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Rh this bit wears rapidly if boulders or interbedded strata of shale, etc., be encountered in the gravels and clays. In 1908 the Sharp and Hughes cone-bit (fig. 6) was introduced to replace the fish-tail bit for boring in rock, and it has been widely used in some of the petroleum and natural gas-fields. The bit consists of a pair of hardened steel cones, with serrated or toothed surfaces, revolving on bronze bear- ings. As the drill rod rotates, the cones roll under pressure upon the rock, crushing and chipping it away. When dull, the cones are readily replaced. These bits in some cases bore as much as too ft. of hole in 24 hours. In boring oil wells through soft strata or shat- tered rock, trouble is often caused by the caving of the sides of the hole before the lining or casing pipe is put in. To overcome this difficulty," mud-laden " water may be used in the boring operations instead of the ordinary wash-water. Almost any finely divided, clayey material that will remain in suspension for a considerable period of time may be employed. The mixture of mud and water should have a specific gravity high enough to afford a counter- pressure that will support the walls of the hole; a specific gravity of about 1-33 is usually sufficient. The pressures in the bore-hole are thus kept in approximate equilibrium until the hole is lined with pipe (casing), or until screens can be set in the oil-bearing strata and cemented in place. This method was introduced in Europe, about 1895, for sinking cylindrical mine shafts in unstable, water-bearing soil by boring. About 1901 it was adopted for boring oil wells in Texas and Louisiana, and since 1906 has been successfully used in a large number of cases.

BIBLIOGRAPHY. General: Peele, Mining Engineers' Handbook, (1918). Methods of Mining: Trans. Am. Inst. Mining Rngs., vol. lv., pp. 118, 137, 218, 240, 397. Coal Mine Explosions: Trans. Inst. Mining Engs. (Great Britain), vol. xlix., p. 721; Bureau of Mines (U.S.A.), Tech. Papers Nos. 56, 71, 169; Bulletins Nos. 20, 56, 72, 102 ; J. Taffanel, Essais sur les inflammations de Poussieres, etc., April 1910, Aug. 1911; Report Commission on British Coal Dust Experiments, 1910. Mine Hygiene: Peele, Min. Engs.' Handbook, pp. 1369-81, 1397-1407; Glaister and Logan, Gas Poisoning in Mining (1914); Trans. Am. Inst. Min. Engs., Feb. 1921. Rock Drills: Peele, Compressed Air Plant, 4th ed. chaps. 20 and 21. Electric Hoisting: Trans. Am. Inst. Elec. Engs., June 28 1917; Eng. & Min. Jour., N.Y., Sept. 22 1906, p. 537; Nov. 19 1910, p. 1014. Underground Shovelling Machines: Peele, Min. Engs.' Handbook, pp. 673-4. Storage-Battery Locomotives : Trans. Am. Inst. Min. Engs., vol. li., p. 223; Colliery Engr., Oct. 1914, p. 121 ; Coal Age, Sept. 19 1918, p. 548; July 15 1920, p. II. Gasoline Locomotives: Eng. & Min. Jour., Aug. 23 1913, p. 347- Deep Boring: Trans. Am. Inst. Min. Engs., vol. li., pp. 571, 620, 638; Eng. & Min. Jour., Aug. 2 1919, p. 171 ; U.S. Bureau of Mines, Bull. No. 134. (R. P.*) MINING, MILITARY (see 10.714). Until the invention of explosives military mining resembled ordinary mining, in that it consisted of driving galleries or tunnels under the enemy's defences, either with the object of penetrating to the interior of the place attacked, or of undermining the walls and causing them to collapse. The latter method was probably used by Joshua at the siege of Jericho, while he distracted the attention of the defenders by a military promenade round the walls.

When the miner was put in possession of gunpowder he used it first for blowing up the enemy's defences, and later for destroying his galleries and for making craters as points of departure for trenches. The term " mine " in military language thus came gradually to mean, not a system of underground galleries as in civil life, but a charge of explosive operated underground or under water.

The position of military mining in 1910 is described in 10.716; contrary to expectation the first events of the World War seemed to indicate that mining was unnecessary, because the effect of the new heavy howitzers proved to be sufficient in themselves for the reduction of fortresses. But when the armies were settled down in opposing systems of trenches the heavy howitzers lost their preponderating role, and in the deadlock that followed old-fashioned methods reasserted themselves for trench warfare, such as grenades, trench mortars, and mining.

On the British front mining was started by the Germans. Before the end of 1914 there were two definite cases of mines under British trenches, and shallow mining systems were started in reply. It soon became evident that there was a probability of mining on an extensive scale, and that the existing R.E. units would be quite unable to supply the necessary personnel. It was proposed that coal-miners should be specially enlisted for the work, and their first detachment arrived in France in Feb. 1915. The formation of " tunnelling companies " followed, and during 1915 there was mine warfare all along the front.

In the early phases mining was chiefly a question of local efforts, partly defensive to counteract enemy mines, and partly directed against close objectives such as strong points in the enemy's trenches or a sniper's post. The mine systems were shallow, being rarely deeper than 20 feet. By June 1915 there was an enormous number of them, and the demands made upon the tunnellers could not be met. The need of a centralized con- trol began to be apparent, not only to coordinate the local schemes, but in order that plans of larger scope might be devel- oped in harmony with strategical ideas. Already in Aug. 1915 the first deep-mining system had been started, against Hill 60, with an object which was more than local; and questions of organiza- tion, of instruction, of intelligence and provision of stores all made direct touch with G.H.Q. essential.

Accordingly in Dec. 1915 a mining staff was appointed with an inspector of mines at G.H.Q., and a controller of mines at each army headquarters. The duties of the inspector of mines were defined as follows:

(a) Preparation, under the instruction of the general staff, of mining schemes which were intended to have a bearing on the principal operations of the campaign, and examination of mining schemes prepared by the armies.

(6) Inspection, for the information of the commander-in-chief, of the progress of all mining work.

(c) Advising the engineer-in-chief on general questions affecting the personnel, organization and equipment of the tunnelling com- panies.

The officers of the tunnelling companies were chiefly mining engineers drawn from all parts of the world, many of whom had already had experience in the trenches. The men included, be- sides ordinary coal-miners, a number of specialists known as " clay kickers." These men worked on their backs with cutting tools fastened to their feet. Each man was strapped on a plank which was pushed forward as his excavation advanced, and in this manner, in good ground, he was able to work in a very small gallery and make rapid progress.

Since a good deal of unskilled work was required for removal of soil, transport of materials, etc., a number of infantry were attached to the tunnelling companies. Thus a tunnelling company on the higher establishment had 569 officers and men, and with attached infantry might be 1,000 strong. By the end of June 1916 there were 25 imperial companies and seven overseas com- panies at work, with a total strength of about 25,000 of all ranks. A new departure was the attachment of geologists to the mining staff; their advice was especially valuable in connexion with the water levels, which in chalk country varied as much as 30 ft. between summer and winter. In the German army the arrangements were much more elaborate, each army having a complete geological staff.

In addition to this it must be noted that for the first time in history the British army had immediate access to scientific brains at home, and could utilize at a moment's notice the most advanced manufacturing facilities of the country. The effect of this showed itself particularly in the development of listening apparatus. In former wars the human ear unaided had to listen for the sounds of the enemy working underground, and estimate his distance and direction; in this war specialists in England were able to turn their attention to the production of listening apparatus as soon as the requirement was made known.

It naturally follows that the headquarters staff, with a very large and highly skilled personnel at their disposal, unlimited resources, and the prospect of a long period of trench warfare, began to conceive operations far in advance of anything that had been contemplated in war.

Shallow mine systems had still to be continued to protect the British trenches, but at the same time deep galleries were started from 50 to 100 ft. below the surface directed against special objects. Naturally the two systems were sometimes worked together. The operations at The Bluff, between St. Eloi and Hill 60, illustrate this. At this point there was a prolonged German offensive from Oct. 1915 to Sept. 1916. Working with a shallow system the Germans blew a large crater in Jan. 1916, which caused many casualties and temporarily destroyed the British.