ioned
and arranged it exactly like the corresponding lever in a locomotive.
Placed vertically, thus, the engines remain motionless. Thrown forward,
thus, the engines will turn ahead. And thrown backward, thus, they will
turn astern. That is simple enough. And so is this," directing
attention to a dial on his left hand which stood facing him. The dial
had a single hand which was obviously intended to travel over a
carefully graduated arc of ninety degrees painted on the dial-face, and
which, in addition to the graduations, was marked in the proper
positions with the words "Stop;" "Quarter speed;" "Half speed;" "Full
speed;" and also with two arrows pointing in opposite directions marked
"On" and "Off" respectively. Just beneath the dial was a small wheel
with a crank-handle projecting from one of its spokes, and on this
crank-handle the professor now laid his hand.

"This," he said, "regulates the valve which admits vapour into the
engine; and the dial-hand shows the extent to which the valve is opened.
Turn the wheel in the direction of the arrow marked `On'--thus, and you
admit vapour into the engine. You will observe that, as I turn the
wheel, the hand on the dial travels over the arc and indicates the
extent to which the valve is open. There; now it is fully open, and the
cylinders are full of vapour." Then he quickly reversed the wheel and
sent the index hand back to "Stop," keeping a wary eye on his companions
as he did so.

"These are dangerous things to meddle with," he remarked apologetically.
"The engines are of one hundred thousand horse-power; and, full as the
ship now is of air at the atmospheric pressure, they would drive her
irresistibly along the ground and through all obstacles. I must beg
that none of you will meddle with the machinery until you are fully
acquainted with its tremendous power."

"What is this pendulum-looking affair, professor?" asked the colonel,
pointing to a pendulum the point of which hung in a shallow basin-like
depression thickly studded with needle-points which the pendulum just
cleared by a hair's-breadth.

"That," explained the professor, "is a device for automatically
regulating the balance, or `trim' as you call it, of the ship when she
is floating in the air. You will readily understand that when freed of
air, and thus deprived of weight, as it were, the most trifling matter
will suffice to derange her equilibrium; one of us, walking from side to
side, or from