For a stratum little incli d to the horizon, placed beneath a p .1n, the first thing is to pierce two v rtical shafts, which are usually made t. arrive at two points in the same line o slope, and a gallery is driven to unite th m. It is, in the first place, for the sake o circulation of air that these two pits are ,unk ; one of them, which is also destine. • drainage of the waters, should reach the lowest point of the intended workings.
The excavations of mines are divisible into three principal species ; shafts, gal leries, and chambers. When the width of these excavations is inconsiderable, as is commonly the case with shafts and gal leries, their sides can sometimes stand upright of themselves • but more fre quently they require propped or staved by billets of wood, or by walls built with bricks or stones ; or even by stuffing the space with rubbish. These three kinds of sumort are called umber ing, walling, and filling up.
Timbering is most used. It varies in form for the three species of excavations, according to the solidity of the walls which it is destined to sustain.
The timbering of shafts varies in form, as well as that of galleries, according to the nature and the locality of the ground which they traverse, and the purposes which they are meant to serve. The shafts intended to be stayed with timber are usually square or rectangular, because this form, in itself more convenient for the miner, renders the execution of the timbering more easy. The wood-work consists generally of rectangular frames, the spars of which are about eight inches in diameter, and placed at a distance asunder of from a yard to a yard and a half. The spars are never placed in con tact, except when the pressure of the earth and the waters is very great. The pieces composing the frames are com monly united by a half-check, and the longer of the two pieces extends often beyond the angles, to be rested in the rock. Whether the shaft is vertical or inclined, the frame-work is always placed so that its plane may be perpendicular to the axis of the pit. It happens some times in i nclined shafts that there are only two sides, or .even a single one, which needs to be propped. These are stayed by means of cross beams, which rest at their two ends in the rock. When the frames do not touch one another, strong planks or stakes are fastened be hind them to sustain the ground. To these planks the frames are firmly con nected, so that they cannot slide. In this ease the whole timbering will be sup ported, when the lower frame is solidly fixed, or when the pieces from above pass by its angles to be abutted upon the ground.
In the large rectangular shafts, which serve at once for extracting the ores, for the discharge of the waters, and the de scent of the workmen, the spaces destined for these several purposes are in general separated by partitions, which also serve to increase the strength of the timber ings, by acting as buttresses to the planks in the long sides of the frame-work.
When men penetrate by narrow pas sages into the interior of the earth, their respiration, joined to the combustion of candle and gunpowder, are not long in vitiating the air. The decomposition of wood contributes to the same effect, as also the mineral bed itself, especially in coal mines, by the carburetted hydrogen and carbonic acid evolved, and from the absorption of oxygen by pyrites. In many cases, arsenical and mercurial va pors are disengaged. Hence the necessity of maintaining in subterranean cavities a continual circulation of air, which may renew the atmosphere round the miners. The whole of the means employed to produce this effect, constitutes what is called the ventilation of mines.
These means are divided into natural and artificial. The natural means are the currents produced by the difference of density between the air of mines and the external air ; the artificial are air exhanaters or condensers, fires, Ste.
The temperature of the air of the sub terranean workings surpasses the mean temperature of the place in which the mine is opened. Hence it is lighter in winter, but in summer often heavier than the air of the atmosphere. For this rea son, when the mine presents two open ings at different levels, the air naturally flows out by the most elevated in winter, and by the lowest in summer. We may take advantage of this circumstance, to lead the air into the bottom of even a very long gallery, opening into the side of the mountain, by piercing a shaft into its roof at some distance from the en trance, and dividing the gallery by a horizontal floor into two parts, which have no mutual communication, except at the furthest extremity—the upper part communicating with the shaft, and the under with the mouth of the gallery. If the two compartments have different di mensions, the air in the smaller sooner comes into an equilibrium of temperature with the rock ; and the difference of tem perature of the two compartments is suf ficient to produce a current. If a stream let of water flows through this gallery, it facilitates the flow of the air along the lower compartment. If a mine has several openings situated on the same level, it rarely happens but some peculiar circum stance destroys, during the colds of win ter and the heats of summer, the equili brium of the air. But in spring and autumn, when the external air is nearly of the same temperature with that of the mines, the above-named causes are al most always too feeble to excite an issu ing current. The effect is, however, fre quently obtained by raising over one of the shafts a chimney 20 or 30 yards high, which alone produces the effect of an opening at a different level. It has been remarked that stormy weather usually deranges every system of ventilation.