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