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.
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