“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.”
“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, lifting and dropping, gusts viciously colliding—a mad phantasmagoria of forces!
Wickedly it seizes and shakes the Aeroplane; then tries to turn it over sideways; then instantly changes its mind and in a second drops it into a hole a hundred feet deep, and if it were not for his safety belt the Pilot might find his seat sinking away from beneath him.