allowing that there is no
difference in the power-producing qualities of the material used. But
few of us understand just what the ratio of increase is, or how it is
caused. This proposition is one of keen interest in connection with
aviation.

Let us cite a problem which will illustrate the point quoted: Allowing
that it takes a given amount of gasolene to propel a flying machine a
given distance, half the way with the wind, and half against it, the
wind blowing at one-half the speed of the machine, what will be the
increase in fuel consumption?

Increase of Thirty Per Cent.

On the face of it there would seem to be no call for an increase as the
resistance met when going against the wind is apparently offset by the
propulsive force of the wind when the machine is travelling with
it. This, however, is called faulty reasoning. The increase in fuel
consumption, as figured by Mr. F. W. Lanchester, of the Royal Society of
Arts, will be fully 30 per cent over the amount required for a similar
operation of the machine in still air. If the journey should be made at
right angles to the wind under the same conditions the increase would be
15 per cent.

In other words Mr. Lanchester maintains that the work done by the motor
in making headway against the wind for a certain distance calls for more
engine energy, and consequently more fuel by 30 per cent, than is saved
by the helping force of the wind on the return journey.

CHAPTER XIV. ABOUT WIND CURRENTS, ETC.

One of the first difficulties which the novice will encounter is the
uncertainty of the wind currents. With a low velocity the wind, some
distance away from the ground, is ordinarily steady. As the velocity
increases, however, the wind generally becomes gusty and fitful in its
action. This, it should be remembered, does not refer to the velocity of
the machine, but to that of the air itself.

In this connection Mr. Arthur T. Atherholt, president of the Aero
Club of Pennsylvania, in addressing the Boston Society of Scientific
Research, said:

"Probably the whirlpools of Niagara contain no more erratic currents
than the strata of air which is now immediately above us, a fact hard to
realize on account of its invisibility."

Changes In Wind Currents.

While Mr. Atherholt's experience has been mainly with balloons it is all
the more valuable on this account, as the balloons were at the mercy of
the wind and their varying directions afforded an indisputable g