"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. "Each equalizer has a bore in it half-filled with mercury, working a good deal like a carpenter's level. If the airplane tilts to one side or the other, the mercury will try to keep its level and will immediately flow to the high side of the bore. At each end of this mercury tube there are electrical contact points. As one becomes submerged in the mercury by a tilting of the plane, a connection is made whereby two electro-magnets are energized on that side. One of these magnets closes an exhaust-valve, and the other opens an inlet-valve, in the compressed air tank. At once air is forced into this double cylinder, which you see at the bottom of the stabilizer, filling the half which is to operate its own set of rudders; and a piston begins to work inside. The piston is connected to a toothed rack, as you will note, causing this to turn a sector engaging it. The control wires connect with this sector." "Very clever arrangement; but I don't quite see how, in banking, the ailerons can be brought back automatically to a neutral position as soon as the turn has been completed," ventured Mr. Giddings.