The _hydrophone_ had been in the experimental stage and under trial for a considerable period, but it had not so far developed into an effective instrument for locating submarines, and although trials of the different patterns which had been devised were pushed forward with energy, many months elapsed before it became a practicable proposition. One of the best offensive measures against the enemy submarines, it was realized, was the _mine_, if laid in sufficiently large numbers. Unfortunately, in January, 1917, we did not possess a mine that was satisfactory against submarines. Our deficiency in this respect was clearly shown in the course of some trials which I ordered, when one of our own submarines was run against a number of our mines, with the result that only about 33 per cent. of the mines (fitted, of course, only with small charges) exploded. The Germans were well aware that our mines were not very effective against submarines. We possessed at the time mines of two patterns, and whilst proving unsatisfactory against submarines, they were also found to be somewhat unreliable when laid in minefields designed to catch surface vessels, owing to a defect in the mooring apparatus. This defect was remedied, but valuable time was lost whilst the necessary alterations were being carried out, and although we possessed in April, 1917, a stock of some 20,000 mines, only 1,500 of them were then fit for laying. The position, therefore, was that our mines were not a satisfactory anti-submarine weapon. A _new pattern mine_, which had been designed on the model of the German mine during Sir Henry Jackson's term of office as First Sea Lord in 1916, was experimented with at the commencement of 1917, and as soon as drawings could be prepared orders for upwards of 100,000 were placed in anticipation of its success. There were some initial difficulties before all the details were satisfactory, and, in spite of the greatest pressure on manufacturers, it was not until November, 1917, that mines of this pattern were being delivered in large numbers. The earliest minefields laid in the Heligoland Bight in September and October, 1917, with mines of the new pattern met with immediate success against enemy submarines, as did the minefields composed of the same type of mine, the laying of which commenced in November, 1917, in the Straits of Dover. When it became possible to adopt the system of bringing merchant ships in convoys through the submarine zone under the escort of a screen of destroyers, this system became in itself, to a certain extent, an offensive operation, since it necessarily forced the enemy submarines desirous of obtaining results into positions in which they themselves were open to violent attack by depth charges dropped by destroyers.