Note 94 — PROGRESSION IN OR THROUGH THE AIR. (continued 3)
Reading edition
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.