As they rose with the speed and strength and sureness of a giant eagle, they left the city of William Penn far behind, noted the spot which indicated Lewes, Delaware, as it seemed to flit swiftly beneath them on the flank of the lower bay, then passed Cape May and were out over the open sea again. The moon was now disappearing and it devolved upon Don Harlan, the navigator of the crew, by chart and compass and air-speed indicator (whose information, by the way, is always problematical, for reasons which will be explained in a moment), to guide them safely to their destination.
Now as to one of the present grave difficulties with which the navigators of the air have to contend, especially when flying over bodies of water, which, unlike flights over the ground, give no "landmarks" by which position may be determined.
If there is, let us say, no wind whatever blowing, either with or against the direction of the plane, the air-speed indicator will register one hundred miles per hour speed when the plane is traveling at that rate. But let the plane, with its engines running at the same power, get into the teeth of a seventy-five-mile-an-hour gale, and with a seventy-five mile push back to a hundred mile an hour forward push of the engines, the speed-indicator will still register one hundred miles per hour (that is, air-speed), although the plane will actually be traveling a distance of only twenty-five miles per hour with relation to the ground.
In other words, it is the principle of air pressure, and if there is no adverse air pressure, the indicator will show the exact speed of the plane. But the moment the plane is either augmented or retarded by favorable or unfavorable winds, the air-speed indicator becomes a very unreliable instrument for showing distances traveled: it practically only records the speed of the air pushed past the plane. It is like running at ten miles an hour with a pin-wheel in the hand on a perfectly calm day, and getting a certain velocity of revolutions of the wheel per minute. On another day one might stand still with the pin-wheel and permit the rush of a high velocity of wind to twirl it round with the same speed.
And here is a hint to our youthful readers who are interested in mathematics and things mechanical: Sometime somebody is going to invent an instrument which will record an aeroplane's actual speed with relation to the distance covered above the ground; in other words, which will actually show a speed of only twenty-five miles an hour when a hundred-mile-an-hour engine speed is being reduced to twenty-five by a head-on seventy-five-mile-an-hour gale; and the one who succeeds with that invention not only will make for himself a fortune, but then may turn his attention to the devising of another instrument, equally important, which will show how far a side wind is driving a plane out of its course.
But Don Harlan had trained long and studiously to combat and conquer just such difficulties, and like the seasoned sailor who can look at a clear sky and seem to smell a storm brewing, or a squall coming, he had learned, by some intuition which he could not even attempt to explain, to estimate with almost miraculous accuracy to just what extent the wind was retarding them or blowing them off their course.
He was bending over his charts now, marking off their course, registering the slight wind deviation, when an exclamation from Fred, who sat at all times with the radio earpieces on, attracted the attention of all. With Big Jack and Andy Flures, the pilots, it was indicated merely by the briefest turning of the head, but Don stopped short in his work to watch Fred jotting down a message that was coming mysteriously out of the night.
"Official dispatch," he announced a moment later.
"Follow previous instructions. One remain with plane, other three at my office nine if possible. Repeat."
It was signed by Bronson, head of the air service.