oplane. As a matter of fact, when the machine is travelling at
high speed it trails horizontally astern, but this is immaterial. One
investigator, who strongly disapproves of the trailing aerial, has
carried out experiments with a network of wires laid upon and attached
to the surface of the aeroplane's wings. But the trailing wire is
generally preferred, and certainly up to the present has proved more
satisfactory.

The greatest obstacle, however, is the necessary apparatus. The average
aeroplane designed for military duty is already loaded to the maximum.
As a rule it carries the pilot and an observer, and invariably includes
a light arm for defence against an aerial enemy, together with an
adequate supply of ammunition, while unless short sharp flights are to
be made, the fuel supply represents an appreciable load. Under these
circumstances the item of weight is a vital consideration. It must be
kept within a limit of 100 pounds, and the less the equipment weighs the
more satisfactory it is likely to prove, other things being equal.

The two most successful systems yet exploited are the Dubilier and the
Rouget. The former is an American invention, the latter is of French
origin. Both have been tested by the British Military Aeronautical
Department, and the French authorities have subjected the French system
to rigorous trials. Both systems, within their limitations, have proved
satisfactory.

The outstanding feature of the Dubilier system is the production of sine
waves of musical frequency from continuous current, thus dispensing
with the rotary converter. The operating principle is the obtaining of
a series of unidirectional impulses by a condenser discharge, the
pulsating currents following one another at regular intervals at a
frequency of 500 impulses per second, which may be augmented up to 1,000
impulses per second. The complete weight of such an apparatus is 40
pounds; the electric generator, which is no larger than the motor used
for driving the ordinary table ventilating fan, accounts for 16 pounds
of this total. Under test at sea, upon the deck of a ship, a range of
250 miles has been obtained. The British Government carried out a series
of experiments with this system, using a small plant weighing about 30
pounds, with which communication was maintained up to about 20 miles.

In the French system the Reuget transmitter is employed. The apparatus,
including the dynamo, which is extremely small, weighs i