tations imposed on the motion of an aeroplane by wind velocity, Mr.
Lanchester gave the diagrams shown in Figs. 1 to 4. The circle in each
case was, he said, described with a radius equal to the speed of the
aeroplane in still air, from a center placed "down-wind" from the
aeroplane by an amount equal to the velocity of the wind.

Fig. 1 therefore represented the case in which the air was still, and
in this case the aeroplane represented by _A_ had perfect liberty of
movement in any direction

In Fig. 2 the velocity of the wind was half that of the aeroplane, and
the latter could still navigate in any direction, but its speed against
the wind was only one-third of its speed with the wind.

In Fig. 3 the velocity of the wind was equal to that of the aeroplane,
and then motion against the wind was impossible; but it could move to
any point of the circle, but not to any point lying to the left of the
tangent _A_ _B_. Finally, when the wind had a greater speed than the
aeroplane, as in Fig. 4, the machine could move only in directions
limited by the tangents _A_ _C_ and _A_ _D_.

Matter of Fuel Consumption.

Taking the case in which the wind had a speed equal to half that of the
aeroplane, Mr. Lanchester said that for a given journey out and home,
down wind and back, the aeroplane would require 30 per cent more fuel
than if the trip were made in still air; while if the journey was made
at right angles to the direction of the wind the fuel needed would be 15
per cent more than in a calm. This 30 per cent extra was quite a heavy
enough addition to the fuel; and to secure even this figure it was
necessary that the aeroplane should have a speed of twice that of the
maximum wind in which it was desired to operate the machine. Again, as
stated in the last lecture, to insure the automatic stability of the
machine it was necessary that the aeroplane speed should be largely in
excess of that of the gusts of wind liable to be encountered.

Eccentricities of the Wind.

There was, Mr. Lanchester said, a loose connection between the average
velocity of the wind and the maximum speed of the gusts. When the
average speed of the wind was 40 miles per hour, that of the gusts might
be equal or more. At one moment there might be a calm or the direction
of the wind even reversed, followed, the next moment, by a violent gust.
About the same minimum speed was desirable for security against gusts as
was demanded by other considerations. Si