There would be three “systems,” each consisting of one or more rows of mines just below the surface, dangerous to any craft, and other rows at intermediate and extreme depths, so that, whether running on the surface or at ordinary submergence or as deep as 240 feet, a submarine had the odds against her. In the absence of patrol vessels to drive them down, submarines would naturally run on the surface, and so the rows of upper level mines were made more numerous than those at deeper levels. The stroke of a mine is sudden and powerful, and while a vessel on the surface may survive it, to a submerged submarine it is usually fatal. All classes of vessels shy at a minefield, and that the Germans shared this aversion was shown by captured papers, which made it clear that the submarines dreaded nothing so much as mines. The scheme was unprecedented, and that its great magnitude would involve a mass of detail requiring very careful adjustment was evident on the most cursory examination. Some who heard of it regarded it as impossible, and foolish to attempt. As to the new mines, the very basis of the whole project—since a complete unit would not exist for several months, the statement of Rear Admiral Ralph Earle, Chief of the Naval Bureau of Ordnance, that the mines would be forthcoming in season, had to be based upon tests of the mine only by parts, with the assumption that all would function properly when assembled. Action upon that assurance would at once involve upward of forty million dollars, which made his stand a bold one, inviting unmeasured odium, should the mine after all fail. To await the mine’s final proving, however, would have been fatal to any possibility of beginning the barrage before 1919. The task of laying the barrier would be hazardous in itself, with constant danger of interruption by the enemy. A single minefield in the open sea, or widely separated ones, presented no extreme difficulties, but to lay a series of them so close together as to leave no considerable gaps between, made a problem for which no really practical solution was yet visible.