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