“Such exercises are continued daily: at first only one single wing-beat succeeds, and before the wings can be raised for the second beat, the long, cautiously placed legs are again touching ground. But as soon as this stage is passed, _i.e._ when a second wing-beat is possible without the legs touching the ground, progress becomes very rapid, because the increased forward velocity facilitates flight, and three, four, or more double beats follow each other in one attempt, maybe awkward and unskilled, but never attended by accident, because of the caution exercised by the bird.” Lilienthal was fascinated by the mechanism of the bird’s wing. He and his brother built one machine after another to determine the exact amount of lifting effort that a man could obtain by imitating the wing-beat of a bird. One such apparatus is illustrated in Fig. 19. This had a double set of wings; a wide pair in the centre and narrower ones in front and at the rear. These wings beat alternately, by movements of the operator’s legs; and the machine was suspended by a rope and pulleys from a beam, being counterbalanced by a weight. The tests showed this: that, after some practice in working the wings, a man could raise with them just half the weight of himself and of the machine; but the muscular effort proved so great that he could only maintain this rate of wing-beating for a few seconds. Here, incidentally, a fact may be mentioned: the energy a man can produce, at all events for a prolonged effort, has been estimated at about a quarter of a horse-power; and this--in tests so far made--has been insufficient for the purpose of wing-flapping flight. Lilienthal himself thought that, with some perfect form of apparatus, a man might fly with an expenditure of 1·5 h.p. of energy; but other experimenters have put the minimum power necessary, even if mechanism could be devised, at 2 h.p. And another fact must be remembered: even had Lilienthal been able, with such a machine, actually to raise himself in the air, he would still have had the problem of balancing himself, in addition to the working of his wings. [Illustration: FIG. 19.] [Illustration: FIG. 20.--Lilienthal Kite.]