Principles of mining : valuation, organization and administration; copper, gold, lead, silver, tin and zinc . of the mine, andis, therefore, of a more temporaiy character. entry to the mine. There are four methods of entiy: by tunnel, vertical shaft,inclined shaft, or by a combination of the last two, that is, by ashaft initially vertical then turned to an incline. Combinedshafts are largely a development of the past few years to meetdeep level conditions, and have been rendered possible onlyby skip-winding. The angle in such shafts (Fig. 2) is now gen-erally made on a parabolic curve, and the
Principles of mining : valuation, organization and administration; copper, gold, lead, silver, tin and zinc . of the mine, andis, therefore, of a more temporaiy character. entry to the mine. There are four methods of entiy: by tunnel, vertical shaft,inclined shaft, or by a combination of the last two, that is, by ashaft initially vertical then turned to an incline. Combinedshafts are largely a development of the past few years to meetdeep level conditions, and have been rendered possible onlyby skip-winding. The angle in such shafts (Fig. 2) is now gen-erally made on a parabolic curve, and the speed of winding isthen less diminished by the bend. The engineering problems which present themselves underentry may be divided into those of: — 1. Method. 2. Location. 3. Shape and size. 58 DEVELOPMENT OF MINES. 59 The resolution of these questions depends upon the: — a. Degree of dip of the Output of ore to be provided for. c. Depth at which the deposit is to be attacked. d. Boundaries of the property. e. Surface Cost. SI U- Operating efficiency. h. Prospects of the Fig. 2. —Showing arrangement of the bend in combined shafts. 60 PRINCIPLES OF MINING. From the point of view of entrance, the cooperation of amajority of these factors permits the division of mines into cer-tain broad classes. The type of works demanded for moderatedepths (say vertically 2,500 to 3,000 feet) is very different fromthat required for great depths. To reach great depths, the sizeof shafts must greatly expand, to provide for extended ventila-tion, pumping, and winding necessities. Moreover inclinedshafts of a degree of flatness possible for moderate depths be-come too long to be used economically from the surface. Thevast majority of metal-mining shafts fall into the first class,those of moderate depths. Yet, as time goes on and ore-de-posits are exhausted to lower planes, problems of depth willbecome more common. One thing, however, cannot be toomuch emphasized,
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