known angle of resistance generates a lb.
pressure per square foot at this velocity, probably a waft of a little
more than 4 feet per second would produce this effect, one-tenth part
of which would be the propelling power. The advantage of this mode of
application compared with the former is rather more than ten to one.

'In continuing the general principles of aerial navigation, for the
practice of the art, many mechanical difficulties present themselves
which require a considerable course of skilfully applied experiments
before they can be overcome; but, to a certain extent, the air has
already been made navigable, and no one who has seen the steadiness
with which weights to the amount of ten stone (including four stone,
the weight of the machine) hover in the air can doubt of the ultimate
accomplishment of this object.'

This extract from his work gives but a faint idea of the amount of
research for which Cayley was responsible. He had the humility of the
true investigator in scientific problems, and so far as can be seen
was never guilty of the great fault of so many investigators in this
subject--that of making claims which he could not support. He was
content to do, and pass after having recorded his part, and although
nearly half a century had to pass between the time of his death and the
first actual flight by means of power-driven planes, yet he may be said
to have contributed very largely to the solution of the problem, and his
name will always rank high in the roll of the pioneers of flight.

Practically contemporary with Cayley was Thomas Walker, concerning whom
little is known save that he was a portrait painter of Hull, where
was published his pamphlet on The Art of Flying in 1810, a second and
amplified edition being produced, also in Hull, in 1831. The pamphlet,
which has been reproduced in extenso in the Aeronautical Classics series
published by the Royal Aeronautical Society, displays a curious mixture
of the true scientific spirit and colossal conceit. Walker appears to
have been a man inclined to jump to conclusions, which carried him up to
the edge of discovery and left him vacillating there.

The study of the two editions of his pamphlet side by side shows that
their author made considerable advances in the practicability of his
designs in the 21 intervening years, though the drawings which accompany
the text in both editions fail to show anything really capable
of flight. The great point about