the spring of 1917 the situation which we were facing was that the German submarines were destroying Allied shipping at the rate of nearly 800,000 tons a month. The one thing which was certain was that, if this destruction should continue for four or five months, the Allies would be obliged to surrender unconditionally. The pressing problem was to find methods that would check these depredations and that would check them in time. The convoy system was the one naval plan—the point cannot be made too emphatically—which in April and May of 1917 held forth the certainty of immediately accomplishing this result. Other methods of opposing the submarines were developed which magnificently supplemented the convoy ; but the convoy, at least in the spring and summer of 1917, was the one sure method of salvation for the Allied cause. To have started the North Sea barrage in the spring and summer of 1917 would have meant abandoning the convoy system; and this would have been sheer madness.
Thus in 1917 the North Sea barrage was not an answer to the popular proposal "to dig the Germans out of their holes like rats." We did not have a mine which could be laid in such deep waters in sufficient numbers to have formed any barrier at all; and even if we had possessed one, the construction of the barrage would have demanded such an enormous number that they could not have been manufactured in time to finish the barrage until late in the year 1918. Presently the situation began to change. The principal fact which made possible this great enterprise was the invention of an entirely new type of mine. The old mine consisted of a huge steel globe, filled with high explosive, which could be fired only by contact. That is, it was necessary for the surface of a ship, such as a submarine, to strike against the surface of the mine, and in this way start the mechanism which ignited the explosive charge. The fact that this immediate contact was essential enormously increased the difficulty of successfully mining waters that range in depth from 400 to 900 feet. If the mines were laid anywhere near the surface, the submarine, merely by diving beneath them, could avoid all danger; if they were laid any considerable depth, it could sail with complete safety above them. Thus, if such a mine were to be used at all, we should have had to plant several layers, one under the other, down to a depth of about 250 feet, so that the submarine, at
