"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.
"John and I have provided for that, while Bob is responsible for the electrical features I have just mentioned," said Paul. "You will notice that at the top of the mercury channel there is a dividing wall. A tube runs from the left side of this wall to the right wing of the airplane, also from the right side of the wall to the left wing. At the end of each tube there is what we call a 'venturi tube.' This is a kind of suction device operated by the wind. The wind which blows through the left venturi tube sucks the air out of the right-hand side of the mercury tube, and the right venturi tube sucks the air out of the left-hand side of the mercury tube. The stronger the wind, the greater the suction. Now, when making a turn to the right the left wing must travel faster than the right wing, and so there must be more suction in the left venturi. This produces a greater suction in the right-hand side of the mercury tube, which draws the mercury up on that side and down on the other, until the proper electrical contact is broken and the ailerons are returned to neutral position."
"Can the mechanism be thrown out of gear when desired? I should think such a feature might be desirable," remarked Mr. Giddings.
"Indeed it is desirable, sir," declared Paul. "No red-blooded pilot wishes to sit still and let his machine run itself all the time, no more than an automobilist. That would spoil all the sport. By merely disengaging the automatic pilot's wires here at the sector—the work of a couple of seconds—the airplane is ready for hand control."
"How much does it weigh?" was the gentleman's next query.
"A trifle less than a hundred pounds."
"That oughtn't to handicap an airplane any."
"Not a bit," said Paul.