Firing by Electricity.—The firing of blasts by means of electricity requires that great care should be taken of the wires while tamping, and where dynamite exploders with platinum wires are used the "compound circuit" is most desirable. A paper entitled "On the Simultaneous Ignition of Thousands of Mines," by Julius H. Striedinger, published in the "Transactions" for June, 1877, of the American Society of Civil Engineers, contains much valuable information on the subject.

In charging the drifts the powder (in boxes or kegs) is piled up in rows; two wires, A A and DD (see Fig. 61),

extend along the middle row, the tops of the boxes on which wires rest being removed. The exploders, b, b, b, are inserted in giant-powder cartridges and placed on top of the paper covering the powder.

The wires A A and D D are then connected with the wires Y Y and Z Z', which extend to the battery.

Tamping. Great care should be used to prevent the "blowing-out" of the tamping, which results not only in considerable loss of effect, but often causes great. destruction to property and even to life. It is advisable, when firing blasts by fuse, to tamp nearly the entire main drift. The gravel extracted from the drift is used for this purpose, and should be fairly dry and as free as possible from large stones, which cause great damage in case

of a blow-out. The tamping must be firmly rammed by wooden mauls, so that it will not settle from the roof of the drift. In order to guard against failure through defective fuse it is customary to use two or three lines, which are simultaneously ignited.

Firing by electricity has the advantage of requiring less tamping and of permitting it to be placed in the cross drifts between the two chambers of powder, which are simultaneously fired-a result that could not be effected by fuse. The force from the explosion from the two chambers, acting upon the tamping from opposite sides, prevents its being blown out; and therefore when drifts are fired in this way it is necessary to tamp but a short distance in the cross drifts and but a few feet in the main drift.

Owing, however, to the many failures arising from defective batteries and connections, the miners generally have abandoned the use of the electric battery.

CHAPTER XIV.

TUNNELS AND SLUICES.

Tunnels.-Tunnels are run for the purpose of opening gravel claims (where open cuts are impossible on account of the formation of the ground), and also to afford proper facilities for removing the washed material.

A tunnel should be driven well into the channel before any connection is made with the surface.

Shafts for Tunnels.-The shaft which connects with the headings should be vertical, though in some cases inclines have been used. Its size is determined by the requirements of the work, and varies, for ordinary cases, from 3 by 3 feet to 41⁄2 by 9 feet in the clear. When raising from the tunnel due precaution should be taken against accidents arising from the rush of water, sand, and gravel, which is liable to occur on tapping the bottom of a deposit. A shaft 4% by 9 feet should be divided into two compartments, one of which will serve as a man-way. A compartment 4 by 4 feet in the clear is ample for the water-way.

It may be noted that a vertical shaft, when properly timbered, is the most desirable and economical for opening hydraulic claims, and with drops of 300 feet no trouble has been experienced. There is no difficulty in connecting directly with the tunnel where the work is done well and the mine properly opened. But where washing is going on through a shaft into a tunnel in process of extension, it is convenient to have the shaft located at one side and connected with the tunnel by a short drift. By this means the work in the tunnel can progress while the washing is carried on.

Shaft Timbering. Where a shaft is in hard rock, and no man-way is needed, timbering is unnecessary; but in soft rock or gravel, to avoid any accident or delay the shafts should be strongly timbered, closely lagged, and lined on the inside with blocks (6 to 10 inches thick) to within 8 to 30 feet of the surface, the depth being dependent on the softness of the gravel. This top, being the first washed off, thereby gives the initial grade for the ground sluices. As washing proceeds the upper lining and timbers are removed to enable the material to be drawn into the shaft. A shaft in hard rock can be partitioned for a man-way with stoll-timbers firmly wedged and blocked.

No extraordinary precaution is required for the protection of the bottom of the shaft, the material washed being allowed to drop directly on the bed rock, where it soon wears a hole, in which the large stones from the mine lodge and form a pavement. At the junction of the shaft and the tunnel the latter should be increased in height at least 50 or 75 per cent.

Second Shaft.-With long tunnels it is advisable to sink a second shaft at a convenient distance from the heading. Formerly, as a precautionary measure, a man was placed in the tunnel to watch the washings, and in such cases a second shaft was indispensable. It is now customary, when washing into a shaft, to provide a swinging door over the sluice, about 75 feet below its head, and connected by chain and ropes to a signal on top of the shaft which gives the pipe-men notice in case of overflow.

Should an accident occur at the main shaft by its caving or closing up, the second shaft might afford the necessary facilities for continuing the work. When a line of pipe is carried down the second shaft for the purpose of assisting in opening the closed one, great precaution must be used in piping, particularly if the closed shaft is filled with water. When this expedient has to be resorted to it is usual to place the pipes in position and withdraw the

workmen before the water is turned on; and if the blockade is not broken in a reasonable time the water is shut off, men go down and extend the pipes nearer the blockade, and again the water is turned on, and the operation is continued until the blockade is broken. If the shaft or tunnel is closed by gravel mixed with heavy boulders it is necessary often to employ powder.

First Washing. The first washings through a shaft should be done with care, and the surface within as great a radius as can be conveniently washed and drawn should be cleared on all sides before taking off the top timbers. Attempts to push this preliminary work have frequently caused an over crowding of the shaft, resulting in its filling up or caving. It is therefore essential that the gravel should be run so as to avoid the rush of material from caves.

Size of Tunnel. The size of the tunnel is generally dependent on the size of the sluice. It is usually driven 2 to 3 feet wider than the inside width of the sluice, and 71⁄2 to 8 feet high. These proportions permit the proper construction of the sluice and give sufficient room for the blocks and for the workmen when cleaning up. The grade depends on the topography of the country.

Location of Tunnels.-In locating the mouth of a drainage tunnel (or of an open cut) that point is to be selected from which the sluices, running on the most direct practicable line, with a given grade, can bottom the maximum extent of the "pay channel" at the smallest expense. Due regard should be had to the dump, and allowances made for contingencies arising from changes, such as depressions and holes in the bed-rock.

Where the bed-rock disintegrates on exposure to the air an extra allowance for depth is advisable. This additional depth is a matter of judgment, and is regulated by the character and peculiarities of the bed-rock, extent of ground to be worked, and the position of the shaft. It is always possible to "ease up" the grade; but if the main.