ble 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 principles by
which flight is to be achieved than any which had preceded it--and for
that matter, than many that have succeeded it. Two further extracts from
his work will show the exactness of his observations:--

'When a bird which is in equilibrium throws the centre of resistance of
the wings behind the centre of gravity, then such a bird will descend
with its head downward. This bird which finds itself in equilibrium
shall have the centre of resistance of the wings more forward than
the bird's centre of gravity; then such a bird will fall with its tail
turned toward the earth.'

And again: 'A man, when flying, shall be free from the waist up, that he
may be able to keep himself in equilibrium as he does in a boat, so
that the centre of his gravity and of the instrument may set itself in
equilibrium and change when necessity requires it to the changing of the
centre of its resistance.'

Here, in this last quotation, are the first beginnings of the inherent
stability which proved so great an advance in design, in this twentieth
century. But the extracts given do not begin to exhaust the range of
da Vinci's observations and deductions. With regard to bird flight, he
observed that so long as a bird keeps its wings outspread it cannot fall
directly to earth, but must glide down at an angle to alight--a small
thing, now that the principle of the plane in opposition to the air is
generally grasped, but da Vinci had to find it out. From observation
he gathered how a bird checks its own speed by opposing tail and wing
surface to the direction of flight, and thus alights at the proper
'landing speed.' He proved the existence of upward air currents by
noting how a bird takes off from level earth with wings outstretched and
motionless, and, in order to get an efficient substitute for the
natural wing, he recommended that there be used something similar to
the membrane of the wing of a bat--from this to the doped fabric of an
aeroplane wing is but a small step, for both are equally impervious to
air. Again, da Vinci recommended that experiments in flight be conducted
at a good height from the ground, since, if equilibrium be lost through
any cause, the height gives time to