The immense manufacturing resources of the United States rendered a large production of mines an easy matter, with the result that as soon as the United States Navy produced a reliable type of mine the idea of placing a mine barrage across the northern part of the North Sea which had been previously discussed became a matter of practical politics.

With this end in view a still further addition to the minelaying fleet became necessary, and since the mining would be carried out at leisure in this case and speed was no great necessity for the minelayer owing to the distance of the minefields from enemy waters, an old battleship was put in hand for conversion.
With the enormous increase in the number of mines on order the problem of storage became of importance, including as it did the storage of the very large number, some 120,000, required for the northern barrage.

The Third Sea Lord, Admiral Lionel Halsey, took this matter in hand with characteristic energy, and in conjunction with United States naval officers made all the necessary arrangements.
The United States mines were stored in the vicinity of Invergordon, and the British mines intended for use in the northern barrage were located at Grangemouth, near Leith, where Rear-Admiral Clinton Baker was in charge, as well as in other places, whilst those for use in the Heligoland Bight and Channel waters were stored at Immingham and other southern depots.
The laying of the North Sea mine barrage was not accomplished without very considerable delay, and many difficulties were encountered.

It was originally anticipated that the barrage would be completed in the spring of 1918, but owing to various defects in both British and United States mines which made themselves apparent when the operations commenced, due partly to the great depth of water as well as to other causes, a delay of several months took place; and, even when near completion, the barrage was not so effective as many had hoped in spite of the great expenditure of labour and material involved.

I have not the figures of the number of submarines that the barrage is thought to have accounted for, but it was known to be disappointing.
FLARES In the late summer of 1917 _flares_ were experimented with; they were intended to be used from kite balloons with the object of sighting submarines when on the surface at night.