. that the faster the speed the less the force required to
sustain the planes, and that it would cost less to transport such planes
through the air at a high rate of speed than at a low one. I found
further that one horse-power could carry brass plates weighing two
hundred pounds at the rate of more than forty miles an hour in
horizontal flight.'

When these researches were known and understood, their effect upon the
practical handling of the problem of flight was immediate and decisive.
The aeroplane, or gliding machine, had many rivals; they were all killed
by Professor Langley's researches, which showed that the cheapest and
best way to raise a plane in the air is to drive it forward at a small
upward inclination; and that its weight can be best countered not by
applying power to raise it vertically, but by driving it fast. In the
statistical tables that he prepared he called the upward pressure of the
air _Lift_; the pressure which retards horizontal motion he called
_Drift_. The words make a happy pair, but the word _Drift_ is badly
needed to describe the leeway of an aeroplane in a cross-wind, so that
in England another pair of words, _Lift_ and _Drag_, has been
authoritatively substituted.

From this time onward Langley devoted himself to those other problems,
especially the problems of balance, of mechanical power, and of safety
in taking off and alighting, which had to be solved if he was to make a
machine that should fly. He was much influenced, he says, by a
mechanical toy, produced as early as 1871 by an ingenious Frenchman
called Penaud, and named by its inventor the 'planophore'. This toy,
which weighed only a little over half an ounce, was supported on wings,
and was driven forward by an airscrew made of two feathers. The motive
power was supplied by twisted strands of rubber which, as they
untwisted, turned the airscrew. The wings were set at a dihedral angle,
that is, they were bent upwards at the tips; and fore-and-aft stability
was secured by a smaller pair of wings just in front of the airscrew.
'Simple as this toy looked,' says Professor Langley, 'it was the father
of a future flying machine, and France ought to have the credit of it.'
His own steam-driven flying machine was produced and successfully flown
in 1896. It had two wings and a tail, with a supporting surface in all
of seventy square feet; its total weight was seventy-two pounds; the
engine, constructed by himself, weighed only seven poun