f sufficient amplitude and frequency, would be capable, by
reason of their own "internal work," of sustaining or even raising a
suitably curved surface which was being carried along by the main mean
air stream. This would account for the phenomenon of "soaring." Lord
Rayleigh, discussing the same problem, premises that when a bird is
soaring the air cannot be moving uniformly and horizontally. Then comes
the natural question, Is it moving in ascending currents? Lord Rayleigh
has frequently noticed such currents, particularly above a cliff facing
the wind. Again, to quote another eminent authority, Major Baden-Powell,
on an occasion when flying one of his own kites, found it getting to
so high an angle that it presently rose absolutely overhead, with the
string perpendicular. He then took up a heavy piece of wood, which, when
tied to the string, began to rise in the air. He satisfied himself that
this curious result was solely due to a strong uptake of the air.

But, again, Lord Rayleigh, lending support to Professor Langley's
argument, points out that the apparent cause of soaring may be the
non-uniformity of the wind. The upper currents are generally stronger
than the lower, and it is mechanically possible for a bird, taking
advantage of two adjacent air streams, different in velocity, to
maintain itself in air without effort on its own part.

Lord Rayleigh, proceeding to give his views on artificial flight,
declares the main problem of the flying machine to be the problem of the
aerial plane. He states the case thus:--"Supposing a plane surface to
be falling vertically at the rate of four miles an hour, and also moving
horizontally at the rate of twenty miles an hour, it might have been
supposed that the horizontal motion would make no difference to the
pressure on its under surface which the falling plane must experience.
We are told, however, that in actual trial the horizontal motion much
increases the pressure under the falling plane, and it is this fact on
which the possibility of natural and artificial flight depends."

Ere this opinion had been stated by Lord Rayleigh in his discourse on
"Flight," at the Royal Institution, there were already at work upon the
aero-plane a small army of inventors, of whom it will be only possible
in a future chapter to mention some. Due reference, however, should here
be made to Mr. W. F. Wenham, of Boston, U.S.A., who had been at work on
artificial flight for many years, and to