During the last 50 years a great deal has been said and written in regard to the flight of birds; no other natural phenomenon has excited so much interest and been so imperfectly understood. Learned treatises have been written to prove that a bird is able to develop from ten to twenty times as much power for its weight as other animals, while other equally learned works have shown most conclusively that no greater amount of energy is exerted by a bird in flying than by land animals in running or jumping.
Prof. Langley, who was certainly a very clever observer and a mathematician of the first order, in discussing the subject relating to the power exerted by birds in flight and the old formula relating to the subject, expresses himself as follows:--
"After many years and in mature life, I was brought to think of these things again, and to ask myself whether the problem of artificial flight was as hopeless and as absurd as it was then thought to be. Nature had solved it, and why not man? Perhaps it was because he had begun at the wrong end, and attempted to construct machines to fly before knowing the principles on which flight rested. I turned for these principles to my books and got no help. Sir Isaac Newton had indicated a rule for finding the resistance to advance through the air, which seemed, if correct, to call for enormous mechanical power, and a distinguished French mathematician had given a formula showing how rapidly the power must increase with the velocity of flight, and according to which a swallow, to attain a speed it is known to reach, must be possessed of the strength of a man.
"Remembering the effortless flight of the soaring bird, it seemed that the first thing to do was to discard rules which led to such results, and to commence new experiments, not to build a flying machine at once, but to find the principles upon which one should be built; to find, for instance, with certainty by direct trial how much horse-power was needed to sustain a surface of given weight by means of its motion through the air."
There is no question but what a bird has a higher physical development, as far as the generation of power is concerned, than any other animal we know of. Nevertheless, I think that everyone who has made a study of the question will agree that some animals, such as hares and rabbits, exert quite as much power in running, in proportion to their weight, as a sea-gull or an eagle does in flying.