As the process of sinking a shaft is often extremely slow, from the hardness of the ground (one fathom per week being a very good average, and sometimes half that progress being scarcely attainable), and as the most productive workings of a mine may be greatly impeded for want of another outlet to the surface, expedition is often of the utmort importance to remedy this evil, and it thus becomes highly desirable to accelerate the operation. The perfection which subterra nean surveying. has of late years attained enables the miner to accomplish this object by a very interesting proceass, whenever the workings of the mine have advanced near the spot where a shaft is required. The site of the shaft having been fixed upon and marked out at the surface, the miner, by a series of very accurate measure ments of the length. windings, and direction of the levels, is enabled to ascertain correctly their relative position with regard to this spot, and consequently at whet point each of them approaches nearest to a supposed vertical line penetrating the rock below it. This being ascertained, it is evident that by pursuing the same process still further, he may determine in what direction and to what distance cross-cuts must be driven from each of these points, in order to bring him exactly to this line, or underneath the site of the shaft ; and having arrived there, that excavations exactly corresponding with it, both in form and dimensions,may proceed simultaneously both upwards and downwards from each cross-cut, while the shaft itself is being sunk from the surface, the work thus proceeding from several different points at the same time, as shown in Fig. 3. It is found in practice that the various separate portions may be made to unite with surpri sing exactness ; so much so, that even in very deep shafts, when coin plete, daylight may be seen from the bottom ; and from the great swing of time which the process effects, it is now frequently used in mines of great depth, the working of which is much facilitated by it, as the work of many years may thus be brought within the compass of one or two. A very remarkable instance of a deep Rhein being thus Runk from several points at once occurred at the Consolidated Mines in Cornwall, about thirty years ago, where a perpendicular shaft, 204 fathoms in depth, was completed in less than a twelvemonth, being worked from fifteen different points at once.
As the working of a mine proceeds, the increase of shafts and levels tends to obliterate, in a great degree, the uniformity and simplicity of operations which were at first apparent, the position of these works being entirely regulated I y the irregular distribution of the productive parts rtzof the vein, as developed in their progress. When the depth becomes considerable, many of the first shafts are rendered in a great measure useless, either from being inclined, and thus inconvenient for machinery, or from having passed through the vein at a shallow depth, and thus requiring long crosscuts previous to commencing the deeper levels. Hence, in very deep mines, a double line of shafts will often be found to range along the course of the principal veins ; and some. times even three shafts will be found opposite each other, and inter. Reding the same part of the vein successively at greater depths. In
this case, while the most recent shafts are used for drainage and ex• traction, the older and more shallow ones are often fitted up as " foot• ways," and serve for the partial ascent and descent of the miners. In some of the large mines of Cornwall it is usual to sink two shafts within a few fathoms of each other, one being of large dimensions, and Intended for a drainage or " engincsahaft," the other smaller, and adapted to drawing stuff only. This arrangement Is found more convenient than having a single large shaft arranged for both purposes, for which however one ideate is often made to answer, being divided down the middle by thnbering, and one side appropriated to the pumps and ladders, while the other is occupied by the " kibbles" or iron bucket/ used for drawing the ore and in many awe the rubbish, when the latter cannot be conveniently disposed of underground.
Shafts in this country are generally sunk of a rectangular form, except in our coal minor, where a circular form is often preferred Those intended for the extraction of ores, or " whim-shafts," are com monly six feet by four; those employed for drainage, or " engine ihafte," as they are generally termed, vary from about ids feet by eight to eight by ten, or sometimes are rather larger. In coal-mines the pits are generally about seven or eight feet in diameter, the dimensions varying according to the nature of the ground and the arrangements proposed in them.
As veins are generally found to run nearly parallel, and often at no great distance from each other, and as the neighbourhood of a productive vein is a favourable indication of the contents of others in its vicinity, transverse levels or " cross-cuts" are frequently driven from mines at various depths, with a view to discovering side-veins, or making trial of branches which diverge from the main lode. Should a productive vein be found in the neighbourhood of the first, the most usual mock of working it is by extending levels upon it, at the same depth as flew in the mine from which the cross-cute are driven, commencing at the points where these intersect it. In this case the seine shafts will probably serve for both the old mine and the new one, the one being, in fact, a mere appendage, as it were, to the other. Should the distance of the newly-discovered vein be considerable, it will prevent the workings from being carried on in this manner, both from the length of the cross-cuts, and from the difficulty of ventilation and extraction ; and it will therefore be necessary to sink shafts upon it, and lay it open as a separate mine, in a somewhat similar manner to that which has already been described.
Metalliferous, veins aro often traversed by other veins crossing them nearly at right angles, which seldom contain ore, excepting perhaps near the points of intersection : they are termed " cross-courses," or " cross-veins," and occur in most mines. Cross-cuts are sometimes carried upon these veins, partly to explore their contents, and partly because the work will often proceed more rapidly than when in the solid rock ; but this is not considered so effectual a mode of exploring the ground and discovering new veins as by driving iu the rock itself.