convoys reached the English. Channel they broke up, certain ships going to Plymouth, Portsmouth, Southampton, and other Channel ports, and others sailing to Brest, Cherbourg, Havre, and other harbours in France. In the same fashion, convoys which came in by way of the North Channel split up as soon as they reached the Irish Sea. In other words, the convoys, as convoys, necessarily ceased to exist the moment that they entered these inland waters, and the ships, as individual ships, or small groups of ships, had to find their way to their destinations unescorted by destroyers, or escorted most inadequately. This was the one weak spot in the convoy system, and the Germans were not slow to turn it to their advantage. They now proceeded to withdraw most of their submarines from the high seas and to concentrate them in these restricted waters. In April, 1917, the month which marked the high tide of German success, not far from a hundred merchant ships were sunk in an area that extended about 300 miles west of Ireland and about 300 miles south. A year afterward—in the month of April, 1918—not a single ship was sent to the bottom in this same section of the sea. That change measures the extent to which the convoy saved Allied shipping. But if we examine the situation in inclosed waters—the North Channel, the Irish Sea, St. George's Channel, and the English Channel—we shall find a less favourable state of affairs. Practically all the sinkings of April, 1918, occurred in these latter areas. In April, 1917, the waters which lay between Ireland and England were practically free from depredations; in the spring of 1918, however, these waters had become a favourite hunting ground for submarines; while in the English Channel the sinkings were almost as numerous in April, 1918, as they had been in the same month the year before.
Thus we had to deal with an entirely new phase of the submarine campaign; the new conditions made it practicable to employ light vessels which existed in large numbers, and which could aggressively hunt out the submarines even though they were sailing submerged. The subchaser, when fitted with its listening devices, met these new requirements, though of course not to the desirable degree of precision we hoped soon to attain with still further improved hydrophones and larger vessels of the Eagle class then being built.
