Take sound-ranging for instance, which, with flash-spotting and air photography, has enabled the gunner more and more certainly to locate his enemy's gun while concealing the position of his own.

For "the object of a gun or howitzer is to throw a projectile to some spot the position 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.