question; these were made of paper and wire, and actuated by
springs of steel wire, which caused them to lift themselves in
the air. It is, however, in the theories which he put forward
that da Vinci's investigations are of greatest interest; these
prove him a patient as well as a keen student of the principles
of flight, and show that his manifold activities did not prevent
him from devoting some lengthy periods to observations of bird
flight.

'A bird,' he says in his Treatise, 'is an instrument working
according to mathematical law, which instrument it is within the
capacity of man to reproduce with all its movements, but not
with a corresponding degree of strength, though it is deficient
only in power of maintaining equilibrium. We may say,
therefore, that such an instrument constructed by man is lacking
in nothing except the life of the bird, and this life must needs
be supplied from that of man. The life which resides in the
bird's members will, without doubt, better conform to their needs
than will that of a man which is separated from them, and
especially in the almost imperceptible movements which produce
equilibrium. But since we see that the bird is equipped for many
apparent varieties of movement, we are able from this experience
to deduce that the most rudimentary of these movements will be
capable of being comprehended by man's understanding, and that he
will to a great extent be able to provide against the destruction
of that instrument of which he himself has become the living
principle and the propeller.'

In this is the definite belief of da Vinci that man is capable
of flight, together with a far more definite statement of the