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, 4300 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 1000 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 100 and for 200 ft., a rock temperature of 130° 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. 1000 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 1000 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 200 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 10%, 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 whole life 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
