on of which is _known_." The older
way of knowing was by registration--throwing round after round, and by
the help of aeroplane or other observation of the results, getting
nearer and nearer to the target till the range was exactly found. By
this method, not only is the enemy warned, but your own position is
revealed. The newer method aims at _surprise_--the supreme aim of
modern war.

"The principle of the location of guns by sound," writes an artillery
officer, "is simple enough. Suppose there are two observers in the
British lines, one at each end of a long line. Bisect this base, and
from the middle point draw a line at right angles to the base and
towards the German lines. Now, if a hostile gun fires from a position
on this line, the sound will reach both observers simultaneously. If
the gun fires from a position to the right of the line, the sound will
reach the right-hand observer first, and vice versa. Then, by
measuring exactly the time-interval between the arrival of the sound
at each observation post, the bearing to the gun can be calculated."

"Until quite recently the Germans used four human observers, who timed
the sound intervals with stop watches. The British used six
microphones of a special type, connected electrically with a
photographic-recording apparatus. Instead of stop watches, therefore,
we used a timing device capable of recording the most minute
time-intervals with perfect precision. The whole system was
immeasurably superior to the German, and at least twenty times as
accurate, for the British system was absolutely automatic. It recorded
the arrival of the sound at the various microphones instantaneously on
a permanent record; while the German system, apart from its crude
method of measuring time, was subject to the combined errors of four
human 'microphones.' The British system requires only one forward
observer, placed well ahead of the base, and all he has to do is to
press a button and start the apparatus before the sound reaches the
microphones.

"The photographic record is ready for the computer in from six to ten
seconds, and the gun position can be found and plotted in three or
four minutes.

"Sound ranging also can be used for ranging our own guns with great
accuracy. When a record has been obtained of a hostile gun, all that
need be done is to record the burst of our own shell and give
corrections to our battery until the record of our shell-burst is
identical with that of the