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

by James Bell Pettigrew

These remarks are more especially applicable to the flight of the bat and bird where the wing is made to vibrate more or less perpendicularly (fig. [17], p. 36; figs. [82] and 83, p. 158. Compare with fig. [85], p. 160, and fig. [88], p. 166). If a bird or a bat wishes to fly upwards, its flying surfaces must always be inclined upwards. It is the same with the fish. A fish can only swim upwards if its body is directed upwards. In the insect, as has been explained, the wing is made to vibrate in a more or less horizontal direction. In this case the wing has not to contend directly against gravity (a wing which flaps vertically must). As a consequence it is made to tack upon the air obliquely zigzag fashion as horse and carriage would ascend a steep hill (vide figs. [67] to 70, p. 141. Compare with figs. [71] and 72, p. 144). In this arrangement gravity is overcome by the wing reversing its planes and acting as a kite which flies alternately forwards and backwards. The kites formed by the wings of the bat and bird always fly forward (fig. [88], p. 166). In the insect, as in the bat and bird, the posterior margin of the wing never rises above the horizon so as to make an upward and backward angle with it, as stated by Borelli, Marey, and others (c x a of fig. [114], p. 228).

While Borelli and his successors are correct as to the wedge-action of the wing, they have given an erroneous interpretation of the manner in which the wedge is produced. Thus Borelli states that when the wings descend their posterior margins ascend, the two wings forming a cone whose base is represented by c b e of fig. [113], p. 220; its apex being represented by a f of the same figure. The base of Borelli’s cone, it will be observed, is inclined forwards in the direction of the head of the bird. Now this is just the opposite of what ought to be. Instead of the two wings forming one cone, the base of which is directed forwards, each wing of itself forms two cones, the bases of which are directed backwards and outwards, as shown at fig. 116.

Fig. 116.

In this figure the action of the wing is compared to the sculling of an oar, to which it bears a considerable resemblance.[116] The one cone, viz., that with its base directed outwards, is represented at x b d. This cone corresponds to the area mapped out by the tip of the wing in the process of elevating. The second cone, viz., that with its base directed backwards, is represented at q p n. This cone corresponds to the area mapped out by the posterior margin of the wing in the process of propelling. The two cones are produced in virtue of the wing rotating on its root and along its anterior margin as it ascends and descends (fig. [80], p. 149; fig. [83], p. 158). The present figure (116) shows the double twisting action of the wing, the tip describing the figure-of-8 indicated at b e f g h d i j k l; the posterior margin describing the figure-of-8 indicated at p r n. It is in this manner the cross pulsation or wave referred to at p. [148] is produced. To represent the action of the wing the sculling oar (a b, x s, c d) must have a small scull (m n, q r, o p) working at right angles to it. This follows because the wing has to elevate as well as propel; the oar of a boat when employed as a scull only propelling. In order to elevate more effectually, the oars formed by the wings are made to oscillate on a level with and under the volant animal rather than above it; the posterior margins of the wings being made to oscillate on a level with and below the anterior margins (pp. [150], 151).

Borelli, and all who have written since his time, are unanimous in affirming that the horizontal transference of the body of the bird is due to the perpendicular vibration of the wings, and to the yielding of the posterior or flexible margins of the wings in an upward direction as the wings descend. I am, however, as already stated, disposed to attribute the transference, 1st, to the fact that the wings, both when elevated and depressed, leap forwards in curves, those curves uniting to form a continuous waved track; 2d, to the tendency which the body of the bird has to swing forwards, in a more or less horizontal direction, when once set in motion; 3d, to the construction of the wings (they are elastic helices or screws, which twist and untwist when they are made to vibrate, and tend to bear upwards and onwards any weight suspended from them); 4th, to the reaction of the air on the under surfaces of the wings, which always act as kites; 5th, to the ever-varying power with which the wings are urged, this being greatest at the beginning of the down stroke, and least at the end of the up one; 6th, to the contraction of the voluntary muscles and elastic ligaments; 7th, to the effect produced by the various inclined surfaces formed by the wings during their oscillations; 8th, to the weight of the bird—weight itself, when acting upon inclined planes (wings), becoming a propelling power, and so contributing to horizontal motion. This is proved by the fact that if a sea bird launches itself from a cliff with expanded motionless wings, it sails along for an incredible distance before it reaches the water (fig. [103], p. 186).

The authors who have adopted Borelli’s plan of artificial wing, and who have indorsed his mechanical views of the action of the wing most fully, are Chabrier, Straus-Durckheim, Girard, and Marey. Borelli’s artificial wing, as already explained (p. 220, fig. [113]), consists of a rigid rod (e, r) in front, and a flexible sail (a, o) composed of feathers, behind. It acts upon the air, and the air acts upon it, as occasion demands.

Chabrier’s Views.—Chabrier states that the wing has only one period of activity—that, in fact, if the wing be suddenly lowered by the depressor muscles, it is elevated solely by the reaction of the air. There is one unanswerable objection to this theory—the bats and birds, and some, if not all the insects, have distinct elevator muscles. The presence of well-developed elevator muscles implies an elevating function, and, besides, we know that the insect, bat, and bird can elevate their wings when they are not flying, and when, consequently, no reaction of the air is induced.

Straus-Durckheim’s Views.—Durckheim believes the insect abstracts from the air by means of the inclined plane a component force (composant) which it employs to support and direct itself. In his Theology of Nature he describes a schematic wing as follows:—It consists of a rigid ribbing in front, and a flexible sail behind. A membrane so constructed will, according to him, be fit for flight. It will suffice if such a sail elevates and lowers itself successively. It will, of its own accord, dispose itself as an inclined plane, and receiving obliquely the reaction of the air, it transfers into tractile force a part of the vertical impulsion it has received. These two parts of the wing are, moreover, equally indispensable to each other. If we compare the schematic wing of Durckheim with that of Borelli they will be found to be identical, both as regards their construction and the manner of their application.