FLIGHT 311 insect to darfc through the wind in whatever direction it pleases. The reader has only to imagine figs. 8 and 9 cut out in paper to realize that extensive, inert, horizontal aero-planes 1 in a flying machine would be a mistake. It is found to be so practically, as will be shown by and by. Fig. 9 so cut out would be heavier than fig. 8, and if both were exposed to a current of air, fig. 9 would be more blown about than fig. 8. It id true that in beetles and certain other insects there are the elytra or wing cases thin, light, horny structures which in the act of night are extended horizontally and act as sustainers or gliders, Tho elytra, however, are com paratively long narrow structures which occupy a position in front of the wings, of which they may be regarded as forming the anterior parts. The elytra are to the delicate wings of some insects what the thick anterior margins are to stronger wings. The elytra, moreover, are not wholly passive structures. They can be moved, and the angles made by their under surfaces with the horizon adjusted. Finally, they are not essential to flight, as flight in the great majority of instances is performed without them. The elytra serve as protectors to the wings when the wings are folded upon the back of the insect, and as they are extended horizontally when the insect is flying they contribute to flight indirectly, in virtue of their being carried forward by the body in motion. The manner in which the wings of the insect traverse the air, so as practically to increase the basis of support, raises the whole subject of natural flight. It is necessary, there fore, at this stage to direct the attention of the reader somewhat fully to the subject of flight, as witnessed in the insect, bat, and bird, a knowledge of natural flight preceding, and being in some senses indispensable to, a knowledge of artificial flight. The bodies of flying creatures are, as a rule, very strong, comparatively light, and of an elongated form, the bodies of birds being specially adapted for cleaving the air. Flying creatures, however, are less remarkable for their strength, shape, and comparative levity than for the size and extraordinarily rapid and complicated movements of their wings. To Professor J. Bell Pettigrew is due the merit of having first satisfactorily analysed those movements, and of having reproduced them by the aid of artificial wings. This physiologist in 18G7 2 showed that all natural wings, whether of the insect, bat, or bird, are screws structurally, and that they act as screws when they are made to vibrate, from the fact that they twist in opposite directions during the down and up strokes. He also explained that all wings act upon a common principle, and that they present oblique, kite-like surfaces to the air, through which they pass much in the same way that an oar passes through water in sculling. He further pointed out that the wings of flying creatures (contrary to received opinions, and as has been already indicated) strike downwards and forwards during the down strokes, and upwards and forwards during the up strokes. Lastly, and most important of all, he demonstrated that the wings of flying creatures, when the bodies of said creatures are fixed, describe^wre-q/ -S tracks in space, the figure-of-8 tracks, when the bodies are released and advancing as in rapid flight, being opened out and converted into waved tracks. 1 By the term aero-plane is meant a thin, light, expanded structure intended to float or rest upon the air, and calculated to afford a certain amount of support to any body attached to it. " On the various modes of Flight in relation to Aeronautics," by J. Bell Pettigrew, M.D., F.R.S., &c. (Proceedings of tlie Royal Institution of Great Britain, March 22d, 1867); " On the Mechanical Appliances by which Flight is attained in the Animal Kingdom," by the same author (Transactions of Vie Linnean- Society, vol. xxvi., re; 1 June 6th and 20th, 1867). Professor Pettigrcw s discovery of the figure-of-8 and waved movements, concerning which so much has been said and written, was confirmed some two years after it was made by Professor E. J. Marey 3 by the aid of the "sphygmograph." 4 The movements in question are now regarded as fundamental, from the fact that they are alika essential to natural and artificial flight. ^The following is Professor Pettigrew s description of wings and wing movements published in 1867: " The wings of insects and birds are, as a rule, more or less tri angular in shape, the base of the triangle being directed towards the body, its sides anteriorly and posteriorly. Tiny are also conical on section from within outwards and from before backwards, this shape converting the pinions into delicately-graduated instruments balanced with the utmost nicety to satisfy the requirements of the muscular system on the one hand and the resistance and resiliency of the air on the other. While all wings are graduated as explained, innumerable varieties occur as to their general contour, some being falcated or scythe-like, others oblong, others rounded or circular, some lanceolate, and some linear. The wings of insects may consist either of one or two pairs, the anterior or upper pair, when two are present, being in some instances greatly modified and presenting a corneous condition. They are then known as elytra, from the Greek tvrpov, a sheath. Loth pairs are composed of a duplicature of the integument, or investing membrane, and are strengthened in various directions by a system of hollow, horny tubes, known to entomologists as the neunu or nervures. These ncrvures taper towards the extremity of the wing, and are strongest towards its root and anterior margin, where they supply the place of the arm in bats and birds. The neura; are arranged at the axis of the wing after the manner of a fan or spiral stair, the anterior one occupying a higher position than that farther back, and so of the others. As this arrangement extends also to the margins, the u-inys are more or less twisted upon themselves, and present a certain degree of convexity on their superior or upper surface, and a corresponding concavity on their inferior or under surface, their free edges supplying those fine curves which act with such efficacy upon the air in obtaining the maximum of resistance and the minimum of displacement. As illustrative examples of the form of wings alluded to, those of the beetle, bee, and fly may be cited, the pinions in those insects acting as helices, or twisted levers, and elevating weights much greater than the area of the wings would seem to warrant" (figs. 10 and 11}. Fig. 10. io- 11 ig. 11, Fir,, in. Right winff of the Bcclle (Goliathus micans) when at rest ; seen from above, (Pettiprew, 1807.) Fio. 11. Right wing of tlic Beetle (Goliathus micans) when in motion , seen from behind. This figure shows how the wing twists and untwists when in action, and how it forms u true screw. (Pettigrew, 18G7.) "To confer on the wings the multiplicity of move ments which they require, they arc supplied with double hinge or compound joints, which enable them to move not only in an upward, downward, forward, and backward direction, but also at various in termediate degrees of obliquity. An insect with wings thus hinged may, as far as steadiness of body is concerned, be not inaptly com- 3 Revue dcs Cours Scicntifiques de la France et de Vjfct.ranger, 1S69. 4 The sphygmograph, as its name indicates, is a recording instrument. It consists of a smoked cylinder revolving by means of clock work at a known speed, and a style or pen which inscribes its surface by scratching or brushing away the lamp black. The movements to be registered are transferred to the style or pen by one or more levers, and the pen in turn transfers them to the cylinder, where they appear as legible tracings. In registering the movements of the wings, the tips and margins of the pinions were, by an ingenious modification, employed as the styles or pens. By this arrangement the different parts of the wings were made actually to record their own movements. As will be seen from this account, the figure-of-8 or wave theory of stationary and progressive flight has been made the subject of a rigorous expcriinentum cnicis.
