This may sound a little involved, but a careful study of the accompanying diagrams will make the various movements of the mine and its sinker, after leaving the submarine, quite clear to the lay reader.
There were also other types of mines employed. Some were fitted with an automatic device which was actuated by the pressure of the water at a set depth. These weapons could be expelled from submarines without the necessity of knowing and adjusting the depth at which they were to float below the surface. A mine of this pattern rose up, after discharge from the tube, until the pressure of water on its casing was reduced to 4½ lb. per square inch (the pressure which obtains at a depth of ten feet below the surface[8]), and there the weapon stopped, waiting patiently for its prey.
Another kind of mine was of the floating variety—tabooed by The Hague Convention—which drifted along under the surface with no moorings to hold it in one position.
Now that the reader is familiar with the mines themselves and the actual methods of laying them, we can pass on to a brief review of the German mine-laying policy during the Great War.
The submarine offensive reached its maximum intensity in 1916-1917, during which period no less than 7000 mines were destroyed by the British navy alone.[9] Of this number about 2000 were drifting when discovered. There was, with one small exception, no portion of the coast of the United Kingdom which was not mined at least once during those eventful two years, the unmined area being undoubtedly left clear to facilitate a raid or invasion. About 200 minesweeping vessels were blown up or seriously damaged, but the losses among the Mercantile Marine were kept down to less than 300 ships out of the 5000 sailings which, on an average, took place weekly.
The heavy losses inflicted on the enemy's submarine fleets in 1917 marked the turning of the tide, and from that date onwards there was a steady but sure reduction in the number of mines laid.
During the first twelve months of the intensified submarine war the Germans concentrated their mine-laying on the food routes from the United States, the sea communications of the Grand Fleet off the east coast of Scotland and the line of supply to France. Then, when they commenced to realise the impossibility of starving the sea-girt island, and the weight of the ever-increasing British armies began to tell in the land war, the submarine policy changed to conform with the general strategy of the High Command, and the troop convoy bases and routes were the objects of special attack.
The arrival in Europe of the advance guard of the United States army caused another change in the submarine strategy. From that time onwards the Atlantic routes assumed a fresh importance and became the major zone of operations.
In the first year of the war the U-C boats discharged their cargoes of mines as soon as they could reach their respective areas of operation. The mines were usually laid close together in one field, frequently situated off some prominent headland, or at a point where trade routes converged. Then the enemy learned to respect the British minesweeping and patrol organisation, and endeavoured to lay their "sea-gulls' eggs" in waters which had been recently swept, or where sweeping forces appeared to be weak in numbers.