Animal Locomotion; or, walking, swimming, and flying / With a dissertation on aëronautics

by James Bell Pettigrew

4th, Because, in commenting upon and differing from Borelli, I will necessarily comment upon and differ from all his successors.

As to the Direction of the Stroke, yielding of the Wing, etc.—The Duke of Argyll[111] agrees with Borelli in believing that the wing invariably strikes perpendicularly downwards. His words are—“Except for the purpose of arresting their flight birds can never strike except directly downwards; that is, against the opposing force of gravity.” Professor Owen in his Comparative Anatomy, Mr. Macgillivray in his British Birds, Mr. Bishop in his article “Motion” in the Cyclopedia of Anatomy and Physiology, and M. Liais “On the Flight of Birds and Insects” in the Annals of Natural History, all assert that the stroke is delivered downwards and more or less backwards.

To obtain an upward recoil, one would naturally suppose all that is required is a downward stroke, and to obtain an upward and forward recoil, one would naturally conclude a downward and backward stroke alone is requisite. Such, however, is not the case.

In the first place, a natural wing, or a properly constructed artificial one, cannot be depressed either vertically downwards, or downwards and backwards. It will of necessity descend downwards and forwards in a curve. This arises from its being flexible and elastic throughout, and in especial from its being carefully graduated as regards thickness, the tip being thinner and more elastic than the root, and the posterior margin than the anterior margin.

In the second place, there is only one direction in which the wing could strike so at once to support and carry the bird forward. The bird, when flying, is a body in motion. It has therefore acquired momentum. If a grouse is shot on the wing it does not fall vertically downwards, as Borelli and his successors assume, but downwards and forwards. The flat surfaces of the wings are consequently made to strike downwards and forwards, as they in this manner act as kites to the falling body, which they bear, or tend to bear, upwards and forwards.

So much for the direction of the stroke during the descent of the wing.

Let us now consider to what extent the posterior margin of the wing yields in an upward direction when the wing descends. Borelli does not state the exact amount. The Duke of Argyll, who believes with Borelli that the posterior margin of the wing is elevated during the down stroke, avers that, “whereas the air compressed in the hollow of the wing cannot pass through the wing owing to the closing upwards of the feathers against each other, or escape forwards because of the rigidity of the bones and of the quills in this direction, it passes backwards, and in so doing lifts by its force the elastic ends of the feathers. In passing backwards it communicates to the whole line of both wings a corresponding push forwards to the body of the bird. The same volume of air is thus made, in accordance with the law of action and reaction, to sustain the bird and carry it forward.”[112] Mr. Macgillivray observes that “to progress in a horizontal direction it is necessary that the downward stroke should be modified by the elevation in a certain degree of the free extremities of the quills.”[113]

Marey’s Views.—Professor Marey states that during the down stroke the posterior or flexible margin of the wing yields in an upward direction to such an extent as to cause the under surface of the wing to look backwards, and make a backward angle with the horizon of 45° plus or minus according to circumstances.[114] That the posterior margin of the wing yields in a slightly upward direction during the down stroke, I admit. By doing so it prevents shock, confers continuity of motion, and contributes in some measure to the elevation of the wing. The amount of yielding, however, is in all cases very slight, and the little upward movement there is, is in part the result of the posterior margin of the wing rotating around the anterior margin as an axis. That the posterior margin of the wing never yields in an upward direction until the under surface of the pinion makes a backward angle of 45° with the horizon, as Marey remarks, is a matter of absolute certainty. This statement admits of direct proof. If any one watches the horizontal or upward flight of a large bird, he will observe that the posterior or flexible margin of the wing never rises during the down stroke to a perceptible extent, so that the under surface of the wing on no occasion looks backwards, as stated by Marey. On the contrary, he will find that the under surface of the wing (during the down stroke) invariably looks forwards—the posterior margin of the wing being inclined downwards and backwards; as shown at figs. [82] and 83, p. 158; fig. [103], p. 186; fig. [85] (a b c), p. 160; and fig. [88] (c d e f g), p. 166.

The under surface of the wing, as will be seen from this account, not only always looks forwards, but it forms a true kite with the horizon, the angles made by the kite varying at every part of the down stroke, as shown more particularly at d, e, f, g; j, k, l, m of fig. [88], p. 166. I am therefore opposed to Borelli, Macgillivray, Owen, Bishop, M. Liais, the Duke of Argyll, and Marey as to the direction and nature of the down stroke. I differ also as to the direction and nature of the up stroke.

Professor Marey states that not only does the posterior margin of the wing yield in an upward direction during the down stroke until the under surface of the pinion makes a backward angle of 45° with the horizon, but that during the up stroke it yields to the same extent in an opposite direction. The posterior flexible margin of the wing, according to Marey, passes through a space of 90° every time the wing reverses its course, this space being dedicated to the mere adjusting of the planes of the wing for the purposes of flight. The planes, moreover, he asserts, are adjusted not by vital and vito-mechanical acts but by the action of the air alone; this operating on the under surface of the wing and forcing its posterior margin upwards during the down stroke; the air during the up stroke acting upon the posterior margin of the upper surface of the wing, and forcing it downwards. This is a mere repetition of Borelli’s view. Marey delegates to the air the difficult and delicate task of arranging the details of flight. The time, power, and space occupied in reversing the wing alone, according to this theory, are such as to render flight impossible. That the wing does not act as stated by Borelli, Marey, and others may be readily proved by experiment. It may also be demonstrated mathematically, as a reference to figs. 114 and 115, p. 228, will show.