་

a

borer, may in some kinds of stone remain uninjured for days or even months together, whilst in certain granites and siliceous veinstones it may destroy the edge of 50 borers in succession to make a hole a foot deep. 14. The practice of blasting was always attended with danger, till the introduction of the safety-fuze, by which fire is communicated to the gunpowder in such a manner as to obviate most of the sources of accident. This simple and economical contrivance, the invention of Messrs. Bickford and Smith in Cornwall, consists of a train of powder protected by an envelope of cord, which lies in the hole during the process of tamping, and is cut to such a length as to allow the miners abundant time to retire. In almost all circumstances it should be employed where due regard is paid to the value of human life.

Fig. 4.

d

g

h

Fig. 4 represents the tools above described: a, is the ancient miner's wedge or "iron," still used in the continental mines; b, wedges of different sizes, to be fixed temporarily on the same handle; c, the hammer for driving them; d, is the Cornish pick; e, the wedges or "gads, to be driven by the last;" f, collier's pick; g, h, borers and hammer employed for two-handed boring.

LESSON II.

WORKING OF MINES.

"A land whose stones are iron, and out of whose hills thou mayest dig brass."

1. PRELIMINARY EXCAVATIONS, Shafts and Levels. -A mineral deposit must in the first instance be opened by means of preparatory workings, the arrangement of which will depend on the character and

position of the bed or the lode. The more or less perpendicular openings, like large wells, are termed shafts, whilst the horizontal galleries are called levels or drifts, and when they open to the surface in some valley, or on a hill side, adits.

Where the latter mode can be brought into action, it is advantageous as affording an outlet to the subterranean waters, and particularly so when the situation admits of the level being driven in the actual thickness of the deposit which is to be worked. If the gallery should have to be excavated through barren rock before it arrive at the object of search it is termed a cross-cul.

These levels in well-managed metalliferous mines are about 7 feet high by 4 or 5 feet wide, but in stratified deposits must depend for their dimensions in great part on the thickness of the bed.

2. Where a seam of coal or other bed is to be worked by shafts, they are of course sunk perpendicularly to the required spot; but where a lode is to be explored,

it is often advisable to sink the shaft in the vein itself, following its inclination downwards, and proving its character step by step.

As it advances in depth, galleries are driven out in each direction on the course of the lode, and at intervals below one another, generally of 10 fathoms or 60 feet. A vertical section, which supposes the rock to be entirely removed from one side of the lode, so that we can see it in its full length and depth, will exhibit these particulars more clearly than any description.

In the little mine here represented, there are two shafts, in one of which the ore is drawn out by a windlass, in the other by a horse-whim.

Three levels or galleries have been driven, the uppermost of which, opening into a valley, is the adit, whilst the two lower ones are at the depths of 10 and 20 fathoms respectively below the adit level.

3. So long as the shafts and levels are excavated in firm rock, they may be left without any artificial support,

but when the ground is soft, or much jointed, or liable to decompositions, it becomes necessary by various means to secure the roof and sides against falling in. These precautions must often, particularly where underground water is abundant, be undertaken on a very large scale, and form a serious item in the current expenses of a mine. Wood is the material of most frequent application, and large forests are every year

thus buried in the depths of the earth, whether they are cut, as in most mining districts, from the woods of the neighbouring hills, or are transported, as into Cornwall, hundreds of miles across the seas from the pine-clad heights of Norway.

4. The mode in which timbering is applied depends on the direction of the pressure of the ground, whether it comes from above only, or from the sides as well. Three of the most common forms are exhibited in the subjoined figures. Fig. 6 shows how single pieces of timber, "stemples," are placed across from wall to wall to support a weight above them. Fig. 7 represents the mode of resisting a pressure from three sides, by means of two stanchions or wall-plates," and a "cap" fitted upon them, each side being lined where it is required by planks or "laths" to prevent the falling of small portions of the rock; while fig. 8 represents

66

the mode of supporting the roof, when one side is weak and the other solid and firm.

In

5. The securing of shafts, particularly when of large dimensions, is an operation of greater difficulty. metal mines the shafts are rectangular, and divided generally into two portions by a "casing" or "brattice" of

woodwork, which separates the part where the buckets for raising the minerals pass up and down, from that in which the pumps are placed, and where the men ascend and descend. Several different modes of arranging and jointing the frames of timber are in vogue, but, with all of them, the same principles must be observed, viz., that the wood be so fixed as to oppose in direction the chief pressure or thrust, that the frames be closer together according to the weakness of the ground, even to the extent of being laid in close contact one upon another, and that at intervals the weight of the timbering be supported from below by stout balks or stems termed "bearers," the extremities of which project one or two feet into the firm rock, so as to secure a safe foundation. Stone and brick are also largely employed for the security of subterranean works. Galleries are protected

by arches, either above or all round, according to the character of the pressure which they have to sustain. The shafts or pits of collieries in Britain are generally lined with brick, except where they pass through hard ground; and, with a view to their strength, receive a round or an oval form, with a diameter of from six to sixteen feet. Such a structure, if placed on the surface of the ground, would resemble a lofty tower, often much higher than our tallest church-spires; indeed, if we were to compare some of the deeper shafts with wellknown edifices, we should find the latter sink into insignificance, for the pits of some of our great mines would

Fig. 9.