The above considerations also apply to suggested schemes for shutting submarines in by a network of mines fastened together by wire. Mine-sweepers or mine-bumpers would force a channel through this as easily as through ordinary lines of mines, the only difference being that several being dragged along by the string would be exploded at the same time. Also it is evident that the limitations of this form of defence cannot be carried very far on account of the enormous quantities of material and explosives required. It would be very difficult and dangerous to lay in close proximity to the enemy’s ports and fleets. The process would be slow; the losses certain, and it could in any case block only a very small portion of the 100–mile broad mouth of the Heligoland Bight. Devices of this character may, however, be useful in defending one’s own harbours, and making anchorages submarine-proof, where we can, by our superior strength, prevent our arrangements from being interfered with. A variety of these methods are at present in use, and are being rapidly extended and developed. The following seven principal expedients are being applied:—
(1) Sinking ships with cement or stone in such a way as to form an absolute breakwater and barrier like the mole of a harbour.
(2) Electrical contact mines strung very closely together, and capable of being made dangerous or safe by the current being switched on or off.
(3) Barges or piles, with torpedo nets attached to them.
(4) Loose wire nets of very large mesh, not fastened at either end, which clog round a submarine and entangle it.
(5) Network arrangement, with explosives attached.
(6) Network arrangements, with upright floating spars, which also entangle the submarine.
(7) Long lengths of fishing nets, particularly Portuguese tunny nets, which operate in the same way.
As the war progresses, we shall gradually improvise the torpedo-proof harbours, of which, except on our southern coast, we are now entirely destitute.
I explained to the Cabinet the quantities of mines which we had in store, and which were available in the future. The experience of the last three months seems to justify the partial and limited reliance put by the Admiralty upon mining as a method of warfare.