. MINING AND MILLING MACHIN ERY. Problems in mining may be classified into groups according to their relation to ge ology, engineering, machinery and metallurgy; but as the problems of each group overlap to some extent those of the others, the special con sideration of the subject of mining machinery necessarily involves a general consideration of mining methods and results in their geological, engineering and metallurgical aspects.
Mining methods differ according to the form and geological relations of the mass of ore or other minerals to obtain which the mining opera tions are instituted. These relations outline two general methods — those applicable to °sur face deposits," and the more complicated meth ods required in the working of °underground deposits." Surface deposits are those in which the mass of ore is of considerable superficial extent and lies on or near the surface of the ground. The first step in this case is to uncover the ore by °stripping° off the overlying worthless ma terial called the °burden.* If this consists of soft earth or gravel it may be removed with pick, shovel and wheelbarrow, or by the use of steam shovels and small tram-cars drawn by horses, mules or locomotives. If the burden is too hard to be stripped by any of these methods, blasting operations are usually em ployed. The burden is first blasted off and removed, and the subsequent work of extract ing the mineral thus exposed is carried on by benches or terraces along the hillside, so that the cuts will naturally drain into the pit, and the ore will have a favorable grade for its transportation, in removal. If the pit is lo cated in level or depressed ground the use of pumping machinery will become immediately necessary, entailing a heavy expense at the very beginning of the operations. Open cuts are the simplest and most convenient form of excava tion, but they expose the men and machinery to the weather, and usually necessitate the aban donment of all operations during the winter. Another method of surface mining consists in the employment of water jets for working auriferous gravels. This is technically known as "hydraulic mining?' Water conducted from great distances and elevations is directed against the ore-bearing gravel banks and beds, in the form of powerful jets through large nozzles called "giants' The impact of the water under the great pressure due to its heavy fall washes away the gravelly material of the banks with almost incredible rapidity through sluices where the gold is separated from the sand and gravel by amalgamation with the mercury in the riffles of the sluice boxes. The
sluices usually consist of a series of 12-foot troughs which empty into one another and often form a line of troughs several hundred feet long. The bottoms of the troughs have corrugations called riffles, and are cut out at intervals and replaced by a grating called a "grizzly?) Under the grizzly another broad trough is placed at right angles to the top trough and empties into another trough which runs parallel to the direction of the top trough and forms the continuation of the main sluice. As the material of the bank is washed through the sluice boxes by a strong stream of water the sand and gravel is caught by the grizzly while the gold passes through into the lower trough where it amalgamates with the mercury which is frequently sprinkled into the riffles.
Underground deposits are worked by the use of shafts and tunnels driven through the overlying earth and rock into the "lodes' or veins of ore. In these operations the overlying rock is always supported in place over those portions of the mines where the borings are used as passageways, and the arrangements for ventilation are more and more carefully made as the workings grow deeper and deeper.
The machinery employed in underground mining is not extensive as to variety. The principal tool is the drill with which is formed the hole for the explosive used to break out the metalliferous rock. To drive the drill the pneumatic hammer is the machine commonly used. In some cases steam is the source of power, but the compressed air mechanism is valued above steam because its exhaust sup plies the mine chambers with a continuous sup ply of fresh air. The air-compressor is com monly located above ground at the mouth of the shaft, the air being brought to the drills by a pipe conduit. In mining native metal, as in the case of virgin copper, a chisel is used, driven by the pneumatic hammer. One of the accessory machines of the mine is the drill sharpener, operated usually by steam. This is an adaptation of the power hammer, carrying a die with which is reforged the cutting edges of the drill heated to cherry-redness. The tempering is done by hand. In most under ground mines the water-pump is as much a necessity as in the surface diggings. Hoisting engines are an indispensable part of the outfit and are of various design, adapted to the vary ing conditions.