"The pilot is blind to these pitfalls," went on Paul. "He must control his machine largely by intuition and the sense of feeling, although the veteran airman, John says, can tell a good deal about what to expect from the nature of the earth or clouds below him."
"That's true," averred John. "The closer you are to the earth the more you will feel the 'bumps,' as we call them. They are a whole lot like the waves of the ocean, only invisible, and there will be one straight over every protuberance or depression of size in the surface of the earth. Mountains, hills, houses, lakes, valleys, rivers, forests, all cause bumps or holes in the air up above them. At one thousand feet they are pretty bad. At ten thousand feet they are scarcely noticeable. That's why most pilots prefer to fly high whenever they can."
"What causes the air to act in this way over such configurations?" propounded the publisher.
John looked helpless, and smiled. "You've got me there," he admitted. "I haven't had the opportunity to study aerostatics the same as Paul here. He can probably tell us."
"I'm not through my course yet," reminded his brother, "but I think I can answer that. The air surrounding the earth is a great belt forty or more miles through and is of an even thickness. As our globe sweeps through it, the lower stratum of air naturally sinks down into the valleys and like depressions. This action pulls down the upper stretches of air, thus creating what are termed 'air-pockets' or 'air-holes.' Very dangerous they are, too."
"That is plain enough," declared Bob. "Now, dad, let Paul go on explaining this 'automatic pilot.'"
"If the aviator is enshrouded in fog or tries to sail through a heavy bank of clouds, he is quite likely to lose all sense of direction," continued Paul. "He will not know whether he is banking or traveling on an even keel. Sometimes pilots have come out of a low cloud to find themselves dangerously close to the earth and in an awkward position, perhaps in a steep bank, a side-slip, or even in the terrifying nose-dive, and they have not had time to right themselves before crashing to earth. So you see that before flying can become reasonably safe, some way must be found of keeping the machine automatically on a level keel.
"To operate this stabilizer of ours all the pilot will have to do is to guide the rudder with his feet. The automatic pilot works the elevator and the ailerons. It takes care of 'bumps' and 'holes' and sees that the machine banks at just the right angle on the turns. This makes the operation of an airplane containing the stabilizer even more simple than running a motor-car, because you do not have to worry about going into different speed gears when climbing or descending. You will notice on this drawing that strong piano-wires connect the instruments with all the necessary controlling planes of the machine."
"Instruments?" interrogated Mr. Giddings. "I thought there was but one."
"No; there are two stabilizers, as you will see,—here, and here," was Paul's response, pointing his finger to the parts. "But, as each one is exactly like the other in its construction, only the one has been drawn in detail. The other stabilizer runs lengthwise of the cockpit and takes care of the elevator. Both of these are operated by compressed air, which proceeds from a little tank, right here. The tank is kept supplied by two tubes which lead into it, and each of which joins a small pump operated by a fan which is right here on each side of the fuselage where the onrush of wind will keep it humming as the airplane travels.