ways. The air ships, whose
length varies from fifty to five hundred feet, have rudders for giving
a vertical or a horizontal motion, and several strengthening keels that
prevent leeway when turning. They are entirely on the principle of
birds, maintaining themselves mechanically, and differing thus from the
unwieldy balloon. Starting as if on a circular railway, against the
wind, they rise to a considerable height, and then, shutting off the
batteries, coast down the aerial slope at a rate that sometimes touches
five hundred miles an hour. When near the ground the helmsman directs
the prow upward, and, again turning on full current, rushes up the
slope at a speed that far exceeds the eagle's, each drop of two miles
serving to take the machine twenty or thirty; though, if the pilot does
not wish to soar, or if there is a fair wind at a given height, he can
remain in that stratum of the atmosphere by moving horizontally. He
can also maintain his elevation when moving very slowly, and though the
headway be entirely stopped, the descent is gradual on account of the
aeroplane's great spread, the batteries and motors being secured to the
under side of the deck.

"The motors are so light that they develop two horse power for every
pound of their weight; while, to keep the frames thin, the necessary
power is obtained by terrific speed of the moving parts, as though a
steam engine, to avoid great pressure in its cylinders, had a long
stroke and ran at great piston speed, which, however, is no
disadvantage to the rotary motion of the electric motor, there being no
reciprocating cranks, etc., that must be started and stopped at each
revolution.

"To obviate the necessity of gearing to reduce the number of
revolutions to those possible for a large screw, this member is made
very small, and allowed to revolve three thousand times a minute, so
that the requisite power is obtained with great simplicity of
mechanism, which further decreases friction. The shafts, and even the
wires connecting the batteries with the motors, are made large and
hollow. Though the primary battery pure and simple, as the result of
great recent advances in chemistry, seems to be again coming up, the
best aeroplane batteries are still of the combination-storage type.
These have been so perfected that eight ounces of battery yield one
horse power for six hours, so that two pounds of battery will supply a
horse power for twenty-four hours; a small fifty