I have described one of these thirteen summer excursions, and the description given could be applied to all the rest. Once or twice the periscope of a submarine was sighted--without any disastrous results--but in the main this business of mine-laying was uneventful. Just what was accomplished the chart makes clear. In the summer and autumn months of 1918 the American forces laid 56,571 mines and the British 13,546. The operation was to have been a continuous one; had the war gone on for two years we should probably have laid several hundred thousand; Admiral Strauss's forces kept at the thing steadily up to the time of the armistice; they had become so expert and the barrage was producing such excellent results that we had plans nearly completed for building another at the Strait of Otranto, which would have completely closed the Adriatic Sea. Besides this undertaking the American mine-layer _Baltimore_ laid a mine-field in the North Irish Channel, the narrow waters which separate Scotland and Ireland; two German submarines which soon afterward attempted this passage were blown to pieces, and after this the mine-field was given a wide berth.
Just what the North Sea barrage accomplished, in the actual destruction of submarines, will never be definitely known. We have information that four certainly were destroyed, and in all probability six and possibly eight; yet these results doubtless measure only a small part of the German losses. In the majority of cases the Germans had little or no evidence of sunken submarines. The destroyers, subchasers, and other patrol boats were usually able to obtain some evidences of injury inflicted; they could often see their quarry, or the disturbances which it made on the surface; they could pursue and attack it, and the resultant oil patches, wreckage, and German prisoners--and sometimes the recovered submarine itself or its location on the bottom--would tell the story either of damage or destruction. But the disconcerting thing about the North Sea barrage, from the viewpoint of the Germans, was that it could do its work so secretly that no one, friend or enemy, would necessarily know a thing about it. A German submarine simply left its home port; attempting to cross the barrage, perhaps at night, it would strike one of these mines, or its antenna; an explosion would crumple it up like so much paper; with its crew it would sink to the bottom; and not a soul, perhaps not even the crew itself, would ever know what had happened to it. It would in truth be a case of "sinking without a trace"--though an entirely legitimate one under the rules of warfare. The German records disclosed anywhere from forty to fifty submarines sunk which did not appear in the records of the Allies; how these were destroyed not a soul knows, or ever will know. They simply left their German ports and were never heard of again. That many of them fell victims to mines, and some of them to the mines of our barrage, is an entirely justifiable assumption. That probably even a larger number of U-boats were injured is also true. A German submarine captain, after the surrender at Scapa Flow, said that he personally knew of three submarines, including his own, which had been so badly injured at the barrage that they had been compelled to limp back to their German ports.