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).
[Illustration: FIG. 111.--Cayley’s Flying Apparatus.]
_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.
[Illustration: FIG. 112.--Flying Machine designed by M. de la Landelle.]
In the helicopteric models made by MM. Nadar, Pontin d’Amécourt, and de
la Landelle, the screws (_m n o p q r s t_ of figure) are arranged in
tiers, _i.e._ the one screw is placed above the other. In this respect
they resemble the aëroplanes recommended by Mr. Wenham, and tested by
Mr. Stringfellow (compare _m n o p q r s t_ of fig. 112, with _a b c_
of fig. 110, p. 213). The superimposed screws, as already explained,
were first figured and described by Sir George Cayley (p. 215). The
French screws, and that employed by Mr. Phillips, are _rigid or
unyielding_, and strike the air _at a given angle_, and herein, I
believe, consists their principal defect. This arrangement results in
a ruinous expenditure of power, and is accompanied by a great amount
of slip. The aërial screw, and the machine to be elevated by it, can
be set in motion without any preliminary run, and in this respect
it has the advantage over the machine supported by mere sustaining
planes. It has, in fact, a certain amount of inherent motion, its
screws revolving, and supplying it with active or moving surfaces. It
is accordingly more independent than the machine designed by Henson,
Wenham, and Stringfellow.
I may observe with regard to the system of rigid inclined planes wedged
forward at a given angle in a straight line or in a circle, that it
does not embody the principle carried out in nature.
The wing of a flying creature, as I have taken pains to show, is _not
rigid_; neither does it always strike the air _at a given angle_. On
the contrary, it is capable of moving in all its parts, and attacks
the air at _an infinite variety of angles_ (pp. 151 to 154). Above
all, the surface exposed by a natural wing, when compared with the
great weight it is capable of elevating, is remarkably small (fig. 89,
p. 171). This is accounted for by the length and the great range of
motion of natural wings; the latter enabling the wings to convert large
tracts of air into supporting areas (figs. 64, 65, and 66, p. 139). It
is also accounted for by the multiplicity of the movements of natural
wings, these enabling the pinions to create and rise upon currents of
their own forming, and to avoid natural currents when not adapted for
propelling or sustaining purposes (fig. 67, 68, 69, and 70, p. 141).
If any one watches an insect, a bat, or a bird when dressing its wings,
he will observe that it can incline the under surface of the wing at
a great variety of angles to the horizon. This it does by causing the
posterior or thin margin of the wing to rotate around the anterior or
thick margin as an axis. As a result of this movement, the two margins
are forced into double and opposite curves, and the wing converted into
_a plastic helix_ or _screw_. He will further observe that the bat and
bird, and some insects, have, in addition, the power of folding and
drawing the wing towards the body during the up stroke, and of pushing
it away from the body and extending it during the down stroke, so as
alternately to diminish and increase its area; arrangements necessary
to decrease the amount of resistance experienced by the wing during its
ascent, and increase it during its descent. It is scarcely requisite
to add, that in the aëroplanes and aërial screws, as at present
constructed, no provision whatever is made for suddenly increasing or
diminishing the flying surface, of conferring elasticity upon it, or of
giving to it that infinite variety of angles which would enable it to
seize and disentangle itself from the air with the necessary rapidity.
Many investigators are of opinion that flight is a mere question of
levity and power, and that if a machine could only be made light enough
and powerful enough, it must of necessity fly, whatever the nature of
its flying surfaces. A grave fallacy lurks here. Birds are not more
powerful than quadrupeds of equal size, and Stringfellow’s machine,
which, as we have seen, only weighed 12 lbs., exerted _one-third of a
horse power_. The probabilities therefore are, that flight is dependent
to a great extent on the nature of the flying surfaces, and the mode of
applying those surfaces to the air.
_Artificial Wings_ (Borelli’s Views).--With regard to the production of
_flight by the flapping of wings_, much may and has been said. Of all
the methods yet proposed, it is unquestionably by far the most ancient.
Discrediting as apocryphal the famous story of Dædalus and his waxen
wings, we certainly have a very graphic account of artificial wings in
the De Motu Animalium of Borelli, published as far back as 1680, _i.e._
nearly two centuries ago.[107]
[107] Borelli, De Motu Animalium. Sm. 4to, 2 vols. Romæ, 1680.
Indeed it will not be too much to affirm, that to this distinguished
physiologist and mathematician belongs almost all the knowledge we
possessed of artificial wings up till 1865. He was well acquainted with
the properties of the wedge, as applied to flight, and he was likewise
cognisant of the flexible and elastic properties of the wing. To him
is to be traced the purely mechanical theory of the wing’s action. He
figured a bird with artificial wings, each wing consisting of _a rigid
rod in front_ and _flexible feathers_ behind. I have thought fit to
reproduce Borelli’s figure both because of its great antiquity, and
because it is eminently illustrative of his text.[108]
[108] De Motu Animalium, Lugduni Batavorum apud Petrum Vander. Anno
MDCLXXXV. Tab. XIII. figure 2. (New edition.)
[Illustration: FIG. 113.--Borelli’s Artificial Bird.]
The wings (_b c f_, _o e a_), are represented as striking vertically
downwards (_g h_). They remarkably accord with those described by
Straus-Durckheim, Girard, and quite recently by Professor Marey.[109]
[109] Revue des Cours Scientifiques de la France et de l’Etranger.
Mars 1869.
Borelli is of opinion that flight results from the application of an
inclined plane, which beats the air, and which has a wedge action.
He, in fact, endeavours to prove that a bird wedges itself forward
upon the air by the perpendicular vibration of its wings, the wings
during their action forming a wedge, the base of which (_c b e_) is
directed towards the head of the bird; the apex (_a f_) being directed
towards the tail. This idea is worked out in propositions 195 and 196
of the first part of Borelli’s book. In proposition 195 he explains
how, if a wedge be driven into a body, the wedge will tend to separate
that body into two portions; but that if the two portions of the body
be permitted to react upon the wedge, they will communicate _oblique
impulses_ to the sides of the wedge, and expel it, base first, in a
straight line.
Following up the analogy, Borelli endeavours to show in his 196th
proposition, “that if the air acts obliquely upon the wings, or the
wings obliquely upon the air (which is, of course, a wedge action), the
result will be _a horizontal transference of the body of the bird_.”
In the proposition referred to (196) Borelli states--“If the expanded
wings of a bird suspended in the air shall strike the undisturbed air
beneath it with a motion _perpendicular to the horizon_, the bird will
fly _with a transverse motion_ in a plane parallel with the horizon.”
In other words, if the wings _strike vertically downwards_, the bird
will fly _horizontally forwards_. He bases his argument upon the belief
that the anterior margins of the wings are _rigid and unyielding_,
whereas the posterior and after parts of the wings are _more or less
flexible_, and readily give way under pressure. “If,” he adds, “the
wings of the bird be expanded, and the under surfaces of the wings be
struck by the air _ascending perpendicularly to the horizon_, with
such a force as shall prevent the bird gliding downwards (_i.e._ with
a tendency to glide downwards) from falling, it will be urged _in
a horizontal direction_. This follows because the two osseous rods
(virgæ) forming the anterior margins of the wings resist the upward
pressure of the air, and so retain their original form (literally
extent or expansion), whereas the flexible after-parts of the wings
(posterior margins) are pushed up and approximated to form a cone, the
apex of which (_vide_ _a f_ of fig. 113) is directed towards the tail
of the bird. In virtue of the air playing upon and compressing the
sides of the wedge formed by the wings, the wedge is driven forwards
in the direction of its base (_c b e_), which is equivalent to saying
that the wings carry the body of the bird to which they are attached
_in a horizontal direction_.”
Borelli restates the same argument in different words, as follows:--
“If,” he says, “the air under the wings be struck by the flexible
portions of the wings (_flabella_, literally fly-flaps or small fans)
with a motion perpendicular to the horizon, the sails (vela) and
flexible portions of the wings (flabella) will yield in an upward
direction, and form a wedge, the point of which is directed towards the
tail. Whether, therefore, the air strikes the wings from below, or the
wings strike the air from above, the result is the same--the posterior
or flexible margins of the wings _yield in an upward direction_, and in
so doing urge the bird in a _horizontal direction_.”
In his 197th proposition, Borelli follows up and amplifies the
arguments contained in propositions 195 and 196. “Thus,” he observes,
“it is evident that the object of flight is to impel birds upwards,
and keep them suspended in the air, and also to enable them to wheel
round in a plane parallel to the horizon. The first (or upward flight)
could not be accomplished unless the bird were impelled upwards by
frequent leaps or vibrations of the wings, and its descent prevented.
And because the downward tendency of heavy bodies is perpendicular to
the horizon, the vibration of the plain surfaces of the wings must be
made by striking the air beneath them in a direction perpendicular to
the horizon, and in this manner nature produces the suspension of birds
in the air.”
“With regard to the second or transverse motion of birds (_i.e._
horizontal flight) some authors have strangely blundered; for they
hold that it is like that of boats, which, being impelled by oars,
moved horizontally in the direction of the stern, and pressing on
the resisting water behind, leaps with a contrary motion, and so are
carried forward. In the same manner, say they, the wings vibrate
towards the tail with a horizontal motion, and likewise strike against
the undisturbed air, by the resistance of which they are moved forward
by a reflex motion. But this is contrary to the evidence of our sight
as well as to reason; for we see that the larger kinds of birds, such
as swans, geese, etc., never vibrate their wings when flying towards
the tail with a horizontal motion like that of oars, but always bend
them downwards, and so describe circles raised perpendicularly to the
horizon.[110]
[110] It is clear from the above that Borelli did not know that the
wings of birds strike _forwards_ as well as downwards during the down
stroke, and _forwards_ as well as upwards during the up stroke. These
points, as well as the twisting and untwisting figure-of-8 action of
the wing, were first described by the author. Borelli seems to have
been equally ignorant of the fact that the wings of insects vibrate
in a more or less horizontal direction.
“Besides, in boats the horizontal motion of the oars is easily made,
and a perpendicular stroke on the water would be perfectly useless,
inasmuch as their descent would be impeded by the density of the water.
But in birds, such a horizontal motion (which indeed would rather
hinder flight) would be absurd, since it would cause the ponderous bird
to fall headlong to the earth; whereas it can only be suspended in the
air by constant vibration of the wings _perpendicular to the horizon_.
Nature was thus forced to show her marvellous skill in producing a
motion which, by one and the same action, should suspend the bird
in the air, and carry it forward in a horizontal direction. This is
effected by striking the air below perpendicularly to the horizon, but
with oblique strokes--an action which is rendered possible only by the
flexibility of the feathers, for the fans of the wings in the act of
striking acquire the form of a wedge, by the forcing out of which the
bird is necessarily moved forwards in a horizontal direction.”
The points which Borelli endeavours to establish are these:--
First, That the action of the wing is a wedge action.
Second, That the wing consists of two portions--_a rigid_ anterior
portion, and a _non-rigid_ flexible portion. The rigid portion he
represents in his artificial bird (fig. 113, p. 220) as consisting of
_a rod_ (_e r_), the yielding portion of _feathers_ (_a o_).
Third, That if the air strikes the under surface of the wing
perpendicularly in a direction from below upwards, the flexible portion
of the wing will yield in an upward direction, and form a wedge with
its neighbour.
Fourth, Similarly and conversely, if the wing strikes the air
perpendicularly from above, the posterior and flexible portion of the
wing will yield and be forced in an upward direction.
Fifth, That this _upward yielding_ of the posterior or flexible margin
of the wing results in and necessitates _a horizontal transference_ of
the body of the bird.
Sixth, That to sustain a bird in the air the wings must strike
_vertically downwards_, as this is the direction in which a heavy body,
if left to itself, would fall.
Seventh, That to propel the bird in a horizontal direction, the wings
must descend in a perpendicular direction, and the posterior or
flexible portions of the wings _yield in an upward direction_, and in
such a manner as virtually to communicate _an oblique action_ to them.
Eighth, That the feathers of the wing are _bent in an upward direction_
when the wing _descends_, the upward bending of the elastic feathers
contributing to the horizontal travel of the body of the bird.
I have been careful to expound Borelli’s views for several reasons:--
_1st_, Because the purely mechanical theory of the wing’s action is
clearly to be traced to him.
_2d_, Because his doctrines have remained unquestioned for nearly two
centuries, and have been adopted by all the writers since his time,
without, I regret to say in the majority of cases, any acknowledgment
whatever.
_3d_, Because his views have been revived by the modern French school;
and
_4th_, Because, in commenting upon and differing from Borelli, I will
necessarily comment upon and differ from all his successors.
_As to the Direction of the Stroke, yielding of the Wing, etc._--The
Duke of Argyll[111] agrees with Borelli in believing that the wing
invariably strikes _perpendicularly downwards_. His words are--“Except
for the purpose of arresting their flight birds can never strike except
_directly downwards_; that is, against the opposing force of gravity.”
Professor Owen in his Comparative Anatomy, Mr. Macgillivray in his
British Birds, Mr. Bishop in his article “Motion” in the Cyclopedia
of Anatomy and Physiology, and M. Liais “On the Flight of Birds and
Insects” in the Annals of Natural History, all assert that the stroke
is delivered _downwards_ and more or less _backwards_.
[111] “Reign of Law”--Good Words, 1865.
To obtain an _upward_ recoil, one would naturally suppose all that is
required is a _downward_ stroke, and to obtain an _upward and forward_
recoil, one would naturally conclude a _downward and backward_ stroke
alone is requisite. Such, however, is not the case.
In the first place, a natural wing, or a properly constructed
artificial one, cannot be depressed either _vertically downwards_, or
_downwards and backwards_. It will of necessity descend _downwards and
forwards in a curve_. This arises from its being flexible and elastic
throughout, and in especial from its being carefully graduated as
regards thickness, the tip being thinner and more elastic than the
root, and the posterior margin than the anterior margin.
In the second place, there is only one direction in which the wing
could strike so at once _to support and carry the bird forward_. The
bird, when flying, is a body in motion. It has therefore acquired
momentum. If a grouse is shot on the wing _it does not fall vertically
downwards_, as Borelli and his successors assume, but _downwards and
forwards_. The flat surfaces of the wings are consequently made to
strike downwards and forwards, as they in this manner act as kites
to the falling body, which they bear, or tend to bear, _upwards and
forwards_.
So much for the direction of the stroke during the descent of the wing.
Let us now consider to what extent the posterior margin of the wing
yields in _an upward direction_ when the wing descends. Borelli does
not state the exact amount. The Duke of Argyll, who believes with
Borelli that the posterior margin of the wing is elevated during the
down stroke, avers that, “whereas the air compressed in the hollow of
the wing cannot pass through the wing owing to the closing upwards of
the feathers against each other, or escape forwards because of the
rigidity of the bones and of the quills in this direction, it passes
backwards, and in so doing _lifts by its force the elastic ends of the
feathers_. In passing backwards it communicates to the whole line of
both wings a corresponding push forwards to the body of the bird. The
same volume of air is thus made, in accordance with the law of action
and reaction, _to sustain the bird and carry it forward_.”[112] Mr.
Macgillivray observes that “to progress _in a horizontal direction_
it is necessary that the downward stroke should be modified _by
the elevation in a certain degree of the free extremities of the
quills_.”[113]
[112] “Reign of Law”--Good Words, February 1865, p. 128.
[113] History of British Birds. Lond. 1837, p. 43.