A. A.
sombre caverns rose to their feet and stood to
attention:
'Eyes front!'
'Sit down/ he said.
They were very cheerful, the troglodytes. Under
the dank earth they sung, wrote letters innumerable,
and waited for soup and the night.
Outside, beneath an awning of plaited straw> the
spotters, who are replaced every half-hour, watched
at the four points of the compass. The Lieutenant
put on a show for us. The spotter signalled an
imaginary aeroplane and with remarkable speed the
guns were ready to fire.
'Target on V
Tire!'
The shooting is directed by the trembling needle
of the predictor, that astounding apparatus with the
brain of a dazzling arithmetician. To reach an
aeroplane that is moving at a vaguely determined
height at a speed of three to four hundred miles an
hour presents a problem worthy of an Einstein, a
problem only to be solved in four dimensions. The
guns of the A. A. fire upon an aeroplane in the future.
To the predictor, a metal box larded with frames,
the officer submits the elements of the problem: the
direction and speed of the wind, the height of the
machine as determined by the telemeter, the speed of
the aeroplane, et cetera. An equation has to be
established between these data and the direction of
49