FLIGHT 317 to flap in what the operator believes to be a strictly vertical downward direction, the tip of the wing, in spite of him, will dart forwards between two and three feet the amount of forward movement being regulated by the rapidity of the down stroke. This is a very striking experiment. The same thing happens with a properly con- -structed artificial wing. The down stroke with the artifi cial as with the natural wing is invariably converted into an oblique, downward, and forward stroke. No one ever saw a bird in the air flapping its wings towards its tail. The old idea was tliat the wings pushed the body of the bird in an upward and forward direction ; in reality the wings do not push but pull, and in order to pull they must always be in advance of the body to be flown. If the wings did not themselves fly forward, they could not possibly cause the body of the bird to fly forward. It is the wings which fly the bird, and not the converse. It only remains to be stated that the wing acts as a true kite, during both dowu. and up strokes, its under concave or biting surface, in virtue of the forward travel communi cated to it by the body of the flying creature, being closely applied to the air, during both its ascent and its descent. This explains how the wing furnishes a persistent buoyancy alike when it rises and when it falls (fig. 30). FIG. 30 shows the kite-like action of the wing during the down and up strokes, how the angles made by the wing with the horizon (a, l>) vary at every stage of these strokes, and how the wing evades the superimposed air during the up stroke, and seizes the nether air during the down stroke. In this figure the spaces between the double dotted lines (ay, ib) represent the down strokes, the single dotted line (h, i) representing the up stroke. The kite-like surfaces and angles made by the wing with the horizon (a, b) during the down strokes are indicated at cdefg, j k I in, those made during the up strokes being in dicated at g ft i. As the down and up strokes run into each other, and the con vex surface of the wing is always directed upwards and the concave surface downwards, it follows that the upper surface of the wing evades in a great measure the upper air, while the under surface seizes the nether air. It is easy to understand from this figure how the wing always flying forwards furnishes a persistent buoyancy. (Pettlgrew, 1870.) The natural kite formed by the wing differs from the artifi cial kite only in this, that the former is capable of being moved in all its parts, and is more or less flexible and elastic, whereas the latter is comparatively rigid. The flexibility and elasticity of the kite formed by the natural wing are ren dered necessary by the fact that the wing, as already stated, is practically hinged at its root and along its anterior margin, an arrangement which necessitates its several parts travel ling at different degrees of speed, in proportion as they are removed from the axes of rotation. Thus the tip travels at a higher speed than the root, and the posterior margin than the anterior margin. This begets a twisting diagonal movement of the wing on its long axis, which, but for the elasticity referred to, would break the wing into fragments. The elasticity contributes also to the continuous play of the wing, and insures that no two parts of it shall reverse at exactly the same instant. If the wing was inelastic, every part of it would reverse at precisely the same moment, and its vibration would be characterized by pauses or dead points at the end of the down and up strokes which would ba fatal to it as a flying organ. The elastic properties of the wing are absolutely essential, when the mechanism and movements of the pinion are taken into account. A rigid wing can never be an effective flying instrument. The kite-like surfaces referred to in natural flight are those upon which the constructors of flying machines very properly ground their hopes of ultimate success. These surfaces may be conferred on artificial wings, aeroplanes, aerial screws, or similar structures ; and these structures, if we miy judge from what we find in nature, should be of moderate size and elastic. The power of the flying organs will be increased if they are driven at a comparatively high speed, and particularly if they are made to reverse and re ciprocate, as in this case they will practically create the currents upon which they are destined to rise and advance. The angles made by the kite-like surfaces with the horizon, should vary according to circumstances. They should be small when the speed is high, and vice versa. This, as stated, is true of natural wings. It should also be true of artificial wings and their analogues. There is no escap ing from natural laws. A knowledge of natural laws alone will enable us to construct (it is to be hoped in the imme diate future) the much-desired flying machine. Having explained as far as space would permit how water differs from air, how the sailing ship differs from the balloon, and how the balloon differs from the flying creature and flying machine constructed on the living type, and having further explained the peculiarities of wings and wing movements as witnessed in natural flight, we are now in a position to enter upon a consideration of artificial wings and wing movements, and of artificial flight and flying machines. We begin with artificial wings. The first properly au thenticated account of an artificial wing was given by Bori-lli in 1G70. This author, distinguished alike as a physiologist, mathema tician, and mechanician, describes and figures a bird with artificial wings, each of which consists of a rigid rod in front anJ flexible feathers behind. The wings are represented as striking vertically down wards, as the annexed FIG. 31. Borelli s bird with artificial wings. . ,. ,, ..-. ,,.. re, anterior margin of the right winir. duplicate of Borelusngu re consisting of a rigid rod ; o a, posterior margin of the right wing, consisting of flexible feathers; b c, anterior, and /, posterior margins of the left wing saun as the right; d, tail of the bird; rg,dfr, vertical direction of the down stroke of the wing. (Borelli, 1G70.) shows (fig. 31). Borelli was of opinion that flight resulted from the application of an in clined plane, which beats the air, and which has a wedgo action. He. in fact, endeavours to prove that a bird wedges itself forward upon the air by the perpendicular vibration of its wings, the wings during their action forming a wedge, the base of which (c b e) is directed towards the head of the bird, the apex (af) being directed towards the tail (d). In the 19Gth proposition of his work (De mohi Animalium, Leyden, 1685) he states that " If the expanded wings of a bird suspended in the air shall strike the undisturbed air beneath it with a motion perpendicular to the horizon, the bird will fly with a transverse motion in a piano parallel with the horizon." " If," he adds, "the wings of the bird be expanded, and the under surfaces of the wings be struck by the air asccndiny perpendicularly to the hv.-izon with such a force as shall prevent the bird gliding downward? (i.e., with a tendency to glide downwards) from falling.it will be urged in a horizontal direction." The same argument is re-stated in different words as under : "If the air under the wings be struck by the flexible portions of the wings (ftabella, literally fly flaps or small fans) with a motion perpendicular to the horizon, the sails (veld) and flexible portions of the wings (fiabclla) will yield in an upward direction and form a wedge, the point of which is directed towards the tail. Whether, therefore, the air strikes the wings from below, or the wings strike the air from above, the result is the same, the posterior or flexible margins of the wings yield in an itpward direction, and in so doing urge the bird in a horizontal direction." There are three points in Borelli s argument to which it is necessary to draw attention : (1) the direction of the down stroke : it is stated to be vertically doirmcards ; (2) the construction of the anterior margin of the wing : it is stated to consist of a rigid rod ; (3) the function delegated to the posterior margin of the wing : it is said to yield in an upward direction during the down stroke. With regard to the first point. It is incorrect to say the
