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 regain it. This
recommendation, by the way, received ample support in the
training areas of war pilots.

Man's muscles, said da Vinci, are fully sufficient to enable him
to fly, for the larger birds, he noted, employ but a small part
of their strength in keeping themselves afloat in the air--by
this theory he attempted to encourage experiment, just as, when
his time came, Borelli reached the opposite conclusion and
discouraged it. That Borelli was right--so far--and da Vinci
wrong, detracts not at all from the repute of the earlier
investigator, who had but the resources of his age to support
investigations conducted in the spirit of ages after.

His chief practical contributions to the science of
flight--apart from numerous drawings which have still a
value--are the helicopter or lifting screw, and the parachute.
The former, as already noted, he made and proved effective in
model form, and the principle which he demonstrated is that of
the helicopter of to-day, on which sundry experimenters work