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Rh The object of such an arrangement is to increase the probability of the mine, but since in this case the mine is fired out of actual contact with the ship's hull, explosive effect is sacrificed for the gain in probability, a failing common to all types of non-contact mine. (Various types of non-controlled mines are shown in the accompa- nying Plate, figs. 1-5.)

Minefields. Controlled minefields, on account of the complicated nature of material, etc., are applicable only to a limited defence of friendly shores. The mines are usually laid by small special mine- layers in short lines or small groups, all the mines of a line or group being fired simultaneously where these are of the non-contact type.

Non-controlled minefields on the other hand are required on an extensive scale and in all depths of water, both for offence against the enemy and defence of friendly coasts. They may consist of " barrages " to prevent the passage of enemy vessels through definite and particular areas, " independent minefields " to inflict loss on the enemy where there is a reasonable possibility of doing so, and " mined areas," which are built up of individual minefields to inflict loss on the enemy in 'areas which he is obliged to use when his ships put to sea for operations or exercise. Mines are laid in lines which are either continuous or broken up into groups, but variations of a single line are more often used, especially when several mine- layers are taking part in the operation together, the more usual variations being two or more parallel lines, single indented or stepped line and dog's leg line.

There are also some special forms of mining, such as the laying of " connected mines " where two or more non-controlled mines are connected together, or where, as an anti-submarine measure, the mines are suspended in nets. The object of all such systems is to increase probability, but the latter is only gained, in these cases, by complication of the material and the laying of it out. . Minelayers. Various classes of vessels are employed for laying non-controlled minefields: (a) Large minelayers with large carry- ing capacity for laying " barrages." (6) Fast minelayers of moderate capacity for laying ' mined areas " or " independent minefields " in enemy waters, (c) Very fast minelayers and submarine mine- layers for laying small minefields close in to vulnerable points.

Submarine minelayers require special laying apparatus. Surface minelayers are usually provided with narrow-gauge rails running along the deck and ending in a discharge " trap " at the stern. The " sinkers " have two pairs of wheels which fit the rail gauge and each mine rests on top of its own sinker. The mines and sinkers are disposed in long tiers along the rails and as the minelaying proceeds the tiers are gradually hauled aft towards the traps, electrical power being usually employed for this purpose. On nearing the

trap " each " unit " is hauled off the face of the tier in succession and pushed into the " trap " from whence it is let go by order.

The spacing of the mines apart varies according to circumstance, but the least distance at which mines can be spaced apart is limited to the distance at which one mine, if exploded, will not damage or countermine the next adjacent. The spacing on board the minelayer is regulated by time; the interval between successive mines being let go varies according to the speed at which the vessel is steaming and the spacing being used. Where a minelayer has more than one set of rails, it is usual to drop mines alternately from each set ; this is for convenience and gives more time for the loading of each " trap." (H. D. B.)

Depth Charges. A development of submarine mines which came in during the World War is the engine known as a depth charge.

This, as its name implies, is a charge of explosive which is detonated on reaching a given depth.

The explosive is carried in a mild-steel plate " charge case," to which rings are secured at the top and bottom for handling. A primer for detonating the main charge is secured in the centre of the charge case, round which primer lies the great bulk of the explosive charge and immediately above the primer is secured the " pistol." The pistol is arranged to fire the charge at varying depths.

One principle by which a depth charge may be fired consists of admitting water to a chamber containing a hydrostatic diaphragm. The pressure of the sea-water acts on the diaphragm and at the set depth causes a striker to act, thereby exploding the charge. A suit- able safety arrangement is of course provided, and this consists of a safety key which cannot be withdrawn until the depth charge has been adjusted for a depth setting.

Depth charges are carried in the stern of vessels, either in chutes or on a tilting tray, and can be released either hydraulically from the forebridge, or by hand. As the depth charge sinks at a rate of 10 ft. per second, it is clear that the laying vessel must maintain a certain minimum speed to ensure herself against damage by the depth charge she has dropped. This is 10 knots.

In addition to the two methods of carrying and dropping depth charges already mentioned, an alternative is provided in some ships in the form of a depth charge thrower. This consists of a steel barrel and an " expansion chamber." Into the expansion chamber is screwed an explosion tube which on firing sets up a pressure which will throw the depth charge a distance of 40 yds. with a time of flight of four seconds.

The introduction of the depth charge was brought about in 1915 owing to the complete immunity enjoyed by a submarine immediate- ly on submersion, notwithstanding the knowledge of a surface vessel

that the submarine was in its immediate vicinity. The introduction and rapid development of the depth charge entirely removed this sense of security, and quite apart from the destruction of 34 sub- marines actually achieved by this means, it produced a very great moral effect upon hostile submarines, and hampered them in attacks upon surface craft, owing to their perception of the risk of allowing their periscopes to be sighted, and thus drawing down a rain of these depth charges upon them.

Though depth charges generally cannot be depended on to vitally damage a submarine outside a range of about 30 ft. (depending on the weight of the charge), the effect on the nerves of a crew of a series of heavy explosions at a greater distance than this fatal limit is very marked, and may be regarded as one of the great uses of this weapon. In many cases in which British submarines have been subjected to a depth charge attack, the force of the explosion has caused an immense concussion inside the boat. Fig. 8 in the accompanying Plate shows the explosion of a depth charge at 40 feet. (B. A.)

SUDAN (see 26.9). The countries of the western and central Sudan are treated under their distinctive names; the present article deals with the Anglo-Egyptian Sudan, which, following official usage, is -called the Sudan simply.

The area administered by the Sudan Government, enlarged during 1910-6 by the addition of the Lado Enclave and Darfur, was officially given in 1921 as 1,014,000 sq. miles. In the same year the pop. was estimated at over 4,000,000, which compared with an estimate of 1,853,000 in 1905. Nearly half the people are primitive negroid tribes living in the equatorial belt. Khar- tum, including Khartum North, had 39,056 inhabitants in 1921; Omdurman 59,429.

Economic and Social Conditions. The years 1910-2 were years of prosperity, so much so that in April 1912 Lord Kitchener declared that " there is now hardly a poor man in the Sudan." But in that year the country experienced low floods and poor rains, while the 1913 Nile flood was one of the lowest on record. The rains, scanty at the best, failed altogether in some districts. The result was that 1913 and 1914 were years of acute agricultural and trade depression 1914 was described as perhaps the most difficult, from the point of revenue and the economic situation, experienced in the history of the Sudan. Trade suffered another shock with the outbreak of the World War. Abundant rains in the autumn and a high Nile happily resulted in providing the country with an ample supply of food-stuffs. Yet 1915 was little better than 1914. War conditions and the scarcity of shipping had caused a much lessened demand for the produce of the country. The cultivators, who form the majority of the people, " while they had enough to eat, were short of ready money " and hard pressed to pay their taxes. European conditions were indeed closely reproduced in the Sudan so far as commerce and economics were concerned. This was seen in 1916 when a period of comparative prosperity set in, though this was also due in part to abundant rains and a good flood in 1915. "The influence of the war," said the official report issued in 1920, " which had had such a depressing effect on trade in 1914 and 1915, began to operate in the reverse direction, and a great impetus was given to the export trade through an unprecedented demand for Sudan products. The presence of a large body of British troops in Egypt requiring grain and live stock, the demand created on the Arabian coast for Sudan millet, and in England and Allied and neutral countries for cotton and gum enabled all these commodities to be disposed of freely."

The experience of 1916 was repeated in 1917 and 1918 and, although there was a low Nile and poor rains in 1918, the country suffered no serious setback in 1919. Conditions in 1920 were influenced by the world depression in trade, nevertheless the year (following average rains and a medium flood in 1919) proved one of fair prosperity.

In considering the productivity and industry of the country it should be remembered that the Sudan consists of three natural zones, the desert zone in the north, where cultivation is only possible in a narrow strip on either bank of the Nile; a central zone where there are large areas of fertility, including the rainlands of Kassala and of Tokar, the Gezira plain, the pastures and gum-forests of Kordofan; and a southern belt, where the soil is richest and the rain tropical. But this southern