What is of special interest is this. Up to the year 1909 it appeared quite premature to discuss methods of concentrating the fire of several ships on a single distant target, until right methods had been discovered for making sure of hitting it with the guns of a single ship. But by the winter of 1909 there seemed to be sufficient experience to show that a complete solution of the simpler problem was assured, and that the time had come for considering how two or more ships could combine their armament. The difficulty of the matter was soon made obvious. While great guns do not all shoot exactly alike, it is possible to ascertain by experiment the individual differences of all the guns in a single ship, and to vary the sight scales so that, at all critical ranges, they should give identical results. But what can be done for a single battery of eight or ten guns cannot be done by experiment for two units of such batteries. If then two ships are to be employed at the same target, it was the very essence of the matter if two processes were carried on simultaneously to obtain one result, that each process should be so organized as to run as if the other were not going at all. Now ships’ guns at sea can be corrected only from positions high up in the masts. It therefore became clear that if the firing ship allowed a fixed interval, say three or four seconds, to elapse after a sister ship had fired, before sending her own salvo at the enemy, it would be quite easy, by keeping a record of the time of flight of the projectiles, to pick out her own amongst the salvoes falling in rapid succession on the target, so that there should be no possibility of her mixing up her own shells with her neighbours’. It is now many years since it was suggested that gongs driven by a clockwork device, which could be set to the time of flight, would simplify this method of identification. Suppose the time of flight to be twelve seconds, the gong would be set to this interval and the clockwork started into motion simultaneously with the firing of the salvo. The observers watch the target and pay no attention to any shots that fall, except those whose incidence coincided with the ringing of the gong. The essence of this system was the ear-marking, so to speak, of each separate salvo as it went away. But it was manifestly not a principle on which observers placed at a distance from a ship could work. If they were to do their work they must employ some totally different means of identification. Else indirect firing could only be carried on by one ship at a time.