up to mark, and all the control cables in perfect condition
and tension.''
``Very good,'' said the Pilot; and then turning to the
Observer, ``Before we start you had better have a look
at the course I have mapped out.
``A is where we stand and we have to reach B, a hundred
and fifty miles due North. I judge that, at the altitude
we shall fly, there will be an East wind, for although it is
not quite East on the ground it is probably about twenty
degrees different aloft, the wind usually moving round clockways
to about that extent. I think that it is blowing at the
rate of about fifty miles an hour, and I therefore take a line
on the map to C, fifty miles due West of A. The Aeroplane's
speed is a hundred miles an hour, and so I take a line of one
hundred miles from C to D. Our compass course will then
be in the direction A--E, which is always a line parallel to
C--D. That is, to be exact, it will be fourteen degrees off
the C--D course, as, in this part of the globe, there is that
much difference between the North and South lines on the
map and the magnetic North to which the compass needle
points. If the compass has an error, as it may have of a
few degrees, that, too, must be taken into account, and the
deviation or error curve on the dashboard will indicate it.
``The Aeroplane will then always be pointing in a direction
parallel to A--E, but, owing to the side wind, it will be actually
travelling over the course A--B, though in a rather
sideways attitude to that course.
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