in that position by a small metal buoy. Any metallic substance, such as the hull of a submarine, simply by striking this antenna at any point, would produce an electric current, which, instantaneously transmitted to the mine, would cause this mine to explode. The great advantage of this device is at once apparent. Only about one fourth the number of mines required under the old conditions would now be necessary. The Mining Section estimated that 100,000 mines would form a barrier that would be extremely dangerous to submarines passing over it or through it, whereas, under the old conditions, about 400,000 would have been required. This implies more than a mere saving in manufacturing resources; it meant that we should need a proportionately smaller number of mine-laying ships, crews, officers, bases, and supplies--all those things which are seldom considered by the amateur in warfare, but which are as essential to its prosecution as the more spectacular details. I wish to emphasize the fact that, in laying such a barrage, it was not our object to make an absolute barrier to the passage of submarines. To have done this we should have needed such a great number of mines that the operation would have been impossible. Nor would such an absolute barrier have been necessary to success; a field that could be depended upon to destroy one-fourth or one-fifth of the submarines that attempted the passage would have represented complete success. No enemy could stand such losses as these; and the _moral_ of no crew could have lasted long under such conditions.