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Rh  3

1918

tion of a number of minesweepers and torpedo craft, and though only one or two German submarines were lost on the mines their movements were undoubtedly seriously hampered.

In July the Germans suddenly began laying large protective minefields close to the Dutch line of lights, which resulted in the loss of the "Vehement" and "Ariel" on Aug. 1 and led to a great reduction in British mining in the Bight in the last three months of the war. The dangers of the Bight led to an increased use of the Cattegat by German submarines, and opened an excellent opportunity for deep mines in that area. A field was laid off the Skaw in February, and another in April off Lesso, which evidently caused considerable anxiety in Germany, but the work in this area was sporadic and did not form part of a coherent plan. Controlled minefields (that is, fields fired by instrumental means from a post in the vicinity on the passage of a submarine over them) were now being developed, and three submarines succumbed to them in 1918 (UC11, Harwich, June 26; UB109, Folkestone, Aug. 19; UB116, Scapa Flow, Oct. 28).

A large protective minefield of three deep and three shallow lines holding some 9,000 mines was laid in August and Septem- ber off the coast of Yorkshire and Durham, where submarines were particularly troublesome.

Minelaying by the end of the war had developed into one of the most important operations, and, favoured by geographical conditions, played a very important part in later British strategy, being an indispensable feature of the war against the submarine. The enormous number of mines used can be gathered from the fact that in the large minefields in home waters alone, there were some 34,300 mines laid in the Dover area and Narrows of the North Sea (including 9,500 in the Sir Roger Keyes' Folkestone to Gris Nez barrage); the Bight absorbed some 43,000 and the Northern barrage 70,117. There can be little doubt that too little importance was attached to the mine in British pre-war views on naval strategy. There was too great a tendency in those days to interpret war at sea wholly in terms of hitting a target at long ranges with a heavy gun. No modern battleship was sunk in this way, and of the various classes of British ships lost during the war, fewer were sunk by guns than by mines—namely 5 battleships out of 13 (38%), no battle cruisers out of 3, i cruiser out of 13 (7⋅7%), 2 light cruisers out of 12 (16⋅5%), 5 sloops out of 18 (28%), 20 destroyers out of 64 (31%), 4 submarines out of 54 (7⋅5%). In the destruction of German submarines they played a more important part. Of 200 submarines the loss of 43 (or 21⋅4%) was due to mines, and as one-fifth of those unknown (4 out of 17) may be attributed to the same cause it may be accepted that 23% of German submarine losses were due to British mines.

 MINING (see 18.528). While the standard methods of extracting ore from vein deposits, by overhand or underhand stoping, changed little during the period 1910-20, a more definite classification than was formerly possible grew up respecting the application of these methods to given local conditions. Comparatively thin veins, with a steep pitch (dip), are developed by a series of drifts or gangways (levels), and above each of these overhand slopes are opened for extracting the ore, the working being advanced upward and forward. The broken ore is run through chutes (mills or passes) to the level below, in which it is conveyed by small hand-trammed cars, or by mechanical haulage, to the shaft or through a tunnel to the surface. For somewhat thicker veins, especially those with a steep dip, underhand stoping is occasionally employed, most of the holes for blasting being then drilled downward, and the advance is forward and downward towards the haulage level. For flat veins or bedded deposits breast stoping is used, the details of which resemble those of underhand stoping. In all of these methods, the roof of the deposit (hanging wall) is supported by pillars of ore, by timber posts or stulls, by square-set tim- bering, or by masses of waste ore and rock (rilling) carried by stulls. Sometimes slopes are completely filled with waste rock.

General Classification of the A bove Methods. Narrow, steep-dipping veins: open, overhand or underhand slopes, the roof being supported by timbers or pillars. Wide, steep-dipping veins: open, underhand slopes, with pillars or ore. Flat beds or veins: breast sloping, or room-and-pillar working, in which little or no timber is used.

For full illuslraled details and variations, see Mining Engineers' .Handbook, pp. 493-598 (John Wiley & Sons, 1918).

Shrinkage Slopes, a variety of overhand slopes, for both narrow and wide, steeply dipping veins, have in recenl years been more widely employed than formerly. In them the broken ore is allowed to accumulate until the slope is compleled, Ihus making limbering or olher artificial support for Ihe walls of Ihe slope unnecessary. Since rock when broken increases in bulk, from 25 to 40% of the ore is drawn from the slope as it ad- vances to leave room at the top for the miners, who stand on the broken ore while drilling. Obviously, this method is appli- cable only when the inclination and width of the vein are great enough to allow the broken ore to slide down freely on the slope floor (footwall). Finally, after all the ore has been drawn off, the slope is allowed lo cave, or is filled wilh wasle. Shrink- age-sloping is often employed in connexion with other methods in the same deposit.

Caving Systems. The prototype of these, long employed in certain British iron-mines, is known as the North of England caving system. In the United States caving was first used for mining the soft iron ores of northern Michigan. More recently, it has been extensively applied to the iron deposits of the Mesabi district, Minnesota, and to some large copper deposits of the south-western part of the United Stales. The most important requirements for the successful application of caving methods are: (i) massive deposits of relatively cheap minerals; (2) ore-bodies of large horizontal dimensions, overlaid by a capping varying in character from earthy soil or glacial drift to firm rock; (3) large-scale work. There are three distinct methods: top slicing, block caving and sub-level caving. The salient features of all are: (a) horizontal subdivision of the ore-body into floors; (b) subdivision of each floor into small sections (slices or blocks), which are mined separately; (c) delivery of the broken ore through raise-chutes to the haulage-ways below, and thence to the hoisting-shafls; (d) as Ihe ore is removed, the overlying capping must gradually cave and settle. Formerly, a method called " bottom slicing " was occasionally employed, in which the mining of the successive floors was begun at the botlom of the ore-body, instead of the top, as in top slicing. It is now almosl obsolele. The decade 1910-20 was marked by a much wider use of the slicing and caving methods, especially for mining large low-grade deposits of disseminaled copper ore. More deposils of this type were developed and worked in very recent years than ever before. Many variations in practice and changes in details were inlroduced lo suit the given dimensions of ore-body, ils deplh below the surface, and the physical and mineralogical character of the ore and capping. All this brought a more definite understanding of the applicability and limitalions of Ihe caving syslems, as delermined by Ihe exisling local conditions.

Some prominent examples of the newer mines, in which dif- ferent forms of slicing and caving have been adopted, follow.