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.