Page:Encyclopædia Britannica, Ninth Edition, v. 9.djvu/325

 F L I G II T .... &quot;The wing of the bat bears considerable resemblance to that of the insect, inasmuch as it consists of a delicate, semi-transparent, continuous membrane, supported in divers direc tions, particularly towards its anterior margin, by a system of stays or stretchers which confer upon it the degree of rigidity requisite for flight. It is, as a rule, deeply concave on its under or ven tral surface, and in Fio. 17 HieTit wing of the Hat (Phyllocina graciHj). this respect re- 1)ol sal aspect as seen from above. (Pettigrew, 18G7.) semhles the wing of the heavy-bodied birds. The movement of the bat s wing in extension i.s a spiral one, the spiral running alter nately from below upwards and forwards and from above down wards and back wards. The action of the wing of the bat, and the move ments of its com ponent bones, are essentially the same Fl(, ls ._ni R ht wing of the Bat (Pliyllocina grariUs). as in the bird &quot; Dorsal and ventral aspects, as seen from behind, (li^s. 17 and 18). These show the screw-like configuration of the wing, &quot; Trip vimr !1IU a so S 1OW l ow tnc wing twists and untwists . .. .. . during its action. (Pettigrew, 1867.) strikes the air pre cisely as a boy s kite would if it were jerked by its string, the only difference being that the kite is pulled forwards upon the wind by the string and the hand, whereas in the insect, bat, and bird the wing is pttshrd forwards on the wind by the weight of the body and the power residing in the pinion itself&quot; (fig. 19). 1 Fir,, in. The Cape Burn -owl (Stris capensis), showing Hie kite-like surfaces pre sented by the ventrul aspect of the wings and body in flight. (I ettigrew, 1867.) The figure-of-8 and kite-like action of the wing referred to lead us to explain how it happens that the wing, which in many instances is a comparatively small and delicate Pettigrcw s more recent memoirs 2 will serve to explain the rationale (figs. 20, 21, 22, aud 23). As will be seen from these figures, the wing during its vibration sweeps through a comparatively very large space. This space, as already explained, is practically a sclid basis of support for the wing and for the flying animal. The wing attacks the air in such a manner as virtually to have no slip, this for two reasons. The wing reverses instantly and acts as a kite during nearly the entire down and up strokes. The angles, moreover, made by the wing with the horizon during the down and up strokes are at no two intervals the same, but (and this is a remarkable cir cumstance) they are always adapted to the speed at which the wing is travelling for tbe time being. The increase and decrease in the angles made by the wing as it hastens to and fro are due partly to the resistance offered by the air, and partly to the mechanism and mode of application of the wing to the air. The wing, during its vibrations, rotates upon two separate centres, the tip rotating round the root of the wing as an axis (short axis of wing), the posterior margin rotating around the anterior margin (long axis of wing). The wing is really eccentric in its nature, a remark which applies also to the rowing feathers of the bird s wing. The compound rotation goes on throughout the entire down and up strokes, and is intimately associated with the power which the wing enjoys of alternately seizing and evading the air. The compound rotation of the wing is greatly facilitated by the wing being elastic and flexible. It is this -which causes the wing to twist and untwist diagonally on its long axis when it is made to vibrate. The twisting referred to is partly a vital and partly a mechanical act ; that is, it is occasioned in part by the action of the muscles and in part by the greater resistance experienced from the air by the tip and posterior margin of the wing as compared with the root and anterior margin, the resistance experienced by the tip and posterior margin causing them to reverse always subsequently to the root and anterior margin, which has the effect of throwing the anterior and posterior margins of the wing into figure-of-8 curves, as shown at figs. 9, 11,. 12, 16, 18, 20, and 21. The compound rotation of the wing, as seen in the bird, is represented in fig. 24. Fios. 20, 21, 22, and 23 show the urea mapped out by the left wing of the Wasp when the insect is fixed and the wing made to vibrate. These fiinires illus trate tlie various angles made by the wing with the horizon as it hastens to and fro. and show how the wing reverses and reciprocates, nrd how it twists upon Itself in opposite directions, and describes a figure-of-8 track in space. Figs. 20 and 22 represent the forward or down stroke (a be defy), fies. 21 and 23 the backward or up stroke (y h ij k I a). The terms forward nnd back strokes are here employed with reference to the head of (he insect, x, r, line to represent the horizon. If fig. 22, representing the, down or forward stroke, be placed upon fig. 23 representing the up or backward stroke, it will be seen that the iriny rro.w.t Us oicn track more or less completely at every stage of the down and upstrokes. (Pettigrew, 1870.) organ, can yet attack the air with such vigour as to extract from it the recoil necessary to elevate and propel the flying creature. The accompanying figures from one of Professor 1 &quot; On the various modes of Flight in relation to Aeronautics&quot; (Pro ceedings of tJie Royal Institution of Great Britain, March 22d, 1867) ; &quot; Oa the Mechanical Appliances by which Flight is attained in the Animal Kingdom&quot; (Transactions df the Linnean Society, vol. xxvi., r.-ad June 6th and 20th, 1867), by J. Bell Pettigrew, M.D., F.R.S., Professor of Medicine and Anatomy, University of St Andrews. i Fin. 24. Wing of bird wilh its root (a, b) cranked foiwimN. n,k. slmit axis of wing (axis for tip of wing, h); c, &amp;lt;l, long axis (axis for posterior margin of wing, h, i,j, t, I) ; ?), n, short axis of rowing feathers of wing; r, i, long axis of rowing feathers of wing. The rotation of the rowing feathers on their long axis (they are eccentrics) enables them to open or separate during the up, and close or come together during the down strokes. t/,(ip, concave shape pre sented by the under surface of the wing. (Pettigrew, 1870.) Not the least curious feature of the wing movements is- the remarkable power which the wing possesses of making and utilizing its own currents. Thus, when the wing descends it draws after it a strong current, which, being met by the wing during its ascent, greatly increases the efficacy of the up stroke. Similarly and conversely when the wing ascends, it creates an upward current, which, being met by the wing when it descends, powerfully contributes to tlm 2 &quot; On the Physiology of Wings ; being an analysis of the movement* by which flight is produced in the Insect, But, and Bird &quot; ( Trans, liny. i Soc. Edin., vol. xxvi.). IX. .&amp;lt;o