The submarine, which had formerly been able to stop the unarmed merchantman and sink him at leisure, after a few combats with an armed merchantman began to be very wary and to depend almost entirely upon his torpedoes. It was not always easy for the submarine to get in a position where her torpedo would be effective, and the merchantman was carefully directed, if attacked, to pursue a zig-zag irregular course, and at the same time endeavor to hamper the submarine by shooting as near her periscope as possible. Along the sea coasts and at certain points in the English Channel great nets were used effectively. Submarines, however, toward the end of the war were made sufficiently large to be able to force their way through these nets, and net-cutting devices were also used by them with considerable effect. The best way to destroy the submarines seemed to be in a direct attack by flotillas or destroyers. By the end of the war the whole process of sinking or destroying submarines had been thoroughly organized. Practically every portion of the seas near Great Britain and France was carefully watched and the appearance of a submarine immediately reported. As the submarine would only travel at a certain well-understood speed during a given time, it was possible to calculate, after the locality of one was known, about how far from that point it would be found at any later period. Destroyers were therefore sent circling around the point where the submarine had been discovered, enlarging their distance from the center every hour. In the course of time the submarine would be compelled to come up for air, and then, if luck were with the destroyer, it might find its foe before it was seen itself. Having discovered the submarine the destroyer immediately endeavored to ram, dropping depth bombs at the point where they supposed the enemy to be. These bombs were so constructed that at a certain depth in the water they would explode, and the force of the explosion was so great that even if they did not strike the submarine they would be sure to damage it seriously, sometimes throwing the submarine to the surface partly out of water, and at other times driving her to come to the surface herself ready to surrender. In many cases it was not necessary to use the depth bomb at all. The gunners on board the destroyers had become extraordinarily expert, and though a shot might destroy the periscope of a submarine without doing much damage, most submarines carrying extra periscopes to use if necessary, yet it was soon found that it was possible by the use of plunging shells to do effective damage. Plunging shells are somewhat similar in their operation to bombs. Such a shell falling just short of a periscope and fused to burst both on contact and at a certain depth was extremely likely to do damage.