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. 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 nonoblique 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.1 If the wing did not strike

1 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,

downwards and forwards, it would act at a manifest disadvantage:

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,

"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 im pulse 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. and 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.

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,1 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."2 Birds of Flight divisible into four kinds :

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1st. Such as have heavy bodies and short wings with a rapid movement (fig. 59, p. 126).

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

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

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.)—Or ginal.

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

vantage 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,1 and is wielded with great precision and power, particularly at the shoulder.

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 rota tion 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

1 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.