(_Ibis_, April 1872), refers to some of the experiments detailed above,
and endeavours to frame a theory of flight, which differs in some
respects from my own. His remarks are singularly inappropriate, and
illustrate in a forcible manner the old adage, “A little knowledge is
a dangerous thing.” If Captain Hutton had taken the trouble to look
into my memoir “On the Physiology of Wings,” communicated to the Royal
Society of Edinburgh, on the 2d of August 1870,[71] fifteen months
before his own paper was written, there is reason to believe he would
have arrived at very different conclusions. Assuredly he would not have
ventured to make the rash statements he has made, the more especially
as he attempts to controvert my views, which are based upon anatomical
research and experiment, without making any dissections or experiments
of his own.
[71] “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. of Edinburgh, vol. xxvi.
_The Wing area decreases as the Size and Weight of the Volant Animal
increases._--While, as explained in the last section, no definite
relation exists between the weight of a flying animal and the size
of its flying surfaces, there being, as stated, heavy bodied and
small-winged insects, bats, and birds, and the converse; and while, as
I have shown by experiment, flight is possible within a wide range, the
wings being, as a rule, in excess of what are required for the purposes
of flight; still it appears, from the researches of M. de Lucy, that
there is a general law, to the effect that the larger the volant animal
the smaller by comparison are its flying surfaces. The existence
of such a law is very encouraging as far as artificial flight is
concerned, for it shows that the flying surfaces of a large, heavy,
powerful flying machine will be comparatively small, and consequently
comparatively compact and strong. This is a point of very considerable
importance, as the object desiderated in a flying machine is elevating
capacity.
M. de Lucy has tabulated his results, which I subjoin:[72]--
[72] “On the Flight of Birds, of Bats, and of Insects, in reference
to the subject of Aërial Locomotion,” by M. de Lucy, Paris.
+------------------------------------------------------+
| INSECTS. |
+------------------------------+-----------------------+
| | Referred to the |
| | kilogramme |
| NAMES. |= 2lbs. 8oz. 3dwt. 2gr.|
| | Avoird. |
| | = 2lbs. 3oz. 4·428dr. |
+------------------------------+-----------------------+
| | sq. ft. in. |
| | yds. |
|Gnat, | 11 8 92 |
|Dragon-fly (small), | 7 2 56 |
|Coccinella (Lady-bird), | 5 13 87 |
|Dragon-fly (common), | 5 2 89 |
|Tipula, or Daddy-long-legs, | 3 5 11 |
|Bee, | 1 2 74-1/2 |
|Meat-fly, | 1 3 54-1/2 |
|Drone (blue), | 1 2 20 |
|Cockchafer, | 1 2 50 |
|Lucanus} Stag beetle (female),| 1 1 39-1/2 |
| cervus} Stag-beetle (male), | 0 8 33 |
|Rhinoceros-beetle, | 0 6 122-1/2 |
+------------------------------+-----------------------+
+------------------------------------------------------+
| BIRDS. |
+------------------------------+-----------------------+
| | Referred |
| NAMES. | to the |
| | kilogramme. |
+------------------------------+-----------------------+
| | sq. |
| | yds. ft. in. |
| Swallow, | 1 1 104-1/2 |
| Sparrow, | 0 5 142-1/2 |
| Turtle-dove, | 0 4 100-1/2 |
| Pigeon, | 0 2 113 |
| Stork, | 0 2 20 |
| Vulture, | 0 1 116 |
| Crane of Australia, | 0 0 139 |
+------------------------------+-----------------------+
“It is easy, by aid of this table, to follow the order, always
decreasing, of the surfaces, in proportion as the winged animal
increases in size and weight. Thus, in comparing the insects with one
another, we find that the gnat, which weighs 460 times less than the
stag-beetle, has fourteen times more of surface. The lady-bird weighs
150 times less than the stag-beetle, and possesses five times more of
surface. It is the same with the birds. The sparrow weighs about ten
times less than the pigeon, and has twice as much surface. The pigeon
weighs about eight times less than the stork, and has twice as much
surface. The sparrow weighs 339 times less than the Australian crane,
and possesses seven times more surface. If now we compare the insects
and the birds, the gradation will become even much more striking. The
gnat, for example, weighs 97,000 times less than the pigeon, and has
forty times more surface; it weighs 3,000,000 times less than the crane
of Australia, and possesses 149 times more of surface than this latter,
the weight of which is about 9 kilogrammes 500 grammes (25 lbs. 5 oz.
9 dwt. troy, 20 lbs. 15 oz. 2-1/4 dr. avoirdupois).
“The Australian crane is the heaviest bird that I have weighed. It
is that which has the smallest amount of surface, for, referred to
the kilogramme, it does not give us a surface of more than 899 square
centimetres (139 square inches), that is to say about an eleventh part
of a square metre. But every one knows that these grallatorial animals
are excellent birds of flight. Of all travelling birds they undertake
the longest and most remote journeys. They are, in addition, the eagle
excepted, the birds which elevate themselves the highest, and the
flight of which is the longest maintained.”[73]
[73] M. de Lucy, _op. cit._
Strictly in accordance with the foregoing, are my own measurements of
the gannet and heron. The following details of weight, measurement,
etc., of the gannet were supplied by an adult specimen which I
dissected during the winter of 1869. Entire weight, 7 lbs. (minus 3
ounces); length of body from tip of bill to tip of tail, three feet
four inches; head and neck, one foot three inches; tail, twelve inches;
trunk, thirteen inches; girth of trunk, eighteen inches; expanse of
wing from tip to tip across body, six feet; widest portion of wing
across primary feathers, six inches; across secondaries, seven inches;
across tertiaries, eight inches. Each wing, when carefully measured
and squared, gave an area of 19-1/2 square inches. The wings of the
gannet, therefore, furnish a supporting area of three feet three inches
square. As the bird weighs close upon 7 lbs., this gives something like
thirteen square inches of wing for every 36-1/3 ounces of body, _i.e._
one foot one square inch of wing for every 2 lbs. 4-1/3 oz. of body.
The heron, a specimen of which I dissected at the same time, gave a
very different result, as the subjoined particulars will show. Weight
of body, 3 lbs. 3 ounces; length of body from tip of bill to tip of
tail, three feet four inches; head and neck, two feet; tail, seven
inches; trunk, nine inches; girth of body, twelve inches; expanse of
wing from tip to tip across the body, five feet nine inches; widest
portion of wing across primary and tertiary feathers, eleven inches;
across secondary feathers, twelve inches.
Each wing, when carefully measured and squared, gave an area of
twenty-six square inches. The wings of the heron, consequently, furnish
a supporting area of four feet four inches square. As the bird only
weighs 3 lbs. 3 ounces, this gives something like twenty-six square
inches of wing for every 25-1/2 ounces of bird, or one foot 5-1/4
inches square for every 1 lb. 1 ounce of body.
In the gannet there is only one foot one square inch of wing for
every 2 lbs. 4-1/3 ounces of body. The gannet has, consequently, less
than half of the wing area of the heron. The gannet’s wings are,
however, long narrow wings (those of the heron are broad), which
extend transversely across the body; and these are found to be the
most powerful--the wings of the albatross--which measure fourteen feet
from tip to tip (and only one foot across), elevating 18 lbs. without
difficulty. If the wings of the gannet, which have a superficial
area of three feet three inches square, are capable of elevating
7 lbs., while the wings of the heron, which have a superficial area
of four feet four inches, can only elevate 3 lbs., it is evident
(seeing the wings of both are twisted levers, and formed upon a common
type) that the gannet’s wings must be vibrated with greater energy
than the heron’s wings; and this is actually the case. The heron’s
wings, as I have ascertained from observation, make 60 down and 60
up strokes every minute; whereas the wings of the gannet, when the
bird is flying in a straight line to or from its fishing-ground, make
close upon 150 up and 150 down strokes during the same period. The
wings of the divers, and other short-winged, heavy-bodied birds, are
urged at a much higher speed, so that comparatively small wings can
be made to elevate a comparatively heavy body, if the speed only be
increased sufficiently.[74] Flight, therefore, as already indicated,
is a question of power, speed, and small surfaces _versus_ weight.
Elaborate measurements of wing, area, and minute calculations of speed,
can consequently only determine the minimum of wing for elevating the
maximum of weight--flight being attainable within a comparatively wide
range.
[74] The grebes among birds, and the beetles among insects, furnish
examples where small wings, made to vibrate at high speeds, are
capable of elevating great weights.
_Wings, their Form, etc.; all Wings Screws, structurally and
functionally._--Wings vary considerably as to their general contour;
some being falcated or scythe-like, some oblong, some rounded or
circular, some lanceolate, and some linear.[75]
[75] “The wing is short, broad, convex, and rounded in grouse,
partridges, and other rasores; long, broad, straight, and pointed
in most pigeons. In the peregrine falcon it is acuminate, the
second quill being longest, and the first little shorter; and in
the swallows this is still more the case, the first quill being the
longest, the rest rapidly diminishing in length.”--Macgillivray,
Hist. Brit. Birds, vol. i. p. 82. “The hawks have been classed as
noble or ignoble, according to the length and sharpness of their
wings; and the falcons, or long-winged hawks, are distinguished from
the short-winged ones by the second feather of the wing being either
the longest or equal in length to the third, and by the nature of the
stoop made in pursuit of their prey.”--Falconry in the British Isles,
by F. H. Salvin and W. Brodrick. Lond. 1855, p. 28.
[Illustration:
FIG. 61.--Right wing of the Kestrel, drawn from the specimen, while
being held against the light. Shows how the primary (_b_), secondary
(_a_), and tertiary (_c_) feathers overlap and buttress or support
each other in every direction. Each set of feathers has its coverts
and subcoverts, the wing being conical from within outwards, and from
before backwards. _d_, _e_, _f_ Anterior or thick margin of wing.
_b_, _a_, _c_ Posterior or thin margin. The wing of the kestrel is
intermediate as regards form, it being neither rounded as in the
partridge (fig. 96, p. 176), nor ribbon-shaped as in the albatross
(fig. 62), nor pointed as in the swallow. The feathers of the
kestrel’s wing are unusually symmetrical and strong. Compare with
figs. 92, 94, and 96, pp. 174, 175, and 176.--_Original._]
All wings are constructed upon a common type. They are in every
instance carefully graduated, the wing tapering from the root towards
the tip, and from the anterior margin in the direction of the posterior
margin. They are of a generally triangular form, and twisted upon
themselves in the direction of their length, to form a helix or screw.
They are convex above and concave below, and more or less flexible
and elastic throughout, the elasticity being greatest at the tip and
along the posterior margin. They are also moveable in all their parts.
Figs. 61, 62, 63 (p. 138), 59 and 60 (p. 126), 96 and 97 (p. 176),
represent typical bird wings; figs. 17 (p. 36), 94 and 95 (p. 175),
typical bat wings; and figs. 57 and 58 (p. 125), 89 and 90 (p. 171), 91
(p. 172), 92 and 93 (p. 174), typical insect wings.
In all the wings which I have examined, whether in the insect, bat,
or bird, the wing is recovered, flexed, or drawn towards the body
by the action of elastic ligaments, these structures, by their mere
contraction, causing the wing, when fully extended and presenting its
maximum of surface, to resume its position of rest and plane of least
resistance. The principal effort required in flight is, therefore, made
during extension, and at the beginning of the down stroke. The elastic
ligaments are variously formed, and the amount of contraction which
they undergo is in all cases accurately adapted to the size and form
of the wing, and the rapidity with which it is worked; the contraction
being greatest in the short-winged and heavy-bodied insects and birds,
and least in the light-bodied and ample-winged ones, particularly such
as skim or glide. The mechanical action of the elastic ligaments, I
need scarcely remark, insures an additional period of repose to the
wing at each stroke; and this is a point of some importance, as showing
that the lengthened and laborious flights of insects and birds are not
without their stated intervals of rest.
[Illustration:
FIG. 62.--Left wing of the albatross. _d_, _e_, _f_ Anterior or
thick margin of pinion. _b_, _a_, _c_ Posterior or thin margin,
composed of the primary (_b_), secondary (_a_), and tertiary (_c_)
feathers. In this wing the first primary is the longest, the primary
coverts and subcoverts being unusually long and strong. The secondary
coverts and subcoverts occupy the body of the wing (_e_, _d_), and
are so numerous as effectually to prevent any escape of air between
them during the return or up stroke. This wing, which I have in my
possession, measures over six feet in length.--_Original._]
All wings are furnished at their roots with some form of universal
joint which enables them to move not only in an upward, downward,
forward, or backward direction, but also at various intermediate
degrees of obliquity. 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 and the down stroke, and gradually decreasing in an opposite
direction during flexion and the up stroke.
[Illustration:
FIG. 63.--The Lapwing, or Green Plover (_Vanellus cristatus_, Meyer),
with one wing (_c b_, _d´ e´ f´_) fully extended, and forming a
long lever; the other (_d e f_, _c b_) being in a flexed condition
and forming a short lever. In the extended wing the anterior or
thick margin (_d´ e´ f´_) is directed _upwards_ and _forwards_
(_vide_ arrow), the posterior or thin margin (_c_, _b_) _downwards_
and _backwards_. The reverse of this happens during flexion, the
anterior or thick margin (_d_, _e_, _f_) being directed _downwards_
and _forwards_ (_vide_ arrow), the posterior or thin margin (_c b_)
bearing the rowing-feathers _upwards_ and _backwards_. The wings
therefore twist in opposite directions during extension and flexion;
and this is a point of the utmost importance in the action of all
wings, as it enables the volant animal to rotate the wings on and
off the air, and to present at one time (in extension) resisting,
kite-like surfaces, and at another (in flexion) knife-like and
comparatively non-resisting surfaces. It rarely happens in flight
that the wing (_d e f_, _c b_) is so fully flexed as in the figure.
As a consequence, the under surface of the wing is, as a rule,
inclined upwards and forwards, even in flexion, so that it acts
as a kite in extension and flexion, and during the up and down
strokes.--_Original._]
In the insect the oblique surfaces are due to the conformation of the
shoulder-joint, this being furnished with a system of check-ligaments,
and with horny prominences or stops, set, as nearly as may be, at right
angles to each other. The check-ligaments and horny prominences are
so arranged that when the wing is made to vibrate, it is also made to
rotate in the direction of its length, in the manner explained.
In the bat and bird the oblique surfaces are produced by the spiral
configuration of the articular surfaces of the bones of the wing, and
by the rotation of the bones of the arm, forearm, and hand, upon their
long axes. The reaction of the air also assists in the production of
the oblique surfaces.
[Illustration:
FIG. 64. FIG. 64 shows left wing (_a_, _b_) of wasp in the act of
twisting upon itself, the tip of the wing describing a figure-of-8
track (_a_, _c_, _b_). From nature.--_Original._]
[Illustration: FIG. 65. FIG. 66.
FIGS. 65 and 66 show right wing of blue-bottle fly rotating
on its anterior margin, and twisting to form double or figure-of-8
curves (_a b_, _c d_). From nature.--_Original._]
That the wing twists upon itself structurally, not only in the insect,
but also in the bat and bird, any one may readily satisfy himself by a
careful examination; and that it twists upon itself during its action
I have had the most convincing and repeated proofs (figs. 64, 65, and
66). The twisting in question is most marked in the posterior or thin
margin of the wing, the anterior and thicker margin performing more
the part of an axis. As a result of this arrangement, the anterior or
thick margin cuts into the air quietly, and as it were by stealth,
the posterior one producing on all occasions a violent commotion,
especially perceptible if a flame be exposed behind the vibrating
wing. Indeed, it is a matter for surprise that the spiral conformation
of the pinion, and its spiral mode of action, should have eluded
observation so long; and I shall be pardoned for dilating upon the
subject when I state my conviction that it forms the fundamental
and distinguishing feature in flight, and must be taken into account
by all who seek to solve this most involved and interesting problem
by artificial means. The importance of the twisted configuration
or screw-like form of the wing 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 nature; in fact, one entire rowing
feather is equivalent--morphologically 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 pinion may consequently be
removed without materially affecting either its form or its functions.
This is proved by making sections in various directions, and by
finding, as has been already shown, that in some instances as much as
two-thirds of the wing may be lopped off without visibly impairing
the power of flight. The spiral nature of the pinion is most readily
recognised when the wing is seen from behind and from beneath, and when
it is foreshortened. It is also well marked in some of the long-winged
oceanic birds when viewed from before (figs. 82 and 83, p. 158), and
cannot escape detection under any circumstances, if sought for,--the
wing being essentially composed of a congeries of curves, remarkable
alike for their apparent simplicity and the subtlety of their detail.
_The Wing during its action reverses its Planes, and describes a
Figure-of-8 track in space._--The twisting or rotating of the wing
on its long axis is particularly observable during extension and
flexion in the bat and bird, and likewise in the insect, especially
the beetle, cockroach, and such as fold their wings during repose. In
these in extreme flexion the anterior or thick margin of the wing
is directed downwards, and the posterior or thin one upwards. In the
act of extension, the margins, in virtue of the wing rotating upon
its long axis, reverse their positions, the anterior or thick margins
describing a spiral course from below upwards, the posterior or thin
margin describing a similar but opposite course from above downwards.
These conditions, I need scarcely observe, are reversed during flexion.
The movements of the margins during flexion and extension may be
represented with a considerable degree of accuracy by a figure-of-8
laid horizontally.
[Illustration: FIG. 67. FIG. 68. FIG. 69. FIG. 70
FIGS. 67, 68, 69, and 70 show the area mapped out by the left wing of
the wasp when the insect is fixed and the wing made to vibrate. These
figures illustrate the various angles made by the wing as it hastens
to and fro, how the wing reverses and reciprocates, and how it twists
upon itself and describes a figure-of-8 track in space. Figs. 67 and
69 represent the forward or down stroke; figs. 68 and 70 the backward
or up stroke. The terms forward and back stroke are here employed
with reference to the head of the insect.--_Original._]
In the bat and bird the wing, when it ascends and descends, describes
a nearly vertical figure-of-8. In the insect, the wing, from the
more oblique direction of the stroke, describes a nearly horizontal
figure-of-8. In either case the wing reciprocates, and, as a rule,
reverses its planes. The down and up strokes, as will be seen from this
account, cross each other, as shown more particularly at figs. 67, 68,
69, and 70.
In the wasp the wing commences the down or forward stroke at _a_ of
figs. 67 and 69, and makes an angle of something like 45° with the
horizon (_x x´_). At _b_ (figs. 67 and 69) the angle is slightly
diminished, partly because of a rotation of the wing along its anterior
margin (long axis of wing), partly from increased speed, and partly
from the posterior margin of the wing yielding to a greater or less
extent.
At _c_ the angle is still more diminished from the same causes.
At _d_ the wing is slowed slightly, preparatory to reversing, and the
angle made with the horizon (_x_) increased.
At _e_ the angle, for the same reason, is still more increased; while
at _f_ the wing is at right angles to the horizon. It is, in fact, in
the act of reversing.
At _g_ the wing is reversed, and the up or back stroke commenced.
The angle made at _g_ is, consequently, the same as that made at a
(45°), with this difference, that the anterior margin and outer portion
of the wing, instead of being directed _forwards_, with reference to
the head of the insect, are now directed _backwards_.
During the up or backward stroke all the phenomena are reversed, as
shown at _g h i j k l_ of figs. 68 and 70 (p. 141); the only difference
being that the angles made by the wing with the horizon are somewhat
less than during the down or forward stroke--a circumstance which
facilitates the forward travel of the body, while it enables the
wing during the back stroke still to afford a considerable amount of
support. This arrangement permits the wing to travel backwards while
the body is travelling forwards; the diminution of the angles made by
the wing in the back stroke giving very much the same result as if the
wing were striking in the direction of the travel of the body. The
slight upward inclination of the wing during the back stroke permits
the body to fall downwards and forwards to a slight extent at this
peculiar juncture, the fall of the body, as has been already explained,
contributing to the elevation of the wing.