COAL been found in the induced draught produced by jets of compressed air or high-pressure water blowing into ejectors. With a jet of ^^Q-inch area, a pipe discharging 1| gallon of water per minute at 165 lb pressure per square inch, a circulation of 850 cubic feet of air per minute was produced at the end of a level, or about five times that obtained from an equal volume of air at 60 K) pressure. The increased resistance, due to the large extension of workings from single pairs of shafts, the ventilating currents having often to travel several miles to the upcast, has led to great increase in the size and power of ventilating fans, and engines from 250 to 500 H.P. are not uncommonly used for such purposes. Electric driving from central-power stations has been found to be well suited for this particular use. The numerous forms of safety-lamps employed in fiery mines have received several additions in late years, and old forms have been improved and modified to meet the requirements of safety in air-currents travelling at a high velocity. Prominent among the new forms is the Hepplewhite-Gray lamp, which has a conical glass surrounding the light, with a gauze chimney, protected by an outer metal cylinder; the air supply to the flame is carried downwards through three tubes forming the standards of the cage. This, in addition to giving a good light overhead owing to the shape of the glass, is peculiarly sensitive to gas, and therefore valuable in testing for fire-damp. Other approved lamps are the Deflector and those of Marsaut & Mueseler when specially bonneted to resist extra high-speed currents. The illuminant now generally used in Great Britain is a mixture of rape oil with half its volume of petroleum, which is more suitable than vegetable or animal oil alone. In Germany Wolfs lamp, burning benzoline or petroleum spirit upon an asbestos wick, is very popular, as giving a much better light than oil. Special care is, however, required in filling, so that no free liquid may be left in the holder; the spirit must be entirely absorbed by a filling of sponge, and any superfluous quantity poured off. Portable electric lamps, supplied by accumulators or dry batteries, have been introduced into coal-mines; but owing to the weight and cost, their use is as yet very restricted. For the use of exploring parties after explosions, where irrespirable gases are encountered and compressed air or oxygen must be carried, they are especially valuable, as light is obtained without any demand on the air supply. Fire-damp, when present in the air, lengthens the flame of an ordinary safety-lamp, but the effect is not apparent with less than about 2|- per cent, of gas; and for more delicate testing, special lamps with non-luminous flames are adopted. In Pieler’s lamp, which is of the ordinary Davy form, alcohol is burned on a silk wick, and a screen is provided so that the flame can be hidden. When exposed in air containing ^ per cent., a cap of 1^ inch is formed, which increases to 2 inches with J per cent., and with I j- per cent, the lamp is filled with a deep blue glow. Another and more useful method is that of Dr. F. Clowes, who uses a hydrogen flame 0’4 inch long, obtained by attaching a cylinder containing compressed hydrogen to an ordinary safety-lamp; the gas is turned into the oil flame, which is for the time extinguished, and relighted when the observation is finished. As little as 0'2 per cent, of gas can be detected by this method. The danger arising from the presence of coal dust in the air of dry mines, with or without the addition of fireCoal dust ^amP» has> since it was first pointed out by Professor W. Galloway, been made the subject of special inquiries in the principal European countries interested in coal mining; and although certain points are still debatable, the fact is generally admitted as one
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calling for special precautions. The conclusions arrived at by the Royal Commission of 1891, which may be taken as generally representative of the views of British colliery engineers, are as follows :— 1. The danger of explosion when gas exists in very small quantities is greatly increased by the presence of coal dust. 2. A gas explosion in a fiery mine may be intensified or indefinitely propagated by the dust raised by the explosion itself. 3. Coal dust alone, without any gas, may cause a dangerous explosion if ignited by a blown-out shot; but such cases are likely to be exceptional. 4. The inflammability of coal dust varies with different coals, but none can be said to be entirely free from risk. 5. There is no probability of a dangerous explosion being produced by the ignition of coal dust by a naked light or ordinary flame. Danger arising from coal dust is best guarded against by systematically sprinkling or watering the main roads leading from the working faces to the shaft, where the dust falling from the trams in transit is liable to accumulate. This may be done by water-carts or hose and jet, but preferably by finely divided water and compressed air distributed from a network of pipes, carried through the workings. This is now generally done, and in some countries is compulsory, when the rocks are deficient in natural moisture. According to Behrens, the quantity of water required to keep down the dust in a. mine raising 850 tons of coal in a single shift was 28,8 tons, apart from that required by the jets and motors. The distributing network extended to more than 30 miles of pipes, varying from 31,- inches to 1 inch in diameter. In all British coal-mines, when gas in dangerous quantities has appeared within three months, and in all places that are dry and dusty, blasting is prohibited, except with permitted explosives, whose composition and properties have been examined at the testing station at the Royal Arsenal, Woolwich. A list of those sanctioned is published by the Home Office. They are mostly distinguished by special trade names, and are mainly of two classes—those containing ammonium nitrate and nitro-benzole or nitro-naphthalene, and those containing nitro - glycerine and nitro - cellulose, which are essentially weak dynamites. The safety property attributed to them is due to the depression of the temperature of the flame or products of explosion to a point, below that necessary to ignite fire-damp or coal dust in air from a blown-out shot. Hew explosives that are found to be satisfactory when tested are added to the list from time to time, the composition being stated in all cases. The most noticeable feature in the arrangements for draining modern collieries is the general abandonment of surface engines, with heavy wooden or iron rods in the shaft, in favour of high-speed engines “ placed underground, and supplied with power either by steam sent down from the surface, or in a less direct manner by water circulating under high pressure or by electric transmission. Compressed air may also be used, but is mostly restricted to small installations, on account of its low mechanical efficiency. The earlier underground steam-pumps were very wasteful machines, on account of the low steam pressures available and the loss by condensation in the steam conduit pipes, but with improvements in construction and the adoption of multiple expansion in several cylinders with high initial steam pressure, the fuel consumption has been reduced nearl}’ to the level of that of good surface or marine engines. Several engines of this class of considerable size have been erected in the deep Westphalian pits, e.g., one of 1900 to 2000 H.P., lifting a maximum quantity of 17 tons of water per minute 1300 feet high, with an expenditure of
