r 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.

The conclusion, that 'the motive force in birds' win