These German airships were held together by rib-like skeletons of
steel and aluminium and a stout inelastic canvas outer-skin,
within which was an impervious rubber gas-bag, cut up by
transverse dissepiments into from fifty to a hundred
compartments. These were all absolutely gas tight and filled
with hydrogen, and the entire aerostat was kept at any level by
means of a long internal balloonette of oiled and toughened silk
canvas, into which air could be forced and from which it could be
pumped. So the airship could be made either heavier or lighter
than air, and losses of weight through the consumption of fuel,
the casting of bombs and so forth, could also be compensated by
admitting air to sections of the general gas-bag. Ultimately
that made a highly explosive mixture; but in all these matters
risks must be taken and guarded against. There was a steel axis
to the whole affair, a central backbone which terminated in the
engine and propeller, and the men and magazines were forward in a
series of cabins under the expanded headlike forepart. The
engine, which was of the extraordinarily powerful Pforzheim type,
that supreme triumph of German invention, was worked by wires
from this forepart, which was indeed the only really habitable
part of the ship. If anything went wrong, the engineers went aft
along a rope ladder beneath the frame. The tendency of the whole
affair to roll was partly corrected by a horizontal lateral fin
on either side, and steering was chiefly effected by two vertical
fins, which normally lay back like gill-flaps on either side of
the head. It was indeed a most complete adaptation of the fish
form to aerial conditions, the position of swimming bladder,
eyes, and brain being, however, below instead of above. A
striking, and unfish-like feature was the apparatus for wireless