She went to the bottom again, and after half an hour succeeded in getting clear of the net.

Meanwhile the Zeppelin had collected a force of trawlers and destroyers, and the submarine was hunted for fourteen hours by this force, assisted by the airship.

During this period she succeeded in sinking one of the German destroyers, and was eventually left unmolested.
For a correct appreciation of submarine warfare it is necessary to have a clear idea of the characteristics and qualities of the submarine herself, of the numbers possessed by the enemy, and of the rate at which they were being produced.

It is also necessary, in order to understand the difficulty of introducing the counter measures adopted by the Royal Navy, to know the length of time required to produce the vessels and the weapons which were employed or which it was intended to employ in the anti-submarine war.
The German submarines may be divided into four classes, viz.: Submarine cruisers, U-boats, U.B.-boats, U.C.-boats.

There were several variations of each class.
The earlier _submarine cruisers_ of the "Deutschland" class were double-hulled vessels, with a surface displacement of 1,850 tons, and were about 215 feet long; they had a surface speed of about 12 knots and a submerged speed of about 6 knots.