In the European War, the outstanding anti submarine offensive project was the closing of the North Sea by a mine barrage extending from the Orkney Islands to the territorial waters of Norway, together with a barrage across the Straits of Dover. The plan to close the North Sea, and thereby deny enemy sub marines free access to the Atlantic from Ger man bases, had its inception in the United States Bureau of Ordnance in April 1917, immediately following the entrance of the United States into the war. At this time there had not been developed anywhere a type of mine suitable for the Scotland-Norway line, whereon the depths of water are as great as 900 feet and where a prohibitive number of mines of the then exist ing type would have been required to mine this line from the surface to a depth of 250 to 300 feet. A special mine-loading plant, with a capacity of more than 1,000 mines a day, was established near Yorktown, Va., by the Navy Department, in accordance with Congressional authorization, about 11,000 acres of land hav ing been secured for the purpose. An immense number of mines were stowed at this depot, and it included a mine-loading plant, a mine assembling plant and storage for a large amount of high explosives. More than a score of cargo vessels were placed in service as mine carriers to transport mines to Europe. Many mine layers and auxiliary vessels were fitted out and assigned to the execution of the proj ect. The manufacture of the large number of mines required was a work of unprecedented magnitude and necessitated unusual methods.
It was clearly impossible, in the time available, to manufacture the mine as a whole in any plant ; therefore, the mine was divided into its many component parts and the parts were sepa rately produced in a large number of industrial factories throughout the country.
The submarine mine was invented by Bush nell in 1777, improved by Fulton in 1812, fur ther improved by Colt by the addition of the electric element in 1863, carried forward to a complete and practical system of mine defense by Gen. Henry L. Abbott, United States army, and since 1901 further perfected by the United States Coast Artillery corps in its use to the present efficient system of submarine defense of harbors, the planting and exploding of the mines at the right time and with precision be ing a most difficult problem, the mathematical features of which are very exacting. See NAVAL MINES and TORPEDOES.
Bibliography.—Biles J. H.,
Protection of Battleships Against Submarine Attack' (En gineering 1914) ; Currey, E. H.,
Menace of the Torpedo' (New York 1914) ; Degouy, Admiral,
Submarine War of 1917) (in Revue des Deux Mondes 1917) ; Domville-Fife, C. W.,