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fly-wheel, all on a common shaft. This set is in constant motion, though not at constant speed. From the generator the current goes to a hoist motor, which drives a pair of drums on the drum shaft. At the beginning of a hoisting cycle, the hoist motor receives current from the motor-generator set ; but, after the descending cage has reached a point where the trip can be completed by the weight of the rope, the hoist motor is driven by the drum, and therefore supplies current to the generator of the motor-generator set. Thus, part of the recovered power is stored in the flywheel, while the remainder is expended in driving the induction motor as a generator, thereby causing it to deliver current to the external circuit or power service. The fly-wheel cuts down the peaks of the load curve. Since 1915, a number of these plants have been erected ; they are costly and suit- able only where the hoisting is nearly continuous and high peak loads are heavily penalized in the power service.

Underground Haulage. For locomotive haulage, the electric trolley system was in 1920 still first in importance; next to this were the compressed-air locomotives. Storage-battery locomotives, though invented many years ago, were rarely used until about 1911, and in 1920 were employed to a limited extent only. Their construc- tion is simple, and, as they carry their power with them, they have the advantage of being able to operate wherever track is laid, with- out the necessity of stringing trolley wire. They are best suited to short hauls and light service, as for gathering individual cars from the working places and making them up into trains on the main haulage lines. Maximum speed is about 5 m. per hour, and easy track gradients are necessary. Their chief disadvantage is high first cost. A few combined trolley and storage-battery mine locomotives have been built, but they are unlikely to have a wide application. Gasoline locomotives were introduced before 1905, but were not much used until about 1912. Like storage-battery and compressed- air locomotives, they have the advantage of carrying their own power. Ordinary speeds range from 4 to 10 m. per hour. Although reasonable in first cost and running expenses, gasoline locomotives can be employed' underground only where there is abundant and active ventilation, because their exhaust usually contains enough carbon monoxide gas to require a high degree of dilution. Their consumption of gasoline at full load is, say, 0-7 to 1-2 Ib. per H.P. ; considerably more at half speed and load.

Shovelling Machines, for loading broken coal or ore underground, were introduced about 1907. The first was the Thew machine, a dipper shovel of small size, operated by electricity or compressed air and suitable for use in slopes in a flat-lying deposit, or in a tunnel. A later design, the Myers- Whaley, consists of a large scoop, which is thrust into the pile of broken ore or rock, then lifted and dumped backward onto a short travelling conveyer, for loading into a mine car in the rear. This machine occupies but little space, and can be used in a large mine tunnel or drift. In 1915 two of them were installed in the ends of a long haulage drift, 14 ft. wide by 10 ft. high, in the Crown Mines, Transvaal. The Halby shoveller resembles the Myers-Whaley. Other machines, especially for loading coal, have recently been invented, and are undergoing working tests. Mechanically considered, shovelling machines are unquestionably successful. Interest in them has been stimulated by the greatly increased rates of wages now prevailing in most mining regions. It has been proved, however, that, where wages are low, they cannot compete with hand loading.

Machine Drills underwent important changes during 1910-20, especially in the development of the " hammer " drills, which for many kinds of service have largely replaced standard types of piston machines. In the hammer drill, the bit is held stationary in the front end of the machine, and is struck a rapid succession of blows by the reciprocating piston-like hammer. As the bit does not reciprocate, its cutting edge being always in contact with the rock, except during the slight rebound caused by each blow of the hammer, the sludge or cuttings tend to pack in the bottom of the hole. Hence, unless some automatic means be provided for keeping the hole clean, part of the useful effect of the hammer blows would be lost. To keep the hole clean while drilling, most hammer drills use hollow bits; that is, there is a small hole longitudinally through the axis of the bit. Through this hole, a jet of compressed air or water is discharged in the bottom of the hole, thus driving out the cuttings. When compressed air is used, and the rock is dry, the dust discharged from the mouth of the hole is annoying and hurtful to the drill-runner. Hence, most hammer drills use a water jet. The water is delivered under pressure from a 15-gai. to l8-gal. tank, through a short length of hose. Pres- sure in the tank is produced by connecting it by another hose with the compressed-air pipe.

FIG. 3.

Hammer drills, made by all the principal rock-drill manufacturers, are of three general forms: (a) Large machines (fig. 3), correspond-

FIG. 5.

ing in size and weight with ordinary piston drills, mounted on tripod or column and used for the same kinds of work; (b) the small D-handle and cross-handle drills for making holes pointing down- ward, as in shaft-sinking (fig. 4) ; (c) machines having an automatic air-feed standard, used chiefly for holes directed steeply upward, as in most stoning operations (fig. 5). Machines of classes (b) and (c) have the advantages of lower first cost, of being operated by one man instead of two, and of eliminating the time lost in cleaning out the hole and in " setting up," as for the standard piston drills and class (a) hammer drills. In most rocks and ores these hammer drills, therefore, do faster work than piston drills.

Deep Boring. In recent years, for prospecting by boring, there has been a great increase in the use of the "churn drill "; that is, a drop drill, suspended by a rope from the operating machinery on the surface, and similar in many respects to the standard oil-well drilling plant. For boring holes deep- er than, say, 75 or 100 ft., the churn drill has practically superseded the old method of rod-boring, formerly com- mon in Europe. In general, for deep boring, the oil-well "rig," the churn drill and the diamond drill divide the field among them. During the decade 191020, many oil and natural gas wells were bored to depths of 4,000 to 5,000 ft., and a few exploratory holes (in Pennsylvania and West Virginia) reached depths of 7,000 to 7,350 feet. For holes of a few hundred ft. in depth, and when cores are desired, the rotary " shot-boring " method, based upon the old Davis Calyx drill, also came into wider use. The appara- tus consists essentially of a line of hollow rods, carrying a bit, say, 3 in. diameter by 10 in. long, with a narrow slot cut in its lower edge. At intervals during boring, a quantity of small steel shot is fed down through the hollow rod. The shot distribute themselves between the out- FIG. 6.

side of the bit and the walls

of the hole, between the inside of the bit and the core, and under the lower edge of the bit. Due to. the rotation of the bit, the shot are caused to roll forcibly with a milling action against the rock, which is thus ground away. (For full details, see Peele, Mining Engineers' Handbook, Sec. 9, Art. 1 6.)

For rotary boring in unconsolidated strata overlying the solid rock, the " fish-tail," with two cutting edges resembling those of a large carpenter's auger, has long been employed for oil and gas wells. But