the speed increases until at 60 miles an hour
approximately 3 horsepower is needed.

In a machine like the Curtiss the area of wind-exposed surface is about
15 square feet. On the basis of this resistance moving the machine at 40
miles an hour would require 12 horsepower. This computation covers only
the machine's power to overcome resistance. It does not cover the power
exerted in propelling the machine forward after the air pressure is
overcome. To meet this important requirement Mr. Curtiss finds it
necessary to use a 50-horsepower engine. Of this power, as has been
already stated, 12 horsepower is consumed in meeting the wind pressure,
leaving 38 horsepower for the purpose of making progress.

The flying machine must move faster than the air to which it is opposed.
Unless it does this there can be no direct progress. If the two forces
are equal there is no straight-ahead advancement. Take, for sake of
illustration, a case in which an aeroplane, which has developed a speed
of 30 miles an hour, meets a wind velocity of equal force moving in an
opposite direction. What is the result? There can be no advance because
it is a contest between two evenly matched forces. The aeroplane stands
still. The only way to get out of the difficulty is for the operator to
wait for more favorable conditions, or bring his machine to the ground
in the usual manner by manipulation of the control system.

Take another case. An aeroplane, capable of making 50 miles an hour in a
calm, is met by a head wind of 25 miles an hour. How much progress does
the aeroplane make? Obviously it is 25 miles an hour over the ground.

Put the proposition in still another way. If the wind is blowing harder
than it is possible for the engine power to overcome, the machine will
be forced backward.

Wind Pressure a Necessity.

While all this is true, the fact remains that wind pressure, up to
a certain stage, is an absolute necessity in aerial navigation. The
atmosphere itself has very little real supporting power, especially if
inactive. If a body heavier than air is to remain afloat it must move
rapidly while in suspension.

One of the best illustrations of this is to be found in skating over
thin ice. Every school boy knows that if he moves with speed he may
skate or glide in safety across a thin sheet of ice that would not
begin to bear his weight if he were standing still. Exactly the same
proposition obtains in the case of the flying machine.

Th