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612 British effort the German submarine output more than kept pace at first with their destruction. In 1917 the net gain in submarines was approximately 45, but in 1918 the two exactly balanced (74 added, 74 lost). The shipping position depended largely on the irreducible margin which would have fulfilled British needs. This may be taken as 12^ million tons, and in addition there was always some 600,000 tons of British shipping under repair (from enemy and marine damage), requiring a total of, say, 13,000,000 tons (gross). By the end of 1918 there were 3,391 British steam vessels of over 1,000 tons, with a gross tonnage of 14,049,000.

The British shipbuilding capacity remained much the same (about 1-2 million tons a year, 1-310 million tons gross for Jan. to Oct. 1918), but net losses had been reduced to about 33,000 tons gross per month, which meant that the submarine could no long- er attain its object within a reasonable time. It is true that the German -output of submarines would have increased 20 or so monthly in 1919, but there is every reason to believe that the Allied navies could have dealt with it. The really critical time from Aug. 1917 to Dec. 1917 had passed. The submarine cam- paign had failed. On three grey Nov. days they filed along Germany's via dolorosa towards Harwich, bringing to a grim and sordid conclusion one of the most tremendous chapters in the history, not only of naval warfare, but of the world.

Final Tale of German Submarines in Nov. 1918.

Total.

Building and fitting out. Surrendered .... Inspected

2.8 59

12

4.

15 53

7

<;

26 26

7 5

69

I3 ! 26

14

Sunk and interned. Various

66

68 3

57

191

3

ifio

isi

121

441

See Comdr. A. Gayer, Die deutschen U Boole (1920); Schecr, Germany's High Sea Fleet in the War (1920); Archibald Hurd, The Merchant Navy (vol. i., 1921); Comdr. J. G. Bower, Story of our Submarines (1919) ; Henry Newbolt, Submarine and anti- Submarine (1918) ; Comdr. Emile Vedel, Quatre annees de Guerre Sousmarine (I9I9) (A. C. D.) SUBMARINE MINES (see 26.1). It was the Russo-Japanese War 1903-4 which saw the first use of what has been called deep- sea mining that is to say, the application of the submarine mine to strategic and tactical uses quite distinct from its previous application for coast defence; and that war led to the intensifica- tion of development in all maritime countries.

In the World War 1914-8 Great Britain laid a total of 130,389 non-controlled mines, 1,192 controlled mines and 25,983 of a small special type of net mine; in addition, 899 British non- controlled mines were laid by a U.S. minelayer. As showing the growing intensity of mining as the war developed, British mine- layers were engaged on an average number of days in each month of 25 in 1915, si in 1916, n in 1917, and 20 in 1918. A mine barrage across the Dover Straits contained 9,373 mines. The great Northern Barrage from the Orkney Is. to the coast of Norway contained 69,766 mines; of this number 56,033 were American mines and laid by the U.S. minelayers. The British minelayers, who were chiefly employed elsewhere, laid the remainder. British submarines laid 2,469 mines. (See generally

MlNELAYING AND MlNESWEEPING.)

The chief naval war losses in surface ships due to the action of mines were:.

Battleships

Cruisers

Destroyers and Torpedo Boats

Great Britain

5

3

22

France

i

i

5

Russia

i

6

Italy

i

2

United States

i

Japan

I

Germany

2

2O

Austria .,...

3

Turkey

2

Mines, as distinct from depth charges, accounted for the known loss of 35 German (or Austrian) submarines.

The loss of the British mercantile marine due to mines was 673.417 gross tons, besides a loss in fishing vessels of 8,545 gross tons.

Classification of Mines. Submarine mines can be divided into two general types, controlled and non-controlled. These may be again divided, each into two divisions, contact and non-contact, and these may be further sub-divided into three classes, moored, drifting and ground. A ground mine is one which is laid actually on the bottom; it is chiefly useful in shallow waters. Drifting mines may be submerged and oscillate between set depths, may float on the surface, or may be suspended below a float ; they are especially suitable for employment in river warfare. A moored mine which is the type most frequently used, is a buoyant mine anchored to the bottom by a heavy weight or " sinker," the mine being attached to its " sinker " by chain or wire rope. The " sinker " may be automatic in its working and, following an adjustment which is capable of being readily made by the layer, it will take the mine to the desired depth below the surface. The depth adjustment will be made by the layer in accordance with the draft of the enemy's ships.

Controlled mines are those which have their firing source outside the mine and directly controllable by human agency. An electric current, provided by a dynamo or battery, is conveyed to the mines by cables led along the sea bottom from a control station on shore where the current may be switched on or off as desired. In the case of contact controlled mines a break in the controlling circuit, inside the mine, is completed when the mine is struck. Sometimes this is arranged by the crushing of a horn or it may be arranged by mechanism which acts due to the inertia of the blow. In the case of non-contact controlled mines, the firing is accomplished either by the direct observation of the operator or the mines themselves are made their own observers. In the first case the observer follows the enemy vessel through a telescope, which works over a prepared chart having metal strips on it corresponding to the positions of the mines; when a plunger on the training arm attached to the telescope comes into contact with one of the metal strips, the circuit is completed to that particular mine or line of mines and the mines are fired. Where the mines are their own operators, each contains mechanism, such as a microphone, which will pick up the sound of a ship's propellers and will indicate to the operator the moment when he should fire. The observation current from the mine is conveyed to the operator by the same cables that are used to fire the mine.

Controlled mines are specially applicable to the defence of har- bours, where, by nature of their control, passage of friendly ships can be permitted but, at any time if necessary, can be denied. Con- tact controlled mines are used chiefly in side channels, because, al- though they can be put to " safe," they nevertheless foul the ground and friendly ships passing might damage them or tear them from their moorings; used channels have, therefore, to be mined with non- contact controlled mines, moored at a depth below the draft of the deepest draft ship using the channel.

Controlled mines are very costly to install and maintain and they require a large personnel to tend and operate them. (See PLATE, figs. 6 and 7, for types of non-contact controlled mines.)

Non-controlled mines are those which are automatic when once laid. They carry their own firing source or obtain it from the sea and have no further dependence on any human control. Mechanism is usually fitted which renders them safe during laying and for a short time afterwards, or at any time should they break adrift from their moorings. They may also be fitted with mechanism rendering them safe or disposing of them by explosion after a de- termined interval, and unless so fitted they must be swept up when no longer required.

There are several methods by which contact non-controlled mines are fired: (i.) Inertia, where the momentum of the blow displaces a weight or pendulum inside the mine, causing the release of a per- cussion firing mechanism, (ii.) Mechanical lever, where the contact with a vessel displaces a rod or lever on the outside of the mine which first cocks and then releases a percussion firing mechanism, (iii.) Hydrostatic, where the contact with a vessel admits water, usually by the crushing of an external horn, into a valve inside the mine, which acting under the water pressure releases percussion firing mechanism, (iv.) Electrical, which is usually of the well-known " Hertz " horn type, where contact with a vessel crushes an exter- nal horn which contains within it a bichromate solution in a glass tube. When the glass of this latter is broken, the solution flows to the plates of an electric battery, previously inert, situated within the mine at the base of the horn. The solution energizes the bat- tery, which is electrically connected to the mine detonator, thus firing the mine. This type, though electrical in action, carries the energy in a chemical form.

In the case of non-contact non-controlled mines, firing can be accomplished by an observing mechanism, as for instance a micro- phone, within the mine; as a vessel approaches, the sound of her propellers is picked up by the microphone and by means of relay mechanism the mine can be made to fire when a pre-determined intensity of sound has been reached.