hecked over and in the
locker; engine and Aeroplane logbooks written up, signed, and under your
seat; engine revs. 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.

"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."