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.

[Illustration:
A--B, 150 miles,
A--C, 50 miles; direction and miles per hour of wind.
C--D, 100 miles; airspeed of aeroplane.
A--D, Distance covered by aeroplane in one hour.
A--E, Compass course.]

"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."

"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.

In less than fifty yards it takes to the air and begins to climb rapidly
upwards, but how different are the conditions to the calm morning of
yesterday! If the air were visible it would be seen to be acting in the
most extraordinary manner; crazily swirling, li