Bible Encyclopedias
Mining

Working

When the development of a mine has advanced sufficiently the operation of working or extracting the mineral begins. The method to be adopted will vary with the thickness and character of the deposit, with its inclination, and to some extent with the character of the enclosing rocks, the depth below the surface, and other conditions. The safety of the men must be one of the first considerations of the mine operator. In most civilized countries the safety of mine workers is guarded by stringent laws and enforced by the careful supervision of mine inspectors on behalf of the government. The method ,of mining adopted must secure the extraction of the mineral at a minimum cost. The principal item in mining cost is that of labour, which is expended chiefly in breaking down the mineral, either by the use of hand tools or with the aid of powder. Labour is also expended in handling the mineral in the workingplaces and in bringing it to the mine-cars in which it is brought to the surface. Narrow and contracted working-places are to be avoided, as in such places the cost of breaking ground is always large. Economy in handling makes it desirable to bring the mine-cars as near as may be to the point where the mineral is broken. This can be done in inclined deposits, it can often be done by the aid of mechanical appliances, though sometimes at an expense not warranted in the saving in the labour of loading. In steeply inclined beds the working-place can be so arranged that the mineral will fall or slide from the place where it is broken down to the main haulage road. The greatest difficulty is found where the inclination of the deposit is too great to permit the mine-cars to be brought into the working-place and yet not great enough to allow the mineral to fall or slide to a point where it can be loaded.

While it is always desirable to provide large working-places, the size of the working-place is limited by the thickness and Size of strength of the overlying beds forming the roof Working- or hanging wall of the mine. With thick and strong Places. rocks the working-places may sometimes exceed z oo or even 200 ft. in width. Indeed in metal-mines zoo ft. is the usual distance from one level to the next. With weak and thin beds forming the roof the working-places are often not wider than 20 or 3 o ft. as in most coal-mines. While the width of the working-place is thus limited by the strength of the roof, its length is determined by other considerations - namely, the rapidity with which the mining work can be conducted and the length of time it is practicable to keep the working-place open, and also by the increased difficulty of handling the minerals sometimes experienced when the workings reach undue length. In long-wall and in the work of mining pillars the roof will be supported on one side only, the overhanging beds acting as cantilevers. The working-place in such case is considerably narrower than in rooms or stopes, and there is also greater difficulty in supporting the roof because the projecting beds tend to break close to the point of support where the strain is greatest. This tendency is overcome by the use of timber supports so disposed as to ensure the breaking of the overhanging roof at a safe distance from the workingface and prevent the interruption of the work that might otherwise result.

While it is always desirable to work the deposit so as to extract the mineral completely, it frequently happens that this can only be done at greatly increased cost. In Complete the case of cheap and abundant minerals and low- Extraction grade ore deposits it is sometimes necessary to of Mineral. sacrifice a considerable proportion of the mineral, which is left for the support of the overlying strata. A similar sacrifice in the shape of pillars is often necessary to support the surface, either to avoid injury to valuable structures or to prevent a flooding of the mine. As already noted large pillars must always be left to protect shafts, adits and the more important mine-passages necessary for drainage, ventilation and the haulage of mineral. In the early history of mining there was but little attempt at systematic development and working, and the mines were often irregular and tortuous. Fig. 3 is FIG. 3.

an old Mexican silver-mine of this type. In such mines the mineral was carried out on the backs of men, and the water was laboriously raised by a long line of suction-pumps, operated by hand, each lifting the water a few feet only. With but slight modifications permitting the use of pumps and hoistingmachinery equally simple methods of mining may be seen to-day when the deposit is of small extent. Fig. 4 is a portion of a mine which consists of a series of irregular chambers with the roof supported on small pillars left at intervals for the purpose. In the systematic mining of larger deposits, the simplest plan consists in mining large areas by means of numerous working-places under the protection of pillars of mineral left for the purpose, and later mining these pillars systematically, allowing the overlying rock beds to fall and fill the abandoned workings. In shallow mines the pillars are small and the saving of the mineral of minor importance. In deep mines the pillars may furnish the bulk of the product, and the control of the fall of the roof, so as to permit the successful extraction of the mineral, demands a well-schemed plan of operation. In the robbing of pillars, timber is necessary for the support of only a single line of timber and lagging over the level, called the stull. When the roof is weak, or when it is undesirable to leave so much ore in the stopes, false stulls are sometimes erected in the upper part of the stope. The ore below the false stulls can then be drawn out without waiting for the completion of the top stope. When the mineral does not stand well in the pillar it will be necessary to erect a line of timbers with lagging so as to sheathe the under-side of the pillar and prevent level '/?//? ' level drift stops FIG. 7 its falling. It is not desirable to leave large areas standing upon pillars in the mine, and as soon as the work on any level is completed the pillar below should be mined out as far as is safe, and the abandoned portion of the mine allowed to cave in and lessen the weight on the pillars elsewhere. The block or ground between levels is sometimes mined by underhand stoping (fig. 7.). In this case the advanced drift is run underneath the pillar, and the ground below is mined in descending steps. This plan has the advantage of requiring little or no timbering when the mineral is strong enough to stand well in the pillars and when the hanging wall is good. The main haulage tracks are laid at the bottom of the stope, which thus forms the level. In this method of mining the different stopes must be kept close together; otherwise there is much added labour in shovelling the broken ore down to the main level. This method has the advantage of permitting the ore to be sent to the surface as fast as it is mined instead of being left for some months in the stopes for the men to stand upon. It has the disadvantage that the distance from one level to the next cannot usually be more than fifty feet without increasing greatly the chances of injury to the men from falling rock.. The method is then practicable and safe only with exceptionally strong mineral and roof. In metal-mines producing abundant rock-filling the overhand method of stoping, illustrated in fig. 8, is used. In this the stoping contracts run vertically, the roof in the working-places, and later to control the fall of the roof while the pillars are mined. More effective support and control of the roof may be secured by the use of rock-filling alone or with timber. By the use of rock-filling it is even possible to dispense with pillars of mineral; or, if pillars are left, the use of rock-filling greatly facilitates subsequent robbing operations. Rock-filling will be used whenever a large proportion of barren material must be mined with the ore. If rock-filling must be brought from the surface its use will generally be confined to mines in which it is difficult to support the roof in any other way. Rock-filling yields and becomes consolidated under heavy pressure, and therefore does not furnish a rigid support of the overlying strata, but rather a cushion to control and equalize the subsidence.

With soft material, pillars must be large, even at moderate depths below the surface, and it involves less labour to leave long rectangular pillars than to form numerous Pillar- square ones. This leads to the adoption of the room and pillar system so common in coal-mining. Fig. 5 is a mine in a bed of soft iron ore worked by a series ?//?% ry /??i °? ? ?. ??  ? ? ® ? %/r?/  ?? l ?. ® ? ? "% /% % r/i /?/////? ?? ' ii/ r//.i FIG. 5.

of inclined shafts, from which long horizontal rooms branch off right and left.

The usual method of working metal-mines is by overhand and underhand stoping, using rock-filling,, or pillars of mineral to support the roof. Fig. 6 represents a portion of one of the Lake Superior copper-mines worked by overhand stoping. A stope is that portion of the working assigned to a party of miners, and the block of ground is usually A s /u// ,?l'?//?ra  ? ?'% ///i. r ,' i // '?'/ y ?//l/ ?

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divided into three or four stopes at varying heights above the main level, the lowest being known as the cutting-out stope, the others as the first and second back stopes in ascending order. In steep pitching beds sufficient excavated material is allowed to remain in the stope for the support of the machines and men, the excess being drawn out from time to time and loaded into cars. The rest of the mineral is allowed to remain until the stope has so far advanced that its support is no longer needed. This method of mining requires but little timbering, FIG. 8.

and each party of contractors has one or more mills or timbered chutes through which the rich ore is conveyed to the level below and loaded in cars. The ore as mined is hand-picked and the barren material allowed to remain in the stope where it falls. In this method of mining no pillars need be left under the levels, as the rock-filling gives sufficient support to the roof. This method of mining affords the maximum of safety to the miners.

In the working of thick deposits the block of ground between two levels is divided into horizontal sections or floors which are worked either from above downward or from of Thick the bottom upward; in the first case the separate floors are worked by one of the caving systems; in the second, generally with the aid of filling. Fig. 9 illustrates the working of a block of ground by the top-slice caving system. Above, the ground has been completely worked out from the surface, and the space formerly occupied by ore is now filled with the debris of the overlying strata which has caved in above the block of ore now being worked. There is considerable thickness of old timber left from the working of the upper levels. This mat of timber forms a roof under the protection of which the mining of the ore proceeds downward floor by floor. The working-floors are connected by winzes with the main haulage roads below. These winzes serve for ventilation, for the passage of the workmen, and for chutes through which the ore is dumped to the level below. The working out of each floor is conducted much as if it were a bed of corresponding thickness. Haulage roads are driven in the ore so as to divide the floor into areas of convenient size. These separate areas are then mined in small rooms, each room being timbered as in mining under a weak roof rock. The room is driven in this way from one haulage road to another or to the boundary of the ore body. On completion of any room the timbers are withdrawn and the overlying mass of timber and rock is allowed to fall and a new room is started immediately alongside of the one just completed. In this way the whole floor is worked out and the mat of timber and overlying rock is gradually lowered and rests upon the top of the ore forming the floor below. Before abandoning a room it is usual to cover the bottom of the working-place with laggingpoles, which facilitate the mining of the floor below. In this manner one floor after another is worked until the floor containing the main haulage roads of the level below is reached. In the meantime a new level and a system of haulage roads have been driven a hundred feet below, and winzes have been driven upward to connect with the old level which is to be abandoned. The floor containing these old haulage roads now becomes the top slice of the one hundred-foot block of ground below and is mined out as described. Several floors may be mined simultaneously, FIG. 9.

the workings in the upper floor being kept in advance of those below, so as to allow the broken mass above to become consolidated before it is again disturbed by the working places of the next floor. This system permits the complete extraction of the ore at moderate cost and without danger to the men.

The subdrift caving system, fig. io, differs from the top-slice system mainly in the greater thickness given to the working floors, which may be from 12 to 40 ft. in thickness, whereas in the top-slide system the height of the floor is limited by the length of the timbers used in the working-rooms, rarely over S or Io ft. The subdrift system requires a smaller amount of narrow work in excavating the necessary haulage roads, and is therefore better adapted to hard ores in which such narrow work is expensive. The mining of each floor is carried on in sections with small working-places which are first driven of moderate height to their full length and width, leaving a back of ore above and pillars of ore between to support the upper portion of the upper layer or floor. These pillars and the ?

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backof ore above are then mined in retreating back towards the haulage road. The subdrift system is somewhat cheaper than the top-slice system, the output per man being greater.

The bottom-slice caving system of mining begins at the bottom of a hundred-foot block of ground, a floor being excavated under the whole area, leaving pillars of sufficient size to support the ground above. These pillars are then filled with blast holes which are fired simultaneously, permitting the whole block of ground to the level above to drop. A floor is then reopened in this fallen ore, leaving pillars for temporary support which are blasted out as before. This is the cheapest of the three caving systems, but is applicable only when the deposit lies between walls of very solid rock, as otherwise wall rock is liable to cave with and become mixed with ore, which adds greatly to the expense of handling.

When rock filling is available, as when the ore contains much barren material to be left behind in mining, the ore body is divided into blocks of convenient height as above, and these blocks are divided into floors, the bottom floor of each block however being attacked. Each floor is opened up by subsidiary haulage roads and worked out in small rooms which are timbered and filled with broken rock when completed. An adjoining room is next excavated and filled, and thus the whole floor is worked out and replaced with rock-filling. Work is then started on the floor above, the upper floors being connected with the main haulage roads by winzes which are maintained through the filled ground. Several floors can be mined simultaneously, the work in the lower floors being kept well in advance. Instead of mining in horizontal floors the filling method permits the ore to be mined in vertical chambers or slices which extend from one level to the next above and from one wall of the deposit to the other. When a chamber has been excavated and completely filled the slice adjoining is mined out, or at times a block of ground may be left untouched between two filled chambers and then mined out. In the latter case the top-slice caving method will usually be employed for the working of such intervening pillars. In order to lessen the cost of handling the rock-filling, the excavation sometimes takes the form of inclined working-places, parallel to the slope naturally taken by the rock when dumped from above into the working 7 -, ,, --, ,? ,?,ti ? ? Z?:C -? T ?S* ? ° °-',; ? ?-.??;.

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PA place. This method of mining and filling can be used when the work is done in horizontal floors or in transverse chambers. In the United States the Nevada square set system of timbering is used in connexion with rock filling (fig. 11). The use of the heavy timbers and continuous framing which characterize this system facilitates greatly the work of mining and maintaining the haulage roads on the different floors, and gives more rigid support to the unmined portions of the block of ground above. These advantages compensate for the greater first cost. Where each floor is timbered by itself with light timbers, as is the practice on the continent of Europe, the consolidation of the rock-filling under pressure gives rise to considerable subsidence of the unmined ore, which has frequently settled 20 ft. or more before the upper part of the block is reached. This occasions much added expense in the maintenance and retimbering of the haulage roads on the upper floors. The shrinkage of the rock-filling and the settlement of the workings /, Yan be greatly lessened by the use of hard rock with a minimum of fine stuff; but even so the advantage lies with the American system of timbering.

The cost of filling has been greatly reduced by the system of flushing culm, sand, gravel and similar material, through pipes leading from the surface into mine workings. Material as coarse as 1 in. in diameter may be carried long distances underground with the use of little more than an equal volume of water. This method originated in the Pennsylvania anthracite mines in 1887, but has been employed in recent years on a large scale in Silesia, Westphalia and other European coalfields. In some cases it has been found advantageous to quarry and crush rock for the purpose of using it in this way. Examples of other mining methods will be found under Coal.

'Where mineral deposits lie near the surface underground mining may be replaced by open excavations, and the reduced cost of mining makes it possible to remove the overlying soil and rock to considerable depths.

The depth to which open working can be pushed depends upon the size and value of the mineral deposit and upon the expense of removing the over-burden. Open excavations several hundred feet in depth are not uncommon. Where practicable steam shovels are employed, even when it is necessary to break up the material beforehand by blasting. Steam shovels are not well adapted to deep excavation unless provision is made for the rapid handling of the cars when filled. For deep workings the milling method is usually employed, in which the ore is excavated in funnel-shaped pits, each of which connects with underground haulage roads by a shaft. The ore is mined in the ordinary way, by pick and shovel if soft, or by the aid of powder if necessary, and the funnel-shaped bottom of the pit is maintained at such an angle that little or no shovelling is required to bring the excavated material to the shaft. Before the bottom of these pits reaches the level of the haulage roads below, a new set of roads will have been driven at a lower level and connected with the excavations above by the shafts. The cost of mining by the milling method does not greatly exceed the cost of steam-shovel work. For the special methods by which placer deposits are mined see Gold.

Underground Haulage

The excavated material is brought to the hoisting shaft, or sometimes directly to the surface, in small mine cars, moved by men or by animals, or by locomotives or wire-rope haulage. The size, shape and design of the cars depend on the size of the mine passage and of the hoisting compartments of the shafts; on whether the cars are to be trammed by hand or hauled in trains; whether they are loaded by shovel or by gravity from a chute; and whether they are to be hoisted to the surface or used only for underground transport. The cost of underground haulage is lessened by the use of cars of large capacity. In the United States cars in the coal and iron mines hold from 2 to 4 tons. In Europe the capacity ranges from 1000 to Isoo lb, though the tendency is to increase the size of the cars used. In mines of copper, lead and the precious metals, in which the cars are moved by hand, the usual load is from 1200 to 3000 lb. These small cars are constructed so that the load may be dumped by pivoting the car bodies on the trucks. Larger cars are usually dumped by means of rotating or swinging cradles, the car bodies being rigidly attached to the axles or trucks. When loaded by shovel the car is made low to economize labour. Wooden rails, protected by iron straps, are sometimes used on underground roads for temporary traffic; but steel rails, similar to, though lighter than, those employed for railways are the rule. For hand tramming, animal and rope haulage, the rails weigh from 8 to 24 lb per yard, for locomotive haulage 30 to 40 lb. Grades are made, whenever possible, in favour of the load, and of such degree that the power required to haul out the loaded cars shall be approximately equal to that for hauling back the empties, viz. about a of i %. Sharp curves should be avoided, especially for mechanical haulage. Switches for turnouts and branches, &c., are similar to but simpler than those for railways.

In metal mines, where, as a rule, mechanical haulage is inapplicable, the cars are moved by men (trammers). This is expensive, but is made necessary by the small amount of material to be handled at any given point. The average speed is about 200 ft. per minute, and the distances preferably but a few hundred feet. Animal haulage is employed chiefly in collieries and large metal mines; sometimes for main haulage lines, but oftener for distributing empty cars and making up trains for mechanical haulage. In mines operated through shafts the animals are stabled underground, and when well fed and cared for, thrive notwithstanding their rather abnormal conditions of life. Mine cars are sometimes run long distances, singly or in trains, over roads which are given sufficient grade to impart considerable speed by gravity, say from I to 21%. The grades must not be too great for brake control nor for the hauling back of the empty cars. Cars may thus be run through long adits or through branch gangways to some central point for making up into trains. Near the top and bottom of hoisting shafts the tracks are usually graded to permit the cars to be run to and from the shaft by gravity.

Locomotive haulage is applicable to large mines, where trains of cars are hauled long distances on flat or undulating roads of moderate gradients. Steam locomotives have been largely superseded by compressed air or electric locomotives. Compressed air locomotives are provided with cylindrical FIG. I I.

steel tanks charged from a special compressor with air at a pressure of 500 to loo lb per sq. in. The capacity of the tank depends on the power required and the distance to be traversed by a single charge of air.

Y g g The air passes through a reducing valve from the main to an auxiliary tank, in which the pressure is, say, 125 lb, and thence to the driving cylinders. By using compressed air vitiation of the mine air is avoided, as well as all danger of fire or explosion of gas. Electric locomotives usually work on the trolley system, though a few storage battery locomotives have been successfully employed. Trolley haulage lacks the flexibility of steam or compressed air haulage, and is limited to main lines because the wires must be strung throughout the length of the line. By adopting modern non-sparking motors there is but little danger of igniting explosive gas. Electric and compressed air locomotives are durable, easily operated, and can be built to run under the low roofs of thin veins. Their power is proportioned to requirements of load and maximum gradient; the speed is rarely more than 6 or 8 m. per hour. Electric locomotives are in general more economical then either steam or compressed air.

For heavy gradients rope haulage has no rival, though for moderate grades it is often advantageously replaced by electric and compressed air haulage. Gravity or self-acting planes are for lowering loaded cars, one or more at a time, from a higher to a lower level. The minimum grade is that which will enable the loaded cars in travelling down the plane to pull up the empty cars. At the head of the plane is mounted a drum or sheave, and around it passes a rope, one end of which is attached to the loaded cars at the top, the other to the empty cars at the foot. The speed due to the excess of weight on the loaded side is controlled by a brake on the drum. The rope is carried on rollers between the rails. There may be two complete lines of track or three lines of rails, one being common to both tracks, and the cars passing on a middle turnout or " parting "; or a single track with a parting. An engine plane is an inclined road, up which loaded cars are hauled by a stationary engine and rope, the empty cars running down by gravity, dragging the rope after them. This is similar to shaft hoisting, except that the grades are often quite flat. In the tail-rope system of haulage, best adapted for single track roads, there are two ropes - a main and a " tail " rope - winding on a pair of drums operated by an engine. The loaded train is coupled to the main rope, and to the rear end is attached the tail-rope; which reaches to the end of the line, passing there around a large grooved sheave and thence back to the engine. By winding in the main rope the loaded cars are hauled towards the engine, dragging behind them the tail-rope, which unwinds from its drum. The trip being completed, the empty train is hauled back by reversing the engine. The ropes are supported between the rails and guided on curves by rollers and sheaves. High speeds are often attained. Branches, operated from the main line, are readily installed. In the endless rope system the rope runs from a grip wheel on the driving engine to the end of the line, round a return sheave, and thence back to the engine. Chains are occasionally used. The line is double track and the rope constantly in motion, the cars being attached at intervals through its length by clips or clutches; the loaded cars move in one direction, the empties in the other. There are two modes of installing the system: either the rope passes above the cars and is carried by them, resting in the clips, or it is carried under the cars on rollers, the cars being attached by clips or a gripcarriage. (For details see Hughes, Text-book of Coal Mining, pp. 236-272; Hildenbrand, Underground Haulage by Wire Rope.) Rope haulage is widely used in collieries, and sometimes in other mines having large lateral extent and heavy traffic. With the tail-rope system, cars are run in long trains at high speed, curves and branches are easily worked, and gradients may be steep, though undulating gradients are somewhat disadvantageous. In the endless-rope systems cars run singly or in short trains, curves are disadvantageous, unless of long radius, speed is relatively slow, and branch roads not so easily operated as with tail-rope. The tail-rope plant is the more expensive, but for similar conditions the cost of working the two systems is nearly the same. An advantage of the endless system is that the cars may be delivered at regular intervals.

Hoisting

When the mine is worked through shafts, hoisting plant must be installed for raising the ore and handling men and supplies. On a smaller scale hoisting is also necessary for sinking shafts and winzes and for various underground services. As ordinarily constructed, a pair of horizontal cylinders is coupled to a shaft on which are mounted either one or two drums (fig. 12). The diameter of the cylinders is such that each alone is capable of starting the load. As the cranks are set go apart, there is no dead centre, and the engine is able to start under full load from any point of the stroke. This is important in mine hoisting,.

FIG. 12. - Plan of direct-acting hoisting engines, compound Corliss engines and conical drums. Wellman-Seaver-Morgan Co., Cleveland, Ohio, makers.

which is intermittent in character and variable as to power and speed required. The cylinders are generally single-expansion, though compound engines are occasionally used for heavy work. The engine is direct-acting, the drums making one revolution for each double stroke. In geared hoists the drums. are on a separate shaft, driven from the crank-shaft by tooth or friction gearing, and make one revolution for, say, 4 or 5 double strokes. The hoisting speed is therefore slower, and as less engine power is required for a given load the cylinders. are smaller, though making more strokes per minute. Large and powerful geared hoists are not uncommon. The dimensions. of the drum depend on the hoisting speed desired and the depth of shaft or length of rope to be wound. Drums are either cylindrical or conical. Conical drums (fig. 12) tend to equalize the varying load on the engine due to the winding and unwinding of the rope. On starting to hoist, the rope winds from the small towards the large end of the drum, the lever arm, or radius of the coils, increasing as the weight of Winding Engine. rope decreases. A similar equalizing effect is obtained by the use of flat rope and reel, the rope winding on itself like a ribbon. Tapering ropes, tail-ropes suspended from the cages, and other means of equalization, are also employed. If, for a twocompartment shaft, a pair of drums (or a single wide drum) be keyed to the engine shaft, with the ropes wound in opposite directions, the hoisting is " in balance," that is, the cages and cars counterbalance each other, so that the engine has to raise only the useful load of mineral, plus the rope. This arrangement allows no independence of movement: when the loaded cage is being hoisted the empty must be lowered. Independent drums, on the contrary, are loose upon their shaft, and are thrown on or off by tooth or friction clutches. The maximum load on the engine is thus greater and more power is required than for fixed drums. Steam consumption is economized, whenever possible, by throwing in the clutches of both drums and hoisting in balance. Fixed drums are best for mines in which the hoisting is done chiefly from one level; independent drums when there are a number of different levels. Hoisting engines are provided with powerful brakes and frequently with reversing gear. In deep shafts hoisting speeds of 3000 or 3500 ft. per minute are often attained, occasionally as much as 5000 ft.

Formerly hemp and also fibre ropes were commonly used. Except in a few instances these were long ago superseded by ron-wire ropes, which in turn have p been replaced by steel because of its greater strength. For hoisting in deep shafts, and to reduce the weight of rope, tempered-steel wire of very high tensile strength (up to 250,000 or 275,000 lb ultimate strength per sq. in.) is advantageously employed. A i-in. ordinary steel rope has a breaking strength of about 32 tons, which, with a' factor of safety of six gives a safe working load of 54 tons. A i-in. plow-steel rope has breaking and working strengths respectively of at least 48 and 8 tons. Standard round rope (fig. 13) has six strands of 19 wires each and a hemp core. Flat rope is in favour in some districts. It is composed of several fourstranded ropes, without hemp centres, laid side by side, and sewed together by wire (fig. 14). It is not as durable as round rope and is heavier for the same working strength. As the sewing wires soon begin to break, a flat rope must usually be ripped Apart and resewed every six or eight months. Numerous patent ropes, some having wires and strands of special shapes, have been introduced with the idea of improving the wearing properties. Such, for example, are the Lang-lay, locked-coil and flattened strand rope. Hoisting ropes are weakened by deterioration and breakage of the wires, due to corrosion and repeated bending, and should be kept under careful inspection. To prevent excessive bending stresses the diameter of drum and sheave must bear a proper ratio to that of the rope. A ratio of 48 to is the minimum allowable; better 60 to 75 to 1, and for highly tempered steel ropes ratios of 150 to i or more are desirable. To prevent corrosion the rope should be treated at intervals with hot lubricant. With proper care a steel rope should last from two to three years.

A frame of wood or steel, erected at the shaft mouth, and rarely employed except for deep shafts of small cross-section or when the mine cars (tubs) are small, as in many parts of Europe. In many mines the mineral is raised in skips (fig. 18), filled from cars underground and dumping automatically on reaching the surface. Skips are sometimes of very large capacity, holding 5, 7, and even 10 tons of ore; such are used, for example, in several shafts at Butte, Montana, in the Lake Superior copper district, and in South Africa. Fig. 18 is a small skip; the upper illustration showing position for dumping. The lower cut is of a skip for either ore or water; note valve in bottom. Hoisting buckets or kibbles are employed for small (From The Colliery Engineer, May 1897.) FIG. 15. - Head-gear.

carrying the grooved sheaves over 'which the hoisting ropes pass, is known as the head-gear (fig. i 5). In Great Britain and her colonies it is also called the poppet-head or pit head frame; in the United States head-frame or gallows-frame. Though it is small and simple in construction for light work, for heavy hoisting at high speeds massively framed towers, often 80 to ioo ft. in height, are built. Steel frames are more durable than those of wood, and have become common in nearly all mining countries, especially where timber is scarce. A German design is shown in fig. 16. The head-gear is often combined with ore-bins and machinery for breaking and sizing the lump ore previous to shipment to the reduction works.

Cages, running in guides in the shaft, are used for raising the cars of mineral to the surface (fig. 17). They may have one, two or more decks, usually carrying one or two cars on each deck. Multiple-deck cages are FIG. 13. - Standard FIG. 14. - Flat round Rope. Rope.

scale work or temporary service, such as raising the material blasted in sinking shafts. They hold from a few hundred pounds up to i ton. In hoisting from great depths the weight of the rope, which may exceed that of the cage and FIG. 16. - Steel head-gear, modern German type, constructed by Aug. Klonne, Dortmund.

contents, produces excessive variations in the load on the engine difficult to deal with. Moreover, the limit of vertical depth at which rope of even the best quality will support its own weight only, with a proper margin of safety, is, say, io,000 to 12,000 ft.; and with the load the safe working limit of depth would be reached at 7000 to 8000 ft. A number of FIG. 17. - Light steel safety FIG. 18. - Ore and water skips mining cage and car for gold for inclined shaft. Allis and silver mines. WellmanChalmers Co., Milwaukee, Seaver-Morgan Co., Cleveland, Wisconsin, makers. Ohio, makers.

shafts in South Africa, the United States and elsewhere, are already approximating depths of 5000 ft., a few being even deeper. Ropes of tapering section may be used for great depths, but are not satisfactory in practice. 1 Stage hoisting is applicable to any depth. Instead of raising the load in one lift from the bottom of the shaft, one or more intermediate 1 A full discussion of this subject is given in Trans. Ins. Min. and Met., vol. xi.

dumping and loading stations are provided. Each stage has its own engine, rope and cage. The variations in engine load are thus reduced, and incidentally hoisting time is saved.

In shallow mines the men use the ladder-way in going to and from their work. This is sometimes the case also for considerable depths. It is more economical Lowerin g to save the men's strength, however, by raising men. and lowering them with the hoisting engines.

At mines with vertical shafts this is a simple operation. Cages of the size generally used in metal mines will hold from ten to fifteen and occasionally twenty men. The time consumed in lowering the men is shortened by the use of cages having two or more decks. These are common in Europe, and are sometimes employed in the United States and elsewhere in mines where the output is large and the shafts deep and of small cross section. While a shift of men is being lowered the miners of the preceding shift are usually raised to the surface in the ascending cages, the entire shift being thus changed in the time required for lowering. Nevertheless, in very deep and large mines the time consumed in handling the men may make serious inroads on the time available for hoisting ore. At a few mines special man-cages are operated in separate compartments by their own engines for handling part of the men, and for tools, supplies, &c. For inclined shafts, where the mineral is hoisted in skips, the operation of raising and lowering men may not be so simple. Even a large skip will hold but a few men, the speed is slower, and more time is required for the men to get into and out of the skip than to step on and off a cage. Moreover, skips are rarely provided with safety attachments, so that the danger is greater. When the shafts are deep and the number of miners large man-cars are sometimes employed. These are long frames on four wheels, with a series of seats like a section of a theatre gallery. Ordinarily 4 or 5 men occupy each seat, the car accommodating from 20 to 36 men. Such cars are in use at a number of deep inclined shafts in the Lake Superior copper district, where the depths range from 3000 to 5000 ft. or more. At a few mines (since safety catches cannot be successfully applied to man-cars) these conveyances are raised and lowered by separate engines and ropes. To replace the ore-skip expeditiously by the man-car when the shifts are to be changed a crane is often erected over the shaft mouth. At the end of a shift the ore-skip is lifted from the shaft track - the hoisting rope being uncoupled - and the man-car put in its place and attached to the rope. This change may be made in a few minutes.

Formerly, at many deep European mines, and at a few in the United States, men were raised by means of " man-engines." A man-engine consists of two heavy wooden rods (like the rods of a Cornish pumping plant), placed parallel and close to each other in a special shaft compartment, and suspended at the surface from a pair of massive walking beams (or " bobs "). The rods are caused to oscillate slowly by an engine, one rising while the other is falling. Thus they move simultaneously in opposite directions through a fixed length of stroke, say from 10 to 12 ft. At intervals. on the rods are attached small horizontal platforms, only large enough to accommodate two men at a time. As the rods make their measured strokes one of the miners, starting from the surface, steps on the first platform as it rises to the surface landing and is then lowered on the down stroke. At the end of the stroke, when his platform comes opposite to a corresponding platform on the other rod, he steps over on to the latter during the instant of rest prior to the reversal of the stroke, descends with the second rod on this down stroke, steps again at the proper time to a platform of the first rod and so on to the bottom. The men follow each other, one by one, so that in a few minutes all the rod platforms in a deep shaft may be simultaneously occupied by men stepping in unison but in opposite directions from platforms of one rod to the other. Meantime, the men quitting work are ascending in a similar way, as there is room on each platform for two men at a time when passing each other. Man-engines were long used, but are now practically abandoned in both Great Britain and the United States, and few remain in any of the mining regions of the world. Their first cost is great and they are dangerous for new men, as they require constant alertness, presence of mind, and a certain knack in using them. See Trans. Inst. Min. and Met. xi. 334, 345, 380, &c.; also Eng. and Min. Jour. (April 4, 1903), pp. 517 and 518.

Surface Handling, Storage and Shipment of Minerals

To mine ore or coal at minimum cost it is necessary to work the mine plant at nearly or quite its full capacity and to avoid interruption and delays. When the mineral is transported by rail or water to concentration or metallurgical works for treatment, or to near or distant markets for sale, provision must be made for the economical loading of railway wagons or vessels, and for the temporary storage of the mineral product. For short periods the mineral may remain in the mine cars, or may be loaded into railway wagons held at the mine for this purpose. Cars, however, are too valuable to be used in this way for more than a few hours, and it is usual to erect large storage bins at the mine, at concentration works and metallurgical establishments, in which the mineral may be stored, permitting cars, wagons and vessels to be quickly emptied or loaded. In mining regions where 'water transportation is interrupted during certain months of the year the mineral must be stored underground, or in great stock-piles on the surface. In coal mining the market demand varies in different seasons, and surface storage is sometimes necessary to permit regular work at the mines. For coal, iron ore and other cheap minerals, mechanical handling by many different methods is used in loading and unloading railway wagons and vessels, and in forming the stock-piles and reloading the mineral therefrom. (See Conveyor and DocKs; also G. F. Zimmer, Mechanical Handling of Materials, and Engineering Magazine, xiv. 275, xx. 157 and xxi. 657.) Mine Drainage. - A mine which has been opened by an adit tunnel or drift drains itself, so far as the workings above the adit level are concerned. In many mining regions long tunnels have been driven at great expense to secure natural drainage. Under modern mining conditions drainage tunnels have lost much of their former importance. Taking into account the risk attending all mining operations, which make necessary large interest and amortization charges on the cost of a tunnel, it will in most cases be advisable to raise the water to the surface by mechanical means. Drainage channels are provided, usually along the main haulage roads, by which the water flows to a sump excavated at the pump shaft. In driving mine passages thatj are to be used for drainage, care is taken to maintain sufficient gradient. Siphons are sometimes used to carry the water over an undulating grade and thereby save the expense of a deep rock cutting. As the larger part of the water in a mine comes from the surface, the cost of drainage may be reduced by intercepting this surface water, and collecting it at convenient points in the pump shaft from which it may be raised at less cost than if permitted to go to the bottom. Water may be raised from mines by buckets, tanks or pumps. Wooden or steel buckets, holding from 35 to 200 gallons, are employed only for temporary or auxiliary service or for small quantities of water in shallow shafts. Tanks operated by the main hoisting engines, and of capacities up to 50o gallons or more, are applicable under several conditions: (1) When the shaft is deep, the quantity of water insufficient to keep a pump in regular operation, and the hoisting engine not constantly employed in raising mineral, the tank is worked at intervals, being attached temporarily to the hoisting rope in place of the cage. (2) For raising large volumes of water from deep shafts pairs of tanks are operated in balance in special shaft compartments by their own hoisting engine. With an efficient engine the cost per gallon of water is often less than for pumping. (3) For clearing flooded mines. As the water level falls the tanks readily follow it while at work, whereas pumps must be lowered to new positions to keep within suction distance. Self-acting tanks are occasionally built underneath the platforms of hoisting cages. Mine pumps are of two classes: (I) those in which the driving engine is on the surface and operates the pumps by a long line of rods passing down the shaft, commonly known as the Cornish system; (2) direct-acting pumps, in which the engine and pumping cylinders form a single unit, placed close to the point underground from which the water is to be raised. Cornish pumps are the oldest of the machines for draining mines; in fact, one of the earliest applications of the old Woolf and Newcomen engines in the 18th century was to pumps for deep mines. The engine works a massive counter-balanced walking-beam from which is suspended in the shaft a long wooden (or steel) rod, made in sections and spliced together. Attached to the rod by offsets are one or more plunger or bucket pumps, set at intervals in the shaft. All work simultaneously, each raising the water to a tank or sump above, whence it is taken by the next pump of the system, and finally discharged at the surface. The individual pumps are placed several hundred feet apart, so that a series is required for a deep shaft. The speed is slow - from 4 to ro strokes per minute - but the larger sizes, up to 24 in. or more in diameter by io or 12 ft. stroke, are capable of raising millions of gallons per day. Cornish pumps are economical in running expenses, provided the driving engine is of proper design and the disadvantages incurred in conveying steam underground are avoided. Their first cost, however, is high and the cumbersome parts occupy much space in the shaft. Direct-acting pumps, first introduced (1841) by an American, Henry R. Worthington, are made of many different designs. Typically they are steam pumps, the steam and water cylinders being set tandem on the same bed frame, generally without fly-wheel or other rotary parts; they may be single cylinder or duplex, simple, compound or triple expansion, and having a higher speed of stroke are smaller in all their parts than Cornish pumps. For high heads the water cylinders, valves and valve chambers are specially constructed to withstand heavy pressures, water being sometimes raised in a single lift to heights of more than 2000 ft. Condensers are always required for underground pumps. Sinking pumps, designed for use in shafts in process of sinking, are suspended by wire ropes so as to be raised before blasting and promptly lowered again to resume pumping. Electrically driven pumps, now widely used, are convenient and economical. Mine pumps of ordinary forms may be operated by compressed air, and air-lift pumps have been successfully employed. Hydraulic pumping engines, while not differing essentially from steam pumps, must have specially designed valves in the power cylinder on account of the incompressibility of water. They can be used only when a supply of water under sufficient pressure is available for power. Centrifugal pumps, constructed with several stages or sets of vanes, and suitable for high lifts, have been introduced for mine service. When mine water is acid the working parts of the pump must be lined with or made of bronze or other non-corrosive material; or the acid may be neutralized by adding lime in the sump.

Ventilation

The air of a mine is vitiated by the presence of large numbers of men and animals and of numerous lights, each of which may consume as much air as a number of men. In mining operations explosives are used on a large scale and the powder gases contain large quantities of the very poisonous gas, carbon monoxide, a small percentage of which may cause death, and even a minute percentage of which in the air will seriously affect the health. In addition to these sources of contamination the air of the mine is frequently charged with gas issuing from the rocks or from the mineral deposit. For example, carbon dioxide occurs in some mines, and hydrogen sulphide, which is a poisonous gas, in others. In coal-mines we have to deal with " fire-damp " or marsh gas, and with inflammable coal dust, which form explosive mixtures with air and frequently lead to disastrous explosions resulting in great loss of life. The gases produced by such fire-damp or dust explosions contain carbon dioxide and carbon monoxide in large proportion, and the majority of the deaths from such explosions are due to this " after-damp " rather than to the explosion itself. The terrible effects of fire-damp have led to the adoption of elaborate systems of ventilation, as the most effective safeguard against these explosions is the dilution and removal of the fire-damp as promptly and completely as possible. Very large volumes of air are necessary for this purpose, so that in such mines other sources of vitiation are adequately provided against and need not be considered. In metal mines, however, artificial ventilation is rarely attempted, and natural ventilation often fails to furnish a sufficient quantity of air. The examination of the air of metal mines has shown that in most cases it is much worse than the air of crowded theatres or other badly ventilated buildings. This has a serious effect on the health and efficiency of the workmen employed, and in extreme cases may even result in increased cost of mining operations. The ventilation of a mine must in general be produced artificially. In any case whether natural or artificial means be employed, a mine can only be ventilated properly when it has at least two distinct openings to the surface, one an intake or " downcast," the other a chimney serving as an " upcast." Two compartments of a shaft may be utilized for this purpose, but greater safety is ensured by two separate openings, as required by law in most mining countries.

The air underground remains throughout the year at nearly the same temperature, and is warmer in winter and cooler in summer than the outside air. If the two openings to the mine are at different levels the difference in weight of the inside and outside air due to difference in temperature causes a current, and in the winter months large volumes of air will be circulated through the mine from this cause alone. In summer there will be less movement of air and the current will frequently be reversed. In a mine with shafts opening at the same level, natural ventilation once established will be effective during cold weather, as the downcast will have the temperature of the outside air, while the upcast will be filled with the warm air of the mine. In summer this will occur only on cool days and at night. When the temperature of outside and inside air becomes equal or nearly so natural ventilation ceases or becomes insignificant. In a mine with two shafts a ventilating current may result from other conditions creating a difference in the temperature of the air in either shaft - for example, the cooling effect of dropping water or the heating effect of steam pipes. Natural ventilation is impracticable in flat deposits worked by drifts and without shafts.

Ventilation may be produced by heating the air of the mine, as for example, by constructing a ventilating furnace at the bottom of an air shaft. The efficiency of such ventilating furnaces is low, and they cannot safely be used in mines producing fire-damp. They are sometimes the cause of underground fires, and they are always a source of danger when by any chance the ventilating current becomes reversed, in which case the products of combustion, containing large quantities of carbon dioxide, will be drawn into the mine to the serious danger of the men. On account of their dangerous character furnaces are prohibited by law in many countries.

Positive blowers and exhausting apparatus of a great variety of forms have been used in mines for producing artificial ventilation. About 1850, efficient ventilators of the centrifugal type were first introduced, and are now almost universally employed where the circulation of large volumes of air is necessary, as in collieries. The typical mine fan consists of a shaft upon which are mounted a number of vanes enclosed in a casing; the air entering a central side inlet is caught up by the revolving vanes and thrown out at the periphery by the centrifugal force thus generated. " Openrunning " fans have no peripheral casing, and discharge freely throughout their entire circumference; in " closed " fans the revolving part is completely enveloped by a spiral casing opening at one point into a discharge chimney. Fans either force air into or exhaust it from the mine. The inlet opening of the pressure fan is in free communication with the outside air, the discharge connecting with the mine air-way; in the more generally used exhaust fan the inlet is connected with the airway, the fan discharging into the atmosphere. Among the exhaust fans most widely employed is the Guibal. Many others have been introduced, such as the Capell (fig. 19), Rateau, (From Mines and Minerals, March, 1905.) Fm. 19. - Capell Fan.

Schiele, Pelzer, Hanarte, Ser, Winter, Kley, and Sirocco fans. The Waddle may be instanced as an example of the open fans. Slow-speed fans are sometimes of large dimensions, up to 30 and even 45 ft. diameter, discharging hundreds of thousands of cubic feet of air per minute. Occasionally, at very gassy and dangerous collieries, two fans and driving engines are erected at the same air shaft, and in case of accident to the fan in operation the other can be started within a few minutes.

Opposed to the motive force producing the air current is the frictional resistance developed in passing through the mine workings. This resistance is equal to the square of the velocity of the current in feet per minute, Air. multiplied by the total rubbing or friction surface of the air-ways in square feet and by the coefficient of friction. The latter, determined experimentally, varies with different kinds of surfaces of mine workings, whether rough or smooth, timbered or unlined; it ranges from 0.000000001872 to 0.0000000217 lb per sq. ft., the latter being the value usually adopted. A certain pressure of air is required to maintain circulation against the resistance, and for a given volume per minute the smaller and more irregular the mine openings the greater must be the pressure. The pressure is measured by a " water-gauge " and the velocity of flow by an " anemometer." The power required to circulate the air through a mine increases as the cube of the velocity of the air current. To decrease the velocity, when large volumes of air are required, the air passages are made larger, and the mine is divided into sections and the air current subdivided into a corresponding number of independent circuits. This splitting of the air not only lessens the cost of ventilating, but greatly increases its efficiency by permitting the circulation of much larger volumes, and has the added advantage that the effect of an explosion or other accident vitiating the air current is often confined to a single division of the mine, and affects but a small part of the working force. The adjustment of the air currents in the different splits is affected by regulators which are placed in the return air-ways, and act as throttle valves to determine the volume of air in each case. The circulation of air in any given division of the mine is further controlled and its course determined by temporary or permanent partitions (" brattices "), by the erection of stoppings, or by the insertion of doors in the mine passages and by the use of special air-ways (see Coal). In devising a system of ventilation it is customary to subdivide the workings so that the resistance to the ventilating current in each split shall be nearly equal, or so that the desired amount of air shall be circulated in each without undue use of regulating appliances which add to the friction and increase the cost of removing the air. In addition to this it is desirable to take advantage of the natural ventilation, that is, to circulate the air in the direction that it goes naturally, as otherwise the resistance to the movement of the air may be greatly increased. So far as possible, vitiated air is led directly to the shaft instead of passing through other workings; for example, mine stables when used are placed near the upcast shaft and ventilated by an independent split of the ventilating current.

Deep Mining

There has been much speculation as to the depth to which it will be practicable to push the work of mining. The special difficulties which attend deep mining, in addition to the problems of hoisting ore and raising water from great depths, are the increase of temperature of the rocks and the pressure of the overlying strata. The deepest mine in the world is No. 3 shaft of the Tamarack mine in Houghton county, Michigan, which has reached a vertical depth of about 5200 ft. Three other shafts of the Tamarack Company, and three of the neighbouring Calumet and Hecla mine, have depths of between 4000 and 5000 ft. vertical. The Quincy mine, also in Houghton county, has reached a vertical depth of nearly 4000 ft. In England are several collieries over 3000 ft., and in Belgium two are nearly 4000 ft. deep. In Austria three shafts in the silver mines at Prizbram have reached the depth of over 1000 metres. At Bendigo in Australia are several shafts between 3000 and 4000, and one, the Victoria Quartz mine, 43 00 ft. deep. In the Transvaal gold region (South Africa), a number of shafts have been sunk to strike the reef at about 4000 ft. In most cases the deposits worked are known to extend to much greater depths than have been reached. The possibility of hoisting and pumping from great depths has been discussed, and it remains now to consider the other conditions which will tend to limit mining operations in depth - namely, increase of temperature and increase of rock pressure. Observations in different parts of the world have shown that the increase of temperature in depth varies: in most localities the rise being at the rate of one degree for 50 to 100 feet of depth; while in the deep mines of Michigan and the Rand, an increase as low as one degree for each 200 ft. or more has been observed. In the Comstock mines at Virginia City, Nevada, it is possible to continue mining operations at rock temperatures of 130° F. In these mines a constant supply of pure air, about oon cub. ft. per minute, was blown into the hot working places through light iron pipes. The air issuing from these pipes was dry and warm, and served to keep the temperature of the air below 120°, at which temperature it was possible for men to work continuously for half an hour at a time, and for four hours in the day. In some places work was conducted with rock temperatures as high as 158° F., with air 135° F. In these very hot drifts the fatality was large. In the Alpine tunnels, where the air was moist and probably not as pure as in the Comstock mines, great difficulty was experienced in prosecuting the work at temperatures of 90° F. and less. The mortality was large, and it was believed by the engineers that temperatures over 104° would have proved fatal to most of the workmen. Deep mines, however, are generally dry, so that in most cases it will be possible to realize the more favourable conditions of the Comstock mines. Assuming an initial mean temperature of 50° F., and increments of one degree for loo and for 200 ft., a rock temperature of will be reached at 8000 to 16,000 ft. In many deep mines to-day " explosive rock " has been encountered. This condition manifests itself, for example, in mine pillars which are subjected to a weight beyond the limit of elasticity of the mineral of which they are composed. Under such conditions the pillar begins to yield, and fragments of mineral fly off with explosive violence, exactly as a specimen of rock will splinter under pressure in a testing machine. The flying fragments of rock have frequently injured and sometimes killed miners. A similar condition of strain has been observed in deep mines in different parts of the world - perhaps due to geological movements. Assuming a weight of 13 cub. ft. to the ton, then at 6500 ft. the pressure per sq. ft. will be 500 tons, and at 13,000 ft. r000 tons; and as the mineral is mined the weight on the pillars left will be proportionately greater. At such pressures all but the strongest rocks will be strained beyond their limit of elasticity. At depths of loon ft.

and less some of the softer rocks show a tendency to flow, as exhibited by the under-clay in deep coal-mines, which not infrequently swells up and closes the mine passages. In the Mont Cenis tunnel a bed of soft granite was encountered that continued to swell with almost irresistible force for some months. The pressure developed was sufficient to crush an arched lining of two-foot granite blocks. Similar swelling ground is not infrequently met with in metal mines, as, for example, in the Phoenix copper mine in Houghton county, Michigan, where the force developed was sufficient to crush the strongest timber that could be used. In very deep mines this flowing of soft rock will doubtless add greatly to the difficulty of maintaining openings. What may happen in some cases is illustrated by the curious form of accident locally known as a " bump," which occurs in some of the deep coal-mines of England. In one instance (described by F. G. Meacham, Trans. Fed. Inst. M.E. v. 381), the force developed by the swelling under-clay broke through and lifted with the force and suddenness of an explosion a lower bench of coal 8 ft. thick in the bottom of a gangway 12 ft. wide for a length of zoo ft., throwing men and mine cars violently against the roof and producing an air-wave which smashed the mine doors in the vicinity. It is apparent that the combined effect of internal heat and rock pressure will greatly increase the cost of mining at depths of 8000 or 10,000 ft., and will probably render mining impracticable in many instances at depths not much greater.

Mine Administration

In organizing a mining company it must be recognized that mining is of necessity a temporary business. When the deposit is exhausted the company must be wound up or its operations transferred to some other locality. Mining is also subject to the risks of ordinary business enterprises, and to additional risks and uncertainties peculiar to itself. The vast majority of mineral deposits are unworkable, and of those that are developed a large proportion prove unprofitable. In addition mining operations are subject to interruption and added expense from explosions, mine fires, flooding, and the caving-in of the workings. To provide for the repayment from earnings of the capital invested in a mining property and expended in development, and to provide for the depreciation in value of the plant and equipment, an amortization fund must be accumulated during the life of the mine; or, if it be desired to continue the business of mining elsewhere, a similar fund must be created for the purchase, development and equipment of a new property to take the place of the original deposit when that shall be exhausted. If, for example, we assume the life of a given mine at ten years and the rate of interest at 5%, it will be necessary that the property shall earn nearly 13% annually - viz., 5% interest and 8% for the annual payment to the amortization or the reserve fund. To cover the special risks of mining, capital should earn a higher interest than in ordinary business, and if we assume that the sinking-fund be safely invested, we must compute the amortization on a lower basis than 5%. Assuming, for example, the life of the mine at ten years as before, and taking the interest to be earned by the amortization fund at 3%, and that on the investment at io%, we shall find that the annual income should amount to 18.7% per year. These simple business principles do not seem to be generally recognized by the investing public, and mines, whose earning capacity is accurately known, are frequently quoted on the stock markets at prices which cannot possibly yield enough to the purchaser to repay his investment during the probable life of the mine.

Mine Valuation

The value of any property is measured by its annual profits. In the case of mining properties these profits are more or less uncertain, and cannot be accurately determined until the deposit has been thoroughly explored and fully developed. In many instances, indeed, profits are more or less uncertain during the wholelife of the mine, and it is evident that the value of the mining property must be more or less speculative. In the case of a developed mine its life may be predicted in many cases with absolute certainty - as when the extent of the mineral deposit and the volume of mineral can be measured. In other cases the life of the mine, like the value of the mineral, is more or less uncertain. Further, both time and money are required for the development of the mining property before any profit can be realized. Mathematically we have thus in all cases to compute present value on the basis of a deferred as well as a limited annuity. The valuation of mines then involves the following steps: (1) The sampling of the deposit so far as developed, and assaying of the samples taken; (2) The measurement of the developed ore; (3) estimates of the probable amount of ore in the undeveloped part of the property; (4) estimates of probable profits, life of the mine, and determination of the value of the property. Where the deposit is a regular one and the mineral is of fairly uniform richness, the taking of a few samples from widely separated parts of the mine will often furnish sufficient data to determine the value of the deposit. On the other hand in the case of uncertain and irregular deposits, the value of which varies between very wide limits, as, for example - in most metal mines and especially mines of gold and silver - a very large number of samples must be taken - sometimes not more than two or three feet apart - in order that the average value of the ore may be known within reasonable limits of error. The sampling of a large mine of this character may cost many hundreds of pounds. This applies with even greater force to estimates of undeveloped portions of the property. If the deposit is regular and uniform, the value of undeveloped areas may sometimes be predicted with confidence. In the majority of instances, however, the estimates of undeveloped ore contain a large element of uncertainty. In order to determine the probable profit and life of the mine a definite scale of operations must be assumed, the money required for development and plant and for working capital must be estimated, the methods of mining and treating the ore determined, and their probable cost estimated. Where the deposit is uncertain and the element of risk is large, we must adopt a high rate of interest on investments of capital in our computations of value - in some cases as high as 10, 15 or even 20%. Where the deposit is regular and the future can be predicted with some degree of certainty, we may be justified in adopting in some cases possibly as low as 5%. The interest on the annual contribution to the sinking-fund or its equivalent should be reckoned at a low rate of interest, for such funds are assumed to be invested in perfectly safe securities. Allowance must be made for the period of development during which there are no contributions to the sinkingfund and within which no interest is earned on invested capital.

Mining Education

It is necessary to have the work directed by men thoroughly familiar with the characteristics of mineral deposits, and with wide experience in mining. For the purpose of training such men special schools of mining engineering (ecoles des mines, Bergakademie) have been established in most mining countries. A student of mining must receive thorough instruction in geology; he must study mining as practised in different countries, and the metallurgical and mechanical treatment of minerals; and he should have an engineering education, especially on mechanical and electrical lines. As he is called upon to construct lines of transport, both underground and on the surface, works for water-supply and drainage, and buildings for the handling, storage and treatment of ore, he must be trained to some extent as a civil engineer. As a foundation his education must be thorough in the natural and physical sciences and mathematics. In addition there have been established in many countries schools for the education of workmen, in order to fit them for minor positions and to enable them to work intelligently with the engineers. These miners' schools (Bergschule, ecoles des mineurs) give elementary instruction in chemistry, physics, mechanics, mineralogy, geology and mathematics and drawing, as well as in such details of the art of mining as will best supplement the practical information already acquired in underground work. The training of a mining engineer merely begins in the schools, and mining graduates should serve an apprenticeship before they accept responsibility for important mining operations. It is especially necessary that they should gain experience in management of men, and in the conduct of the business details, which cannot well be taught in schools.

Accidents

Mining is an extra-hazardous occupation, and the catastrophes, which from time to time have occurred, have caused agencies to enforce their authority. While in some cases these laws are unnecessarily stringent and tend to restrict the business of mining yet on the whole they have had the effect of reducing greatly the loss of life and injuries of miners where they have been well enforced. This is evident from fig. 20, which shows the number of men killed in the coal and metal mines of Great Britain for a series of years. As will be seen from this diagram the most serious source of death and injury is not found in mine explosions, but in the fall of rocks and mineral in the working places. This danger can be reached only in small degree by laws and inspection; but the safety of the men must depend upon the skill and care of the miners themselves and the officers in charge of the underground work. Great loss of life and injury occur through the ignorance, carelessness and recklessness of the men themselves, who fail to take the necessary precautions for their own safety, even when warned to do so. Mining laws have proved chiefly serviceable in securing the introduction of efficient ventilation, the use of safety-lamps, and of proper explosives, to lessen the danger from fire-damp and coal-dust in the coal-mines, the inspection of machinery for hoisting and haulage, and prevention of accidents due to imperfection in design or in working the machinery.

Fire-damp and dust explosions are caused by the presence of marsh-gas in sufficient quantity to form an explosive mixture, or by a mixture of small percentages of marsh-gas and coal-dust, and in some cases by the presence of coal-dust alone in the air of the mine. Explosive mixtures of marsh-gas and air may be fired by an unprotected light. But when coal-dust is present, and little or no marsh-gas, an initial explosion - such as is produced by a blown-out shot - is required. To guard against explosions from this cause it is necessary to use explosives in moderate quantities and to see that the blast-holes are properly placed, so that the danger of blown-out shots may be lessened. In dry and dusty mines the danger may be greatly lessened by sprinkling the working places and passages, and the removal of the accumulated dust and fine coal. Where large quantities of fire-damp are present, safety-lamps of approved pattern must be used and carefully inspected daily. The use of matches and naked lights of any kind must be prohibited. To lessen the danger from blasting operations the use of special safety explosives is required in Great Britain and some European countries. The use of such explosives decreases to some extent the danger from dust explosions; but experiment shows that no efficient explosive is absolutely safe, if used in excessive quantity, or in an improper manner. Absolute security is impossible, as is proved by the many and serious disasters under the most stringent laws and careful regulations that can be devised.

Mine fires may originate from ordinary causes, but in addition they may result from the explosion of fire-damp or from the accidental lighting of jets of fire-damp issuing from the coal. In some mining districts the coal is liable to spontaneous combustion. A fire underground speedily becomes formidable, not only in coal but also in metal mines, on account of the large quantity of timber used to support the excavations. Underground fires may sometimes be .extinguished by direct attack with water. The difficulty of extinguishing an underground fire in this way is, however, very great, as on account of the poisonous products of combustion it is impossible to attack it except in the rear, and even there the men are always in great danger from the reversal of the FIG. 20. - Death-rate from various classes of accidents in and about all mines in the United Kingdom from 1873 to 1900.

the enactment of laws to protect the lives of the men engaged in air current, or back-draught from the fire. Further, the burning underground work. These laws are enforced by mine inspectors of the timber produces falls of ground, making necessary the excavawho are empowered to call upon the courts and other government tion and removal at times of hundreds of tons of heated rock and burning coal, in order to reach the fire. When direct attack is no longer practicable, it is possible to extinguish the fire by sealing the mine workings, and exhausting the supply of oxygen. It is necessary, however, to keep the mine sealed until the burning timbers, or coal, and the red-hot rocks have become cool, or the fire will again break out. This sometimes requires two or three months. Where an effective sealing of the mine is impracticable it is sometimes possible to extinguish the fire from the outside of the mine by constructing a large reservoir or tank in the upper part of the mine-shaft and suddenly releasing a large volume of water by opening dischargedoors. The mass of water falling down the shaft is converted into spray, which is carried by the force of the fall long distances into the workings. Where the fire is in or near the shaft this method has proved very effective. Mine fires may sometimes be reached by bore-holes sunk for the purpose from the surface, and the burning workings below filled by flushing with culm and water. As a last resort the mine may be flooded with water. This is an expensive operation as it entails the cost of pumping the water out again and repairing the resulting damage. If the fire is in working places to the rise the water may not reach the burning portions of the mine, but will effectually seal them. But sufficient time must be allowed to elapse before pumping out the water, as otherwise the fire may break out again.

Mines may become flooded by the inrush of surface waters in times of great rainfall or sudden floods, or by the undermining of surface waters. The mine workings may also be flooded Flooding of by large bodies of underground water. The surface Mines. floods must be provided with channels of sufficient size to carry them safely past the mine openings, and intercepting ditches should be excavated for this purpose, and dams and embankments constructed to divert the flood waters. That it is possible to work with safety beneath rivers, lakes and even the ocean has been proved in numerous instances; mines in different parts of the world having been extended long distances under the sea. In such cases preliminary surveys should be made to determine the thickness of rock over the proposed workings. Under favourable conditions mining may be conducted under the protection of a few yards of solid rock only, as in the submarine work for the removal of reefs in the harbours of San Francisco and New York. At Silver Islet, Lake Superior, mining was successfully carried on for years under the protection of a coffer dam and an arch of rich silver ore less than 20 ft. thick. At Wheal Cock near St Just in Cornwall the protecting roof was so thin that holes bored for blasting more than once penetrated to the bed of the ocean, and wooden plugs were kept on hand to drive into such holes when this occurred. In storms the boulders could be heard striking each other overhead. When large areas are undermined, as in submarine coal mining, it is best to have several hundred feet of protecting rock. In Great Britain the law requires that the workings shall be protected by 120 ft. of solid strata. When the presence of underground bodies of water is known or suspected, advance bore-holes should radiate from the end of the advancing working place so as to give warning of the position of the body of water, these holes being of such length as to ensure a safe, thickness of solid rock.

The caving in of mine workings results from the excavation of large areas supported upon pillars of insufficient size. While the mine workings are small the overlying rocks support themselves of and the full pressure does not come upon the mine Caving p i llars. As the workings increase in size the pillars Mane Workings. beyond theincreasing of elasticity. til finally When this occurs, strained pillars begin to crack and splinter with a noise like musketry firing, and the roof of the mine shows signs of subsidence. This may con tinue for weeks before the final crash takes place. At first a fall of the roof occurs locally, here and there throughout the mine, and these falls may succeed one another until the settlement of portions of the roof has so far relieved the strain that the remaining areas are supported by the stronger pillars, and by the fallen rock masses. While abundant warning of the caving-in of the workings is thus given in advance it may happen that men are unexpectedly imprisoned by the closing of the main passage ways. The caving-in of the mine, however, is rarely so complete that avenues of escape are not open. In many cases, however, it has been found necessary to reopen the mine through the fallen ground, and even to excavate openings through the solid mineral. The history of mining is full of dramatic episodes of this character.

Accidents from the misuse and careless handling of explosives are unfortunately too frequent in mines. The conditions under Accidents which explosives may be stored, handled and used are from carefully formulated in the mining laws of most states, fro Explosives. but it is almost impossible to secure obedience to these regulations on the part of the miners, who are, as a rule, both careless and reckless in their use of powder. In some states it has become necessary to provide for fines and even imprisonment of men disobeying the regulations regarding explosives.

Mine Hygiene

While mining is not necessarily an unhealthy occupation, miners are subject to certain diseases resulting from vitiated air, and from unusual or special conditions under which at times they are forced to work. Recent investigations have shown an alarming increase in mortality from miners' phthisis in Cornwall, South Africa and elsewhere. This seems to be due to the dust abundantly produced in mining operations, and especially by machine drills when boring " dry " (rising) blast holes. Drill runners, who are compelled to breathe this dusty air daily, furnish most of the sufferers from phthisis. The increased mortality seems to be due to the general tendency toward forced speed in development work, which is secured by rapid drilling, and by an increase in the number of machine drills used in a single working-place. The miners, to save time, often return to their work after blasting before the powdersmoke and dust have been sufficiently removed. It is probable that the carbon monoxide seriously affects the general health and vitality of the men, and renders them more likely to succumb to phthisis. More effective ventilation will materially lessen the deathrate. In the metal mines of Cornwall and Devon special rules are now in force requiring the use of water in drilling, and other precautions, to lessen this danger from dust. In some mines dust seems to have but little effect on the health of the miners; indeed it is even claimed by some that coal dust decreases the mortality from phthisis. On the other hand, as in mining ores containing lead, arsenic and mercury, the dust may be poisonous. The climbing of ladders from deep mines not only lessens the efficiency of the men by reason of fatigue, but often tends to increase the mortality from diseases of the heart. In cold climates men coming from the warm atmosphere of a mine, often in wet clothing, are liable to suffer in health unless proper provision is made for the necessary change of clothing. In such cases the establishment of dressingrooms, properly heated, and connected with the mine by covered passages will be necessary. These " change-houses " are provided with washing and bathing facilities, and arrangements for drying wet clothing. Ankylostomiasis is a disease which finds a congenial habitat in the warm damp atmosphere of mines, and has become a veritable scourge in some mining regions. The disease yields readily to treatment, but is difficult to eradicate from a mine without stringent sanitary regulations to prevent its spread. The care of the health of the working force should be entrusted to competent mine physicians, thoroughly familiar with the conditions under which the miners work, and with the special diseases to which they are subject. The men should be instructed in the laws of sanitation, and in the proper care of injured men.

Mine Law. - Mine law is that branch of the law of real property relating to mineral and mining rights as distinct from rights pertaining to the surface of the ground. Under the common law the owner of the surface possesses all mining rights as well, unless these have been reserved by some previous owner of the property. From very ancient times deposits of gold and silver have in most countries been held as the property of the crown. In public or government land the minerals as well as surface belong to the state, and not infrequently these rights have been separated by law and granted or otherwise disposed of to different owners. It is to the public interest that deposits of mineral should not be permitted to remain idle and undeveloped. This has been recognized from the earliest times, and laws have been framed in all countries for the encouragement of mining enterprise. In many cases the state or the ruler has sought to obtain a share in the profits of mining, or even to work mines for the individual profit of the ruler or of the state. But in most cases it has been found better policy for the state to divest itself of all interest in mining property, and to extend all possible encouragement to those who undertake the development of the mineral wealth of the nation. The mining laws of most civilized states grant the right of free prospecting over the public lands, protect the rights of the discoverer of the mineral deposit during the period of exploration, and provide for the acquisition of mineral property on favourable terms. Striking examples of the far-reaching effect of such laws is shown in the history of the Rocky Mountain region and western coast of the United States, the colonization and development of Australia, and the development of Alaska.

BIBLIOGRAPHY. - See C. Le Neve Foster's Ore and Stone Mining (6th ed., London, 1905), or G. Kohler's Lehrbuch der Bergbaukunde (6th ed., Leipzig, 1903). The following works may also be consulted: Books - Bertolio, Coltivazione delle minere (Milan, 1902); Brown, The Organization of Gold Mining Business (Glasgow, 1897); Brough, Mine Surveying (12th ed., London, 1906); Bulman and Redmayne, Colliery Working and Management (London, 1896); Colomer, Exploitation des mines (Paris, 1899); Curle, The Gold Mines of the World (2nd ed., London, 1902); Demanet, Traite d'exploitation des mines de houille (2nd ed., Brussels, vols. i and ii. 1898, vol. iii. 18 99); Denny, Deep Level Mines of the Rand (London, 1902); Galloway, Lectures on Mining (Cardiff, 1900); Habets, Cours d'exploitation des mines (2nd ed., Liege, vol. i., 1906, vol. ii. 1904); Hatch and Chalmers, The Gold Mines of the Rand (London, 1895); Haton de la Goupilliere, Cours d'exploitation des mines (2nd ed., Paris, vol. i. 1896, vol. ii. 1897); Hoefer, Taschenbuch fiir Bergmanner (Leoben, 1897); Hughes, Coal Mining (4th ed., London, 1900); M. C. Ihlseng, A Manual of Mining (4th ed., New York, 1905); Kirschner, Grundriss der Erzaufbereitung (Leipzig and Vienna, vol. i. 1898, vol. ii. 1899) Lawn, Mine Accounts and Mining Book-keeping (London, 1897); Lupton, Mining (3rd ed., London, 1899); T. A. Rickard, The Sampling and Estimation of Ore in a Mine (New York, 1904); Truscott, The Witwatersrand Goldfields - Banket and Mining Practice (London, 1898; G. F. Williams, The Diamond Mines of South Africa (New York, 1902); Periodical Publications - A nnales des mines de Belgique (Brussels, quarterly); Australian Mining Standard (Melbourne, Sydney and Brisbane, weekly); Engineering and Mining Journal (New York, weekly); Gliickauf (Essen, weekly); Mines and Quarries; General Report and Statistics (London, annually); with details from official reports of colonial and foreign mining departments; Mines and Minerals (monthly, Scranton, Pennsylvania); The Mineral Industry (New York, annually); Transactions of the American Institute of Mining Engineers (New York); The Mining and Scientific Press (weekly, San Francisco); Transactions of the Institute of Mining and Metallurgy (London); Transactions of the Institution of Mining Engineers (Newcastle-on-Tyne). (H. S. M.)