_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 non-oblique 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.[94] If the wing did not strike downwards and _forwards_, it would act at a manifest disadvantage:--
[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.