From the time of Borelli up to the development of the steam
engine it may be said that flight by means of any
heavier-than-air apparatus was generally regarded as impossible,
and apart from certain deductions which a little experiment
would have shown to be doomed to failure, this method of flight
was not followed up. It is not to be wondered at, when
Borelli's exaggerated estimate of the strength expended by birds
in proportion to their weight is borne in mind; he alleged that
the motive force in birds' wings is 10,000 times greater than
the resistance of their weight, and with regard to human flight
he remarks:--

'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