as yet unsolved. The propeller
should always be aft, so that it could abstract energy from the wake
current, and also so that its wash was clear of the body propelled. The
best possible efficiency was about 70 per cent, and it was safe to rely
upon 66 per cent.

Benefits of Soaring Flight.

There was, Mr. Lanchester proceeded, some possibility of the aeronaut
reducing the power needed for transport by his adopting the principle of
soaring flight, as exemplified by some birds. There were, he continued,
two different modes of soaring flight. In the one the bird made use of
the upward current of air often to be found in the neighborhood of steep
vertical cliffs. These cliffs deflected the air upward long before it
actually reached the cliff, a whole region below being thus the seat of
an upward current. Darwin has noted that the condor was only to be found
in the neighborhood of such cliffs. Along the south coast also the gulls
made frequent use of the up currents due to the nearly perpendicular
chalk cliffs along the shore.

In the tropics up currents were also caused by temperature differences.
Cumulus clouds, moreover, were nearly always the terminations of such
up currents of heated air, which, on cooling by expansion in the upper
regions, deposited their moisture as fog. These clouds might, perhaps,
prove useful in the future in showing the aeronaut where up currents
were to be found. Another mode of soaring flight was that adopted by
the albatross, which took advantage of the fact that the air moved
in pulsations, into which the bird fitted itself, being thus able to
extract energy from the wind. Whether it would be possible for the
aeronaut to employ a similar method must be left to the future to
decide.

Main Difficulties in Aviation.

In practical flight difficulties arose in starting and in alighting.
There was a lower limit to the speed at which the machine was stable,
and it was inadvisable to leave the ground till this limit was attained.
Similarly, in alighting it was inexpedient to reduce the speed below the
limit of stability. This fact constituted a difficulty in the adoption
of high speeds, since the length of run needed increased in proportion
to the square of the velocity. This drawback could, however, be
surmounted by forming starting and alighting grounds of ample size.
He thought it quite likely in the future that such grounds would be
considered as essential to the flying machine as a seaport