the unsatisfactory loop.]

[Footnote 10: Deviation Curve: A curved line indicating any errors in the
compass.]

[Footnote 11: A propeller screws through the air, and the distance it
advances during one revolution, supposing the air to be solid, is known
as the pitch. The pitch, which depends upon the angle of the propeller
blades, must be equal to the speed of the aeroplane, plus the slip, and
if, on account of the rarity of the air, the speed of the aeroplane
increases, then the angle and pitch should be correspondingly increased.
Propellers with a pitch capable of being varied by the pilot are the
dream of propeller designers. For explanation of "slip" see Chapter IV.
on propellers.]

[Footnote 12: Getting out of my depth? Invading the realms of fancy?
Well, perhaps so, but at any rate it is possible that extraordinary
speed through space may be secured if means are found to maintain the
impulse of the engine and the thrust-drift efficiency of the propeller
at great altitude.]

[Footnote 13: Box-kite. The first crude form of biplane.]

PART IV

'CROSS COUNTRY

The Aeroplane had been designed and built, and tested in the air, and
now it stood on the Aerodrome ready for its first 'cross-country flight.

It had run the gauntlet of pseudo-designers, crank inventors, press
"experts," and politicians; of manufacturers keen on cheap work and
large profits; of poor pilots who had funked it, and good pilots who had
expected too much of it. Thousands of pounds had been wasted on it, many
had gone bankrupt over it, and others it had provided with safe fat
jobs.

Somehow, and despite every conceivable obstacle, it had managed to
muddle through, and now it was ready for its work. It was not perfect,
for there were fifty different ways in which it might be improved, some
of them shamefully obvious. But it was fairly sound mechanically, had a
little inherent stability, was easily controlled, could climb a thousand
feet a minute, and its speed was a hundred miles an hour. In short,
quite a creditable machine, though of course the right man had not got
the credit.

It is rough, unsettled weather with a thirty mile an hour wind on the
ground, and that means fifty more or less aloft. Lots of clouds at
different altitudes to bother the Pilot, and the air none too clear for
the observation of landmarks.

As the Pilot and Observer approach the Aeroplane the former is clearly
not in the best of tempers. "It's rotten