``The distance we shall travel over the A--B course
in one hour is A--D. That is nearly eighty-seven miles,
so we ought to accomplish our journey of a hundred and
fifty miles in about one and three-quarter hours.
``I hope that's quite clear to you. It's a very simple
way of calculating the compass course, and I always do it
like that.''
``Yes, that's plain enough. You have drafted what
engineers call `a parallelogram of forces'; but suppose you
have miscalculated the velocity of the wind, or that it should
change in velocity or direction?''
``Well, that of course will more or less alter matters,''
replies the Pilot. ``But there are any number of good
landmarks such as lakes, rivers, towns, and railway lines.
They will help to keep us on the right course, and the compass
will, at any rate, prevent us from going far astray when
between them.''
``Well, we'd better be off, old chap. Hop aboard.''
This from the Observer as he climbs into the front seat
from which he will command a good view over the lower
plane; and the Pilot takes his place in the rear seat, and,
after making himself perfectly comfortable, fixing his safety
belt, and moving the control levers to make sure that they
are working freely, he gives the signal to the Engine Fitter
to turn the propeller and so start the engine.
Round buzzes the Propeller, and the Pilot, giving the
official signal, the Aeroplane is released and rolls swiftly
over the ground in the teeth of the gusty wind.
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