to the plane; hence any heel of the machine backwards or forwards
removes the centre of support behind or before the point of
suspension.”--Nicholson’s Journal, vol. xxv. p. 83. When the speed
attained by the bird is _greatly accelerated_, and _the stratum of
air passed over in any given time enormously increased_, the support
afforded by the air to the inclined planes formed by the wings _is
likewise augmented_. This is proved by the rapid flight of skimming
or sailing birds when the wings are moved at long intervals and
very leisurely. The same principle supports the skater as he rushes
impetuously over insecure ice, and the thin flat stone projected
along the surface of still water. The velocity of the movement in
either case prevents sinking by not giving the supporting particles
time to separate.

_The Flight of the Albatross compared to the Movements of a Compass set upon Gimbals._--A careful examination of the movements in skimming birds has led me to conclude that by a judicious twisting or screw-like action of the wings at the shoulder, in which the pinions are alternately advanced towards and withdrawn from the head in a manner analogous to what occurs at the loins in skating without lifting the feet, birds of this order can not only maintain the motion which they secure by a few energetic flappings, but, if necessary, actually increase it, and that without either bending the wing or beating the air.

The forward and backward screwing action of the pinion referred to, in no way interferes, I may remark, with the rotation of the wing on its long axis, the pinion being advanced and screwed down upon the wind, and retracted and unscrewed alternately. As the movements described enable the sailing bird to tilt its body from before backwards, or the converse, and from side to side or laterally, it may be represented as oscillating on one of two centres, as shown at fig. 105; the one corresponding with the long axis of the body (fig. 105, _a b_), the other with the long axis of the wings (_c d_). Between these two extremes every variety of sailing and gliding motion which is possible in the mariner’s compass when set upon gimbals may be performed; so that a skimming or sailing bird may be said to possess perfect command over itself and over the element in which it moves.

Captain Hutton makes the following remarkable statement regarding the albatross:--“I have sometimes watched narrowly one of these birds sailing and wheeling about in all directions for more than an hour, without seeing the slightest movement of the wings, and have never witnessed anything to equal the ease and grace of this bird as he sweeps past, often within a few yards, every part of his body perfectly motionless except the head and eye, which turn slowly and seem to take notice of everything.”[98]

[98] “On some of the Birds inhabiting the Southern Ocean.”--Ibis, 2d
series, vol. i. 1865.

“Tranquil its spirit seem’d and floated slow;
Even in its very motion there was rest.”[99]

[99] Professor Wilson’s Sonnet, “A Cloud,” etc.

As an antithesis to the apparently lifeless wings of the albatross, the ceaseless activity of those of the humming-bird may be adduced. In those delicate and exquisitely beautiful birds, the wings, according to Mr. Gould, move so rapidly when the bird is poised before an object, that it is impossible for the eye to follow each stroke, and a hazy circle of indistinctness on each side of the bird is all that is perceptible. When the humming-bird flies in a horizontal direction, it occasionally proceeds with such velocity as altogether to elude observation.

_The regular and irregular in Flight._--The coot, diver, duck, and goose fly with great regularity in nearly a straight line, and with immense speed; they rarely if ever skim or glide, their wings being too small for this purpose. The woodpecker, magpie, fieldfare and sparrow, supply examples of what may be termed the “irregular” in flight. These, as is well known, fly in curves of greater or less magnitude, by giving a few vigorous strokes and then desisting, the effect of which is to project them along a series of parabolic curves. The snipe and woodcock are irregular in another respect, their flight being sudden, jerky, and from side to side.

_Mode of ascending, descending, turning, etc._--All birds which do not, like the swallow and humming-birds, drop from a height, raise themselves at first by a vigorous leap, in which they incline their bodies in an upward direction, the height thus attained enabling them to extend and depress their wings without injury to the feathers. By a few sweeping strokes delivered downwards and forwards, in which the wings are made nearly to meet above and below the body, they lever themselves upwards and forwards, and in a surprisingly short time acquire that degree of momentum which greatly assists them in their future career. In rising from the ground, as may readily be seen in the crow, pigeon, and kingfisher (fig. 102, p. 183), the tail is expanded and the neck stretched out, so that the body is converted into an inclined plane, and acts mechanically as a kite. The centre of gravity and the position of the body are changed at the will of the bird by movements in the neck, feet, and tail, and by increasing or decreasing the angles which the under surface of the wings makes with the horizon. When a bird wishes to fly in a horizontal direction, it causes the under surface of its wings to make a slight _forward_ angle with the horizon. When it wishes to ascend, the angle is increased. When it wishes to descend, it causes the under surface of the wings to make a slight _backward_ angle with the horizon. When a bird flies up, its wings strike downwards and _forwards_. When it flies down, its wings strike downwards and _backwards_. When a sufficient altitude has been attained, the length of the downward stroke is generally curtailed, the mere extension and flexion of the wing, assisted by the weight of the body, in such instances sufficing. This is especially the case if the bird is advancing against a slight breeze, the effort required under such circumstances being nominal in amount. That little power is expended is proved by the endless gyrations of rooks and other birds; these being continued for hours together. In birds which glide or skim, it has appeared to me that the wing is recovered much more quickly, and the down stroke delivered more slowly, than in ordinary flight--in fact, that the rapidity with which the wing acts in an upward and downward direction is, in some instances, reversed; and this is what we should naturally expect when we recollect that in gliding, the wings require to be, for the most part, in the expanded condition. If this observation be correct, it follows that birds have the power of modifying the duration of the up and down strokes at pleasure. Although the wing of the bird usually strikes the air at an angle which varies from 15° to 30°, the angle may be increased to such an extent as to subvert the position of the bird. The tumbler pigeon, _e.g._ can, by slewing its wings forwards and suddenly throwing back its head, turn a somersault. When birds are fairly on the wing they have the air, unless when that is greatly agitated by a storm, completely under control. This arises from their greater specific gravity, and because they are possessed of independent motion. If they want to turn, they have simply to tilt their bodies laterally, as a railway carriage would be tilted in taking a curve,[100] or to increase the number of beats given by the one wing as compared with the other; or to keep the one wing extended while the other is partially flexed. The neck, feet, and tail may or may not contribute to this result. If the bird wishes to rise, it tilts its entire body (the neck and tail participating) in an upward direction (fig. 59, p. 126; fig. 102, p. 183); or it rises principally by the action of the wings and by muscular efforts, as happens in the lark. The bird can in this manner likewise retain its position in the air, as may be observed in the hawk when hovering above its prey. If the bird desires to descend, it may reverse the direction of the inclined plane formed by the body and wings, and plunge head foremost with extended pinions (fig. 106); or it may flex the wings, and so accelerate its pace; or it may raise its wings and drop parachute-fashion (fig. 55, p. 112; _g_, _g_ of fig. 82, p. 158); or it may even fly in a downward direction--a few sudden strokes, a more or less abrupt curve, and a certain degree of horizontal movement being in either case necessary to break the fall previous to alighting (fig. 107, below). Birds which fish on the wing, as the osprey and gannet, precipitate themselves from incredible heights, and drop into the water with the velocity of a meteorite--the momentum which they acquire during their descent materially aiding them in their subaqueous flight. They emerge from the water and are again upon the wing before the eddies occasioned by their precipitous descent have well subsided, in some cases rising apparently without effort, and in others running along and beating the surface of the water for a brief period with their pinions and feet.

[100] “If the albatross desires to turn to the right he bends
his head and tail slightly upwards, at the same time raising his
left side and wing, and lowering the right in proportion to the
sharpness of the curve he wishes to make, the wings being kept quite
rigid the whole time. To such an extent does he do this, that in
sweeping round, his wings are often pointed in a direction nearly
perpendicular to the sea; and this position of the wings, more or
less inclined to the horizon, is seen always and only when the bird
is turning.”--“On some of the Birds inhabiting the Southern Ocean.”
Ibis, 2d series, vol. i. 1865, p. 227.

FIG. 106.--The Pigeon (_Treron bicincta_, Jerdon), flying downwards
and turning prior to alighting. The pigeon expands its tail both in
ascending and descending.--_Original._]

FIG. 107.--The Red-headed Pochard (_Fuligula ferina_, Linn.)
in the act of dropping upon the water; the head and body being
inclined upwards and forwards, the feet expanded, and the wings
delivering vigorous short strokes in a downward and forward
direction.--_Original._]

_The Flight of Birds referable to Muscular Exertion and Weight._--The various movements involved in ascending, descending, wheeling, gliding, and progressing horizontally, are all the result of muscular power and weight, properly directed and acting upon appropriate surfaces--that apparent buoyancy in birds which we so highly esteem, arising not from superior lightness, but from their possessing that degree of solidity which enables them to subjugate the air,--weight and independent motion, _i.e._ motion associated with animal life, or what is equivalent thereto, being the two things indispensable in successful aërial progression. The weight in insects and birds is in great measure owing to their greatly developed muscular system, this being in that delicate state of tonicity which enables them to act through its instrumentality with marvellous dexterity and power, and to expend or reserve their energies, which they can do with the utmost exactitude, in their apparently interminable flights.

_Lifting-capacity of Birds._--The muscular power in birds is usually greatly in excess, particularly in birds of prey, as, _e.g._ the condors, eagles, hawks, and owls. The eagles are remarkable in this respect--these having been known to carry off young deer, lambs, rabbits, hares, and, it is averred, even young children. Many of the fishing birds, as the pelicans and herons, can likewise carry considerable loads of fish;[101] and even the smaller birds, as the records of spring show, are capable of transporting comparatively large twigs for building purposes. I myself have seen an owl, which weighed a little over 10 ounces, lift 2-1/2 ounces, or a quarter of its own weight, without effort, after having fasted twenty-four hours; and a friend informs me that a short time ago a splendid osprey was shot at Littlehampton, on the coast of Sussex, with a fish 5 lbs. weight in its mouth.

[101] The heron is in the habit, when pursued by the falcon, of
disgorging the contents of his crop in order to reduce his weight.

There are many points in the history and economy of birds which crave our sympathy while they elicit our admiration. Their indubitable courage and miraculous powers of flight invest them with a superior dignity, and secure for their order almost a duality of existence. The swallow, tiny and inconsiderable as it may appear, can traverse 1000 miles at a single journey; and the albatross, despising compass and landmark, trusts himself boldly for weeks together to the mercy or fury of the mighty ocean. The huge condor of the Andes lifts himself by his sovereign will to a height where no sound is heard, save the airy tread of his vast pinions, and, from an unseen point, surveys in solitary grandeur the wide range of plain and pasture-land;[102] while the bald eagle, nothing daunted by the din and indescribable confusion of the queen of waterfalls, the stupendous Niagara, sits composedly on his giddy perch, until inclination or desire prompts him to plunge into or soar above the drenching mists which, shapeless and ubiquitous, perpetually rise from the hissing waters of the nether caldron.

[102] The condor, on some occasions, attains an altitude of six miles.

AËRONAUTICS.

AËRONAUTICS.

The subject of artificial flight, notwithstanding the large share of attention bestowed upon it, has been particularly barren of results. This is the more to be regretted, as the interest which has been taken in it from early Greek and Roman times has been universal. The unsatisfactory state of the question is to be traced to a variety of causes, the most prominent of which are--

_1st_, The extreme difficulty of the problem.

_2d_, The incapacity or theoretical tendencies of those who have devoted themselves to its elucidation.

_3d_, The great rapidity with which wings, especially insect wings, are made to vibrate, and the difficulty experienced in analysing their movements.

_4th_, The great weight of all flying things when compared with a corresponding volume of air.

_5th_, The discovery of the balloon, which has retarded the science of aërostation, by misleading men’s minds and causing them to look for a solution of the problem by the aid of a machine lighter than the air, and which has no analogue in nature.

Flight has been unusually unfortunate in its votaries. It has been cultivated, on the one hand, by profound thinkers, especially mathematicians, who have worked out innumerable theorems, but have never submitted them to the test of experiment; and on the other, by uneducated charlatans who, despising the abstractions of science, have made the most ridiculous attempts at a practical solution of the problem.

Flight, as the matter stands at present, may be divided into two principal varieties which represent two great sects or schools--

_1st_, The Balloonists, or those who advocate the employment of a machine specifically lighter than the air.

_2d_, Those who believe that weight is necessary to flight. The second school may be subdivided into

(_a_) Those who advocate the employment of rigid inclined planes driven forward in a straight line, or revolving planes (aërial screws); and

(_b_) Such as trust for elevation and propulsion to the vertical flapping of wings.

_Balloon._--The balloon, as my readers are aware, is constructed on the obvious principle that a machine lighter than the air must necessarily rise through it. The Montgolfier brothers invented such a machine in 1782. Their balloon consisted of a paper globe or cylinder, the motor power being super-heated air supplied by the burning of vine twigs under it. The Montgolfier or fire balloon, as it was called, was superseded by the hydrogen gas balloon of MM. Charles and Robert, this being in turn supplanted by the ordinary gas balloon of Mr. Green. Since the introduction of coal gas in the place of hydrogen gas, no radical improvement has been effected, all attempts at guiding the balloon having signally failed. This arises from the vast extent of surface which it necessarily presents, rendering it a fair conquest to every breeze that blows; and because the power which animates it is a mere lifting power which, in the absence of wind, must act in a vertical line. The balloon consequently rises through the air in opposition to the law of gravity, very much as a dead bird falls in a downward direction in accordance with it. Having no hold upon the air, this cannot be employed as a fulcrum for regulating its movements, and hence the cardinal difficulty of ballooning as an art.

Finding that no marked improvement has been made in the balloon since its introduction in 1782, the more advanced thinkers have within the last quarter of a century turned their attention in an opposite direction, and have come to regard flying creatures, all of which are much heavier than the air, as the true models for flying machines. An old doctrine is more readily assailed than uprooted, and accordingly we find the followers of the new faith met by the assertion that insects and birds have large air cavities in their interior; that those cavities contain heated air, and that this heated air in some mysterious manner contributes to, if it does not actually produce, flight. No argument could be more fallacious. Many admirable fliers, such as the bats, have no air-cells; while many birds, the apteryx for example, and several animals never intended to fly, such as the orang-outang and a large number of fishes, are provided with them. It may therefore be reasonably concluded that flight is in no way connected with air-cells, and the best proof that can be adduced is to be found in the fact that it can be performed to perfection in their absence.

_The Inclined Plane._--The modern school of flying is in some respects quite as irrational as the ballooning school.

The favourite idea with most is the wedging forward of a rigid _inclined plane_ upon the air by means of a “_vis a tergo_.”

The inclined plane may be made to advance in a _horizontal line_, or made _to rotate_ in the form of a screw. Both plans have their adherents. The one recommends a large supporting area extending on either side of the weight to be elevated; the surface of the supporting area making a very slight angle with the horizon, and the whole being wedged forward by the action of vertical screw propellers. This was the plan suggested by Henson and Stringfellow.

Mr. Henson designed his aërostat in 1843. “The chief feature of the invention was the very great expanse of its sustaining planes, which were larger in proportion to the weight it had to carry than those of many birds. The machine advanced _with its front edge a little raised_, the effect of which was to present its under surface to the air over which it passed, the resistance of which, acting upon it like a strong wind on the sails of a windmill, prevented the descent of the machine and its burden. The sustaining of the whole, therefore, depended upon _the speed at which it travelled through the air, and the angle at which its under surface impinged on the air in its front_.... The machine, fully prepared for flight, was started from the top of an inclined plane, in descending which it attained a velocity necessary to sustain it in its further progress. That velocity would be gradually destroyed by the resistance of the air to forward flight; it was, therefore, the office of the steam-engine and the vanes it actuated simply to repair the loss of velocity; it was made therefore only of the power and weight necessary for that small effect” (fig. 109). The editor of Newton’s Journal of Arts and Science speaks of it thus:--“The apparatus consists of a car containing the goods, passengers, engines, fuel, etc., to which a rectangular frame, made of wood or bamboo cane, and covered with canvas or oiled silk, is attached. This frame extends on either side of the car in a similar manner to the outstretched wings of a bird; but with this difference, that _the frame is immovable_. Behind the wings are two vertical fan wheels, furnished with oblique vanes, which are intended to propel the apparatus through the air. The rainbow-like circular wheels are the propellers, answering to the wheels of a steam-boat, and acting upon the air after the manner of a windmill. These wheels receive motion from bands and pulleys from a steam or other engine contained in the car. To an axis at the stern of the car a triangular frame is attached, resembling the tail of a bird, which is also covered with canvas or oiled silk. This may be expanded or contracted at pleasure, and is moved up and down for the purpose of causing the machine to ascend or descend. Beneath the tail is a rudder for directing the course of the machine to the right or to the left; and to facilitate the steering a sail is stretched between two masts which rise from the car. The amount of canvas or oiled silk necessary for buoying up the machine is stated to be equal to one square foot for each half pound of weight.”

Wenham[103] has advocated the employment of _superimposed planes_, with a view to augmenting the support furnished while it diminishes the horizontal space occupied by the planes. These planes Wenham designates _Aëroplanes_. They are inclined at a very slight angle to the horizon, and are wedged forward either by the weight to be elevated or by the employment of vertical screws. Wenham’s plan was adopted by Stringfellow in a model which he exhibited at the Aëronautical Society’s Exhibition, held at the Crystal Palace in the summer of 1868.

[103] “Aërial Locomotion,” by F. H. Wenham.--_World of Science_, June
1867.

The subjoined woodcut (fig. 110), taken from a photograph of Mr. Stringfellow’s model, gives a very good idea of the arrangement; _a b c_ representing the superimposed planes, _d_ the tail, and _e f_ the vertical screw propellers.

The superimposed planes (_a b c_) in this machine contained a sustaining area of twenty-eight square feet in addition to the tail (_d_).

Its engine represented a third of a horse power, and the weight of the whole (engine, boiler, water, fuel, superimposed planes, and propellers) was under 12 lbs. Its sustaining area, if that of the tail (_d_) be included, was something like thirty-six square feet, _i.e._ three square feet for every pound--the sustaining area of the gannet, it will be remembered (p. 134), being less than one square foot of wing for every two pounds of body.

The model was forced by its propellers along a wire at a great speed, but, so far as I could determine from observation, failed to lift itself notwithstanding its extreme lightness and the comparatively very great power employed.[104]

[104] Mr. Stringfellow stated that his machine occasionally left the
wire, and was sustained by its superimposed planes alone.

The idea embodied by Henson, Wenham, and Stringfellow is plainly that of a boy’s kite sailing upon the wind. The kite, however, is a more perfect flying apparatus than that furnished by Henson, Wenham, and Stringfellow, inasmuch as the inclined plane formed by its body strikes the air at various angles--the angles varying according to the length of string, strength of breeze, length and weight of tail, etc. Henson’s, Wenham’s, and Stringfellow’s methods, although carefully tried, have hitherto failed. The objections are numerous. In the first place, the supporting planes (aëroplanes or otherwise) are not flexible and elastic as wings are, but _rigid_. This is a point to which I wish particularly to direct attention. Second, They strike the air _at a given angle_. Here, again, there is a departure from nature. Third, A machine so constructed must be precipitated from a height or driven along the surface of the land or water at a high speed to supply it with initial velocity. Fourth, It is unfitted for flying with the wind unless its speed greatly exceeds that of the wind. Fifth, It is unfitted for flying across the wind because of the surface exposed. Sixth, The sustaining surfaces are comparatively very large. They are, moreover, passive or dead surfaces, _i.e._ they have no power of moving or accommodating themselves to altered circumstances. Natural wings, on the contrary, present small flying surfaces, the great speed at which wings are propelled converting the space through which they are driven into what is practically a solid basis of support, as explained at pp. 118, 119, 151, and 152 (_vide_ figs. 64, 65, 66, 82, and 83, pp. 139 and 158). This arrangement enables natural wings to seize and utilize the air, and renders them superior to adventitious currents. Natural wings work up the air in which they move, but unless the flying animal desires it, they are scarcely, if at all, influenced by winds or currents which are not of their own forming. In this respect they entirely differ from the balloon and all forms of fixed aëroplanes. In nature, small wings driven at a high speed produce the same result as large wings driven at a slow speed (compare fig. 58, p. 125, with fig. 57, p. 124). In flight a certain space must be covered either by large wings spread out as a solid (fig. 57, p. 124), or by small wings vibrating rapidly (figs. 64, 65, and 66, p. 139).

_The Aërial Screw._--Our countryman, Sir George Cayley, gave the first practical illustration of the efficacy of the screw as applied to the air in 1796. In that year he constructed a small machine, consisting of two screws made of quill feathers (fig. 111). Sir George writes as under:--

“As it may be an amusement to some of your readers to see a machine rise in the air by mechanical means, I will conclude my present communication by describing an instrument of this kind, which any one can construct at the expense of ten minutes’ labour.

“_a_ and _b_ (fig. 111, p. 215) are two corks, into each of which are inserted four wing feathers from any bird, so as to be slightly inclined like the sails of a windmill, but in opposite directions in each set. A round shaft is fixed in the cork _a_, which ends in a sharp point. At the upper part of the cork _b_ is fixed a whalebone bow, having a small pivot hole in its centre to receive the point of the shaft. The bow is then to be strung equally on each side to the upper portion of the shaft, and the little machine is completed. Wind up the string by turning the flyers different ways, so that the spring of the bow may unwind them with their anterior edges ascending; then place the cork with the bow attached to it upon a table, and with a finger on the upper cork press strong enough to prevent the string from unwinding, and, taking it away suddenly, the instrument will rise to the ceiling.”

Cayley’s screws were peculiar, inasmuch as they were superimposed and rotated in opposite directions. He estimated that if the area of the screws was increased to 200 square feet, and moved by a man, they would elevate him. Cayley’s interesting experiment is described at length, and the apparatus figured in Nicholson’s Journal for 1809, p. 172. In 1842 Mr. Phillips also succeeded in elevating a model by means of revolving fans. Mr. Phillips’s model was made entirely of metal, and when complete and charged weighed 2 lbs. It consisted of a boiler or steam generator and four fans supported between eight arms. The fans were inclined to the horizon at an angle of 20°, and through the arms the steam rushed on the principle discovered by Hero of Alexandria. By the escape of steam from the arms, the fans were made to revolve with immense energy, so much so that the model rose to a great altitude, and flew across two fields before it alighted. The motive power employed in the present instance was obtained from the combustion of charcoal, nitre, and gypsum, as used in the original fire annihilator; the products of combustion mixing with water in the boiler, and forming gas charged steam, which was delivered at a high pressure from the extremities of the eight arms. This model is remarkable as being probably the first which actuated by steam has flown to a considerable distance.[105] The French have espoused the aërial screw with great enthusiasm, and within the last ten years (1863) MM. Nadar,[106] Pontin d’Amécourt, and de la Landelle have constructed clockwork models (_orthopteres_), which not only raise themselves into the air, but carry a certain amount of freight. These models are exceedingly fragile, and because of the prodigious force required to propel them usually break after a few trials. Fig. 112, p. 217, embodies M. de la Landelle’s ideas.

[105] Report on the First Exhibition of the Aëronautical Society of
Great Britain, held at the Crystal Palace, London, in June 1868,
p. 10.

[106] Mons. Nadar, in a paper written in 1863, enters very fully into
the subject of artificial flight, as performed by the aid of the
screw. Liberal extracts are given from Nadar’s paper in Astra Castra,
by Captain Hatton Turner. London, 1865, p. 340. To Turner’s handsome
volume the reader is referred for much curious and interesting
information on the subject of Aërostation.