310 FLIGHT not actually fly, but merely dart iuto the air and there sus tain themselves for brief intervals, they afford no real sup port to the theory. The so-called floating animals are well depicted at figs. 5, 6, and 7. Fig. 5. Fig. 6. Ki FIG. 5. The Red-throated Dragon (Draco hxmatopogan). (Pettigrew, ISfiT.) F IO . g The Flying Colugo (Ga/eopithecus volans); also called flying lemur and flying squirrel. (Pettigrew, 1867.) FIG. 7. The Flying Fish (Exocaetus exiliens}. (Pettigrew, 1867.) It has been asserted, and with some degree of plausibility, that a fish lighter than the water might swim, and that a bird lighter than the air might fly; it ought, however, to be borne in mind that, in point of fact, a fish lighter than the water could not hold its own if the water were in the least perturbed, and that a bird lighter than the air would be swept into space by even a moderate breeze without hope of return. Weight and power are always associated in living animals, and the fact that living animals are made heavier than the medium they are to navigate may be regarded as a conclusive argument in favour of weight being necessary alike to the swimming of the fish and the flying of the bird. It may be stated once for all that flying creatures are for the most part as heavy, bulk for bulk, as other animals, and that flight in every instance is the product, not of superior levity, but of weiyld and power directed upon properly constructed flying organs. This fact is important as bearing on the construction of flying machines. It shows that a flying machine need not necessarily be a light, airy structure exposing an immoderate amount of surface. On the contrary, it favours the belief that it should be a compact and moderately heavy and powerful structure, which trusts for elevation and propul sion entirely to its flying appliances whether actively moving wings, or screws, or aero-planes wedged forward by screws. It should attack and subdue the air, and never give the air an opportunity of attacking or subduing it. It should smite the air intelligently and as a master, and its vigorous well-directed thrusts should in every instance elicit an upward and forward recoil. The flying machine of the future, there is reason to believe, will be a veritable example of " multum in jwrvo." It will launch itself in the ocean of air, and will extract from that air, by
- ans of its travelling surfaces however fashioned and
nowever applied the recoil or resistance necessary to devate and carry it forward. Extensive inert surfaces indeed are contra-indicated in a flying machine, as they approximate it to the balloon, which, as has been shown, cannot maintain its position in the air if there are air urrents. A flying machine which could not face air currents would necessarily be a failure. To obviate this difficulty we are forced to fall back upon uw/ht, or rather the structures and appliances which weight represents. These appliances as indicated should not be unnecessarily expanded, but when expanded they should, wherever practicable, be converted into actively moving flying sur faces, in preference to fixed or inert dead surfaces. The question of surface is a very important one in aerostation : it naturally resolves itself into one of active and passive surface. As there are active and passive surfaces in the flying animal, so there are, or should be, active and passive surfaces in the flying machine. Art should follow nature in this matter. The active surfaces in flying creatures are always greatly in excess of the passive ones, from the fact that the former virtually increase in propor tion to the spaces through which they arc made to travel. Nature not only distinguishes between active and passive surfaces in flying animals, but she strikes a just balance between them, and utilizes both. She regulates the sur faces to the strength and weight of the flying creature and the air currents to which the surfaces are to be exposed and upon which they are to operate. In her calculations she never forgets that her flying subjects are to control and not to be controlled by the air. As a rule she reduces the passive surfaces of the body to a minimum ; she likewise reduces as far as possible the actively moving or flying sur faces. While, however, diminishing the surfaces of the flying animal as a whole, she increases as occasion demands the active or wing surfaces by wing movements, and the passive or dead surfaces by the forward motion of the body in progressive flight. She knows that if the wings are driven with sufficient rapidity they practically convert the spaces through which they move into solid bases of support ; she also knows that the body in rapid flight derives support from all the air over which it passes. The manner in which the wing surfaces are increased by the wing move ments will be readily understood from the accompanying illustrations of the blow fly with its wings at rest and in motion (figs. 8 and 9). In fig. 8 the surfaces exposed by FIG. 8. Blow Fly (Musca romitoria) with its wings at rest. (Pettigvcw ) FIG! 9. Ulow Fly with its wings in motion as in flight. (I ettigrew ) the body of the insect and the wings are, as compared with those of fig. 9, trifling. The wind would have much less purchase on fig. 8 than on fig. 9, provided the surfaces exposed by the latter were passive or dead surfaces. But they are not dead surfaces : they represent the spaces occupied by the rapidly vibrating wings, which are actively moving flying organs. As, moreover, the wings travel at a much higher speed than any wind that blows, they are superior to and control the wind ; they enable the
