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.

"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