Although
wireless telegraphy is available for this purpose, it suffers
from certain drawbacks. If the enemy possesses an equipment
which is within range of that of the air-craft and the force to
which it belongs, communications may be nullified by the enemy
throwing out a continuous stream of useless signals which "jamb"
the intelligence of their opponents.
If a message--written in code--or a map is to be dropped from
aloft it is enclosed within a special metallic cylinder, fitted
with a vane tail to ensure direction of flight when launched, and
with a detonating head. This is dropped overboard. When it
strikes the ground the detonator fires a charge which emits a
report without damaging the message container, and at the same
time fires a combustible charge emitting considerable smoke. The
noise attracts anyone in the vicinity of the spot where the
message has fallen, while at the same time the clouds of smoke
guide one to the point and enable the cylinder to be recovered.
This device is extensively used by the German aviators, and has
proved highly serviceable; a similar contrivance is adopted by
French airmen.
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