k, equivalent to that of a 7500-ton
ship necessary to supply lifting power for the metallic frame,
naturally made her unwieldy to handle, unsafe to leave at rest,
outside of a sheltering shed, and a particularly attractive target
for artillery in time of war. Actual action indeed proved that to be
safe from the shells of anti-aircraft guns, the Zeppelins were
forced to fly so high that their own bombs could not be dropped with
any degree of accuracy upon a desired target.

The balloon's frame is made of aluminum, the lightest of metals, but
not the least costly. A curious disadvantage of this construction
was made apparent in the accident which destroyed _Zeppelin IV._
That was the first of the airships to be equipped with a full
wireless outfit which was used freely on its flight. It appeared
that the aluminum frame absorbed much of the electricity generated
for the purpose of the wireless. The effect of this was two-fold. It
limited the radius of operation of the wireless to 150 miles or
less, and it made the metal frame a perilous storehouse of
electricity. When _Zeppelin IV._ met with a disaster by a storm
which dragged it from its moorings, the stored electricity in her
frame was suddenly released by contact with the trees and set fire
to the envelope, utterly destroying the ship.

The balloon frame was divided into seventeen compartments, each of
which held a ballonet filled with hydrogen gas. The purpose of this
was similar to the practice of dividing a ship's hulls into
compartments. If one or more of the ballonets, for any reason, were
injured the remainder would keep the ship afloat. The space between
the ballonets and the outer skin was pumped full of air to keep the
latter taut and rigid. Moreover it helped to prevent the radiation
of heat to the gas bags from the outer envelope whose huge expanse,
presented to the sun, absorbed an immense amount of heat rays.

Two cars were suspended from the frame of the Zeppelin, forward and
aft, and a corridor connected them. A sliding weight was employed
to raise or depress the bow. In each car of the first Zeppelin was
a sixteen-horse-power gasoline motor, each working two screws, with
four foot blades, revolving one thousand times a minute. The engines
were reversible, thus making it possible to work the propellers
against each other and aid materially in steering the ship. Rudders
at bow and stern completed the navigating equipment.

In the first Zeppelins, the