The heaviest flying birds known scarcely reach one quarter of the weight of an average man, and it can be said that one of the most probable causes of the absence in Nature of heavier varieties is the fact that although such birds could fly easily enough, once on tlie wing, they would be unable to leave the ground, owing to’ the enormity of the power the first lift Avould require. The larger and heavier the bird, the less, in relation to its weight, is the power developed in flight, but, on the other hand, the greater is the eft'ort. needed for a. lift from the ground. I do not wish to enlarge upon this question, which has been treated by various authors, and only refer to it because it embodies a principle which we must bear in mind when considering the means of starting flight, namely, that the difference between the power required respectively for the flight and for the start becomes greater as the apparatus increases in weight.