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whole force is raised, and more cordial relations are estab- hed between the employees and the mine management. For ertain mining appliances and machines standard designs grew commoner. There are clearer distinctions as to the applicability of different types of hoisting engines, compressors, ventilating fans, coal undercutting machines, mine cars, ore-chutes, etc,

Blasting. Explosives for blasting rock and ores underwent little change during 1910-20 in general composition, but tests supplied valuable data respecting the disruptive and propulsive forces and the sensitiveness of the various types and grades of dynamite, so hat their suitability for different kinds of service is better under- stood than formerly. Explosives for coal-mines, especially mines in rhich dangerous gases or dust occur, must be so constituted that >rdinary charges will not produce a flame of sufficient intensity and duration to ignite explosive mixtures of gas and air. These comprise the tested " permissible explosives," lists of which are published in jal-mining countries and revised from time to time, to keep them up >date. In Europe their use is required by law. In the United States le lists appear in publications of the Bureau of Mines (a branch j the U.S. Geological Survey). The bureau can only recommend hem, though legal requirements exist in some of the individual tates. The permissible explosives have certain characteristic ngredients: (a) ammonium nitrate; (6) salts containing water of ystallization, which, being liberated and vapourized by the heat
 * the explosion, reduces the flame temperature; (c) organic nitrates

ther than nitro-glycerine, e.g. nitro-starch ; (d) nitro-glycerine, nixed with free water or an excess of carbon. It was formerly sumed that nitro-glycerine compounds and other detonating xplosives were not suitable for collieries, because, due to market requirements, excessive shattering of coal is undesirable (excepffor coke-making); but low-strength, " short-flame " dynamites are now being satisfactorily used. While no explosive can be absolutely safe in gassy mines, those in the permissible lists are relatively safe. As the standard test, an explosive is accepted for the list when a charge of 680 grams (lj Ib.) does not ignite gas or coal dust; it is not accepted if a charge of 250 grams does cause ignition. In 1912, the permissible list of the U.S. Bureau of Mines comprised 96 kinds and grades of safety explosive; in June 1920, the number had increased to 1 75. The United States is the largest user of permissible explo- sives, the quantity consumed having nearly doubled in 1912-20.

Blasting methods were improved by the introduction (1909-10) of " delay-action " electrical fuzes. 1 In work like tunnelling and shatt-sinking, where rounds of holes are best fired in volleys, these special fuzes save time, as the miners need not return to the working place after each volley to prepare for the next blast. The entire round is wired, as if all the holes were to be fired simultaneously, and there is but one application of the current. The groups of holes explode successively, in the desired order and at intervals of about one second, by using " no-delay " fuzes for the first group and " first-delay " and " second-delay " fuzes for the following groups. Construction of delay fuzes: The platinum bridge in the cap shell, between the terminals of the fuze wires, is not embedded in the ful- minating charge itself, so as to explode it directly, but ignites a short piece of slow-burning ordinary fuse, which in turn explodes the fulminate. This gives a delay interval depending on the length of ordinary fuse used. Another new device for the same purpose is the electric fuze-igniter. A special electric cap contains a small charge of fine-grain black powder, beyond which is a piece of ordinary fuse, with a cap on the end to be placed in the dynamite cartridge. For blasting with black powder, no cap is put on the end of the ordinary fuse. Electric squibs, with paper instead of copper shells, are now used to some extent for blasting with black powder.

Mine Hygiene. Improvements made in the years 1910-20 were chiefly in five directions: (i) better ventilation of mine workings; (2) enforcement of dust-prevention regulations; (3) introduction of new types of blasting explosives, so constituted as to minimize the quantity of deleterious gases evolved; (4) adoption of precautions aiming to produce more perfect com- bustion of explosives, and so reduce or prevent the formation of the poisonous carbon monoxide; (5) study and better under- standing of special miners' diseases and their treatment.

Formerly, artificial ventilation by fans or blowers was provided only for collieries, to dilute and sweep out gases emanating from the coal, and to remove the explosive coal dust. In recent years, mechan- ical ventilators have been increasingly used for metalliferous mines also. About the year 1902, the high mortality amongst the miners of some districts began to attract attention. Investigation showed that acute lung trouble (" miner's phthisis " or silicosis) is caused by inhaling silicious dust from drilling in dry rock or ore. In 1903, a Government commission was appointed to study the conditions in the Transvaal gold-mines. Their report led to a demand for better ventilation of the mine workings, and the adoption of water-spraying devices to allay the dust during the operation of drilling. Revised

1 The spelling " fuze " is used for electric blasting, " fuse " for ordinary blasting.

and more stringent regulations went into effect in 1913. Other governmental investigations were made in Cornwall, Australia, and New Zealand, and in the United States by the Bureau of Mines. In 1911, one of the large gold-mining companies in the Transvaal, the Rand Mines (Ltd.), established a department of sanitation, to deal in general with miners' living and working conditions and diseases. The department's activities now cover a large number of the mines of the district, employing between 55,000 and 65,000 men. Marked benefits have resulted from this widespread interest in mine hygiene. For example, tests of the gases from blasting explosives have revealed the extent to which they may vitiate mine air. One pound of stand- ard dynamite produces about 10 cub. ft. of gas, which, due to incom- plete detonation, often contains 25 to 30% of carbon monoxide. Since, for safety, this actively poisonous gas should be diluted to about o-oi of i i, it is evident that natural ventilation can not always be relied upon, and mechanical ventilators have been installed for many metalliferous mines. In recent years, several new types of high explosives have been introduced, so compounded that they produce much less carbon monoxide and methane (CH<) than the standard (" straight ") dynamites. They are therefore particularly useful in poorly ventilated mine workings, as headings where there can be no through ventilation. Furthermore, there has been in- creased insistence on the use of high-strength caps or detonators, since imperfectly detonated dynamites of all kinds produce an excessive amount of carbon monoxide.

Explosions in Coat-Mines. Advances have taken place in the appliances for fighting mine fires, and the modes of preventing and dealing with gas and dust explosions in collieries. Coal-dust explo- sions are generally much more serious in bituminous than in anthra- cite mines. An explosion of anthracite dust does not appear to be self-propagating. Most explosions in anthracite mines are of gas, sometimes aided by presence of dust. Many investigations of coal- dust explosions have been made in Europe since 1880, but some of the phenomena attending their initiation and propagation have long been imperfectly understood. Much light was thrown on the subject by the elaborate experiments conducted by J. Taffanel, at the Licvin testing station, France (beginning in 1907), and by the U.S. Bureau of Mines at their testing plant arid their Bruceton experi- mental mine, near Pittsburgh (since 1909). Amongst the facts demonstrated are: (a) The blasting of a single hole, charged with long-flame explosive (black powder or ordinary dynamite), may cause the ignition of coal dust ; (6) respecting the initiation of an ex- plosion, if enough dry coal dust is present, it is immaterial whether the air at the point of origin is quiescent or moving in either direction ;

(c) quantities of dust as small as 1/5 oz. per cub. ft. of space (or i Ib. per linear ft. of an ordinary gangway) will propagate an explosion;

(d) in presence of sufficient dust, an explosion may be produced at will in a gangwav, even when the roof, sides and floor are wet to the touch; (e) the force of a coal-dust explosion usually increases in violence as it is propagated through a mine working, and may reach its maximum after travelling 500 to 800 ft. from the place of origin ; (f) pressures as high as 120 Ib. per sq. in. have been measured at right angles to the direction of movement of an explosion, the pressure in the line of advance being doubtless much greater.

Rock-dust barriers, for checking or preventing the propagation of coal-dust explosions, were invented by J. Taffanel and modified by G. S. Rice, of the U.S. Bureau of Mines. They consist of series of wide shelves, set across the mine gangway near the roof, each loaded with rock dust. In case of an explosion the shelves are tripped mechanically by the advance force waves, being set to operate at certain air velocities or pressures produced by the explosion. From two to three tons of rock dust are thus discharged in a dense cloud, in front of the advancing explosion wave, and, mixing with the coal- dust-laden atmosphere of the gangway, prevent propagation of the explosion. Several forms of dust barrier are used for different local conditions. Gas helmets and oxygen breathing-apparatus, long used in mine rescue work, were considerably improved after about 1913. Interest in these matters was stimulated by the introduction in several countries of Workmen's Compensation Acts (which hold the employer responsible for injury or death due to accident), and also by the use of poisonous gases in the World War.

'Hoisting Engines. Power plants (generally hydro-electric) have been established in many mining districts, and supply electric cur- rent at cheaper rates than are possible for equivalent steam power. Electric-driven hoists are consequently used in much greater num- bers than formerly. Their control mechanism is now so perfected that they are as manageable as the best steam hoists. The large variations in load, unavoidable in hoisting operations, and very disadvantageous for electric transmission of power, are successfully dealt with by the " equalizing systems " of hoisting, the first of which, the Siemens- 1 Igner, was introduced just previous to 1906. Modifications of it, based chiefly on the mode of control, are the Westinghouse and the Ward-Leonard. The design and operation of all of these plants are based on the principle that, when a motor receives electric current, it will deliver mechanical power; con- versely, when driven by mechanical power, the motor becomes in effect a generator and furnishes electric current. The alternating current usually supplied to a mine is first reduced to about 500 volts, and then goes to a motor-generator set, comprising a shunt or induc- tion motor, which drives a direct-current generator and a heavy