'When, therefore, it is asked whether men may be able to fly by their own strength, it must be seen whether the motive power of the pectoral muscles (the strength of which is indicated and measured by their size) is proportionately great, as it is evident that it must exceed the resistance of the weight of the whole human body 10,000 times, together with the weight of enormous wings which should be attached to the arms. And it is clear that the motive power of the pectoral muscles in men is much less than is necessary for flight, for in birds the bulk and weight of the muscles for flapping the wings are not less than a sixth part of the entire weight of the body. Therefore, it would be necessary that the pectoral muscles of a man should weigh more than a sixth part of the entire weight of his body; so also the arms, by flapping with the wings attached, should be able to exert a power 10,000 times greater than the weight of the human body itself. But they are far below such excess, for the aforesaid pectoral muscles do not equal a hundredth part of the entire weight of a man. Wherefore either the strength of the muscles ought to be increased or the weight of the human body must be decreased, so that the same proportion obtains in it as exists in birds. Hence it is deducted that the Icarian invention is entirely mythical because impossible, for it is not possible either to increase a man's pectoral muscles or to diminish the weight of the human body; and whatever apparatus is used, although it is possible to increase the momentum, the velocity or the power employed can never equal the resistance; and therefore wing flapping by the contraction of muscles cannot give out enough power to carry up the heavy body of a man.' It may be said that practically all the conclusions which Borelli reached in his study were negative. Although contemporary with Lana, he perceived the one factor which rendered Lana's project for flight by means of vacuum globes an impossibility--he saw that no globe could be constructed sufficiently light for flight, and at the same time sufficiently strong to withstand the pressure of the outside atmosphere. He does not appear to have made any experiments in flying on his own account, having, as he asserts most definitely, no faith in any invention designed to lift man from the surface of the earth. But his work, from which only the foregoing short quotations can be given, is, nevertheless, of indisputable value, for he settled the mechanics of bird flight, and paved the way for those later investigators who had, first, the steam engine, and later the internal combustion engine--two factors in mechanical flight which would have seemed as impossible to Borelli as would wireless telegraphy to a student of Napoleonic times. On such foundations as his age afforded Borelli built solidly and well, so that he ranks as one of the greatest--if not actually the greatest--of the investigators into this subject before the age of steam.