official German sources of information the latest types of
Zeppelins are engined by "noiseless" motors. There is nothing remarkable
in this feature, since the modern motor-car virtually answers to this
description, although in this instance quietness is obtained for the
most part by recourse to the sleeve-valve engine. Still, the ordinary
Otto-cycle internal combustion engine can be rendered almost silent by
the utilisation of adequate muffling devices, which, in the Zeppelin,
are more possible of incorporation than in the aeroplane, because the
extra weight imposed by this acquisition is a minor consideration in
comparison with the lifting power of the vessel.

Night operations, however, have not proved eminently successful. The
very darkness which protects the aerial prowler also serves a similar
purpose in connection with its prey. But aerial operations under the
cover of darkness are guided not so much by the glare of lights from
below as betrayal by sound. The difference between villages and cities
may be distinguished from aloft, say at 1,500 to 3,000 feet, by the hum
which life and movement emit, and this is the best guide to the aerial
scout or battleship. The German authorities have made a special study
of this peculiar problem, and have conducted innumerable tests upon the
darkest nights, when even the sheen of the moon has been unavailable,
for the express purpose of training the aerial navigators to discover
their position from the different sounds reaching them from below. In
other words, the corsair in the skies depends more upon compass and
sound than upon compass and vision when operating after dark. The
searchlights with which the Zeppelins are equipped are provided merely
for illuminating a supposed position. They are not brought into service
until the navigator concludes that he has arrived above the desired
point: the ray of light which is then projected is merely to assist the
crew in the discharge of the missiles of destruction.

The Zeppelin, however, owing to its speed, both in the horizontal and
vertical planes, is essentially a unit for daylight operations. The
other airships which Germany possesses, and which for the most part are
of the non-rigid type, are condemned to daylight operations from
the character of their design. Owing to their low speeds they may be
dismissed as impossible aerial vessels for hazardous work and are not
regarded by the German authorities as all-round airships of war