Ventilation.—The ventilation of mines is more generally and more effectually accomplished by a judicious arrangement of the works and frequent communication with the surfaeo than by mechanical means, although it sometimes becomes necessary to resort to the latter. It will be evident that in mines constructed on the principles which have previously been explained, the disposition of the shafts, levels, and winzes is such that the entire workings will always be traversed by currents of fresh air, and it' is, only in particular cases where circum stances prevent the sinking of a shaft in places where it may be required, or where great delay is experienced in effecting the usual communications, that any mechanical process of ventilation is rendered necessary. A very efficient machine for this purpose was invented some years ago by Mr. John Taylor. It consists of a cylindrical exhausting apparatus, which may be fixed at the mouth of a shaft or level, and placed in action by any convenient power, when, by means of a series of tubes connected with it, the foul air is extracted from the interior of the works, and of course the pure atmospheric air rushes in to supply its place, and complete ventilation is thus effected. In sinking shafts, a very simple contrivance is often found sufficient : small wooden pipes made tolerably all-tight are fixed in the pit from near the bottom to six or seven feet above the top, terminating in a funnelahaped wooden box, which is moved round so as always to face the wind. The air mailing in passes down the pipes to the bottom of the abaft, in which a constant current is thus kept up (see Fig. 5). When levels have to be carried to any great distance without any com munication with the atmosphere, they are sometimes divided by a " Dollar," or wooden platform placed a foot or two above the bottom, and in this manner a sufficient current of air is obtained to enable the miners to proceed. In the north of England a contrivance called the "water blast" is often used in driving long levels into rising ground; it consists in putting down either a bore-hole or small shaft near the end of the level, and turning a small stream of water into it, which falls into a cistern placed at the bottom, and is found to carry with it a sufficient current of air to ventilate the works. In coal-mines the ventilation is much more difficult, and, from the rapid generation of explosive pace, more important also. It is generally effected by two shafts, one of which is called the "downcast" and the other the " upeast " shaft, the latter of which occupies the highest situation with reference to the dip of the coal, and has a large furnace con tinually burning near the bottom of it The air being considerably rarefied at this point, an ascending current is formed, which passes upwards to the surface through the upcast shaft with considerable velocity, causing a descending current to pass through the downcast shaft to supply its place, and by a very ingenious arrangement of the workings this current of pure atmospheric air is made to traverse every part of the mine in succession as it passes from one shaft to the other.
Ertraciime—The extraction of the materials from mines will require but little notice here, since it may be stated in general terms merely to require the adaptation of an efficient prime mover to any convenient form of winding apparatus : a purpose for which the steam-engine is very usually employed, although where water-power can be obtained it is equally effective. The horse-whim, or gin, as already noticed, is very useful in small mines or isolated workings, and before the introduction of more powerful machinery was very extensively employed for raising ore and stuff from our mines. In shallow preliminary excavations the common windlass is often employed, and in underground work is much used for various purposes, especially in sinking winzes, and raising the stuff from workings where no regular communications have been opened. The extraction of an extensive mine is enormous: indeed the quantity of ore raised is seldom more than one-third or one-fourth, sometimes indeed a fifth, or less, of the mass of stuff which is brought to the surface. At the Consolidated Mines in Cornwall, the daily extraction is about 200 tons, a Large proportion of which is raised from a depth of from 200 to nearly 300 fathoms. In the coal-mines in the north of England the extraction is still greater ; but here nearly tho whole of the mass raised is more or less valuable, coal-mines being much less incumbered with dead or unproductive works than those of the metals. The South hietton Colliery, in Durham, sends about 600 tons of coal " to bank" daily, and is capable of affording a much larger extraction.
Mechanical Preparation of Ores, or Dressing.—Having now traced the nature and progress of those subterranean works by which access is obtained to mineral and metalliferous deposits, and their produce extracted, we may briefly glance at the mechanical processes of sepa ration, technically termed "dressing," which fall within the province of the miner, and to the carrying on of which a large portion of the surface-works of every mine is devoted. The object of dressing is to separate as far as possible the earthy matter accompanying, and, as before noticed, often mixed up with the ores, from the metallic portion, which is alone valuable ; and the great principle upon which all the varied apparatus and processes which are used in different mines, and in different countries, to effect this purpose, entirely depend, is the difference in specific gravity between earthy and metallic matter, the one being generally double that of the other.
The dressing-floors of amine are always arranged as near the mouths of the principal shafts and levels as possible, the ore being conveyed to them by a small railway ; and they arc always provided with en ade quate supply of water by an artificial channel or "lost" The floor itself is paved, and there are en one or two sides range‘of sheds for the persona employed to work in, and buildings containing the apparatus used in the operation, which commences by picking the ore, which is brought from the mine in large irregular lumps, as blasted or broken from the vein. These lumps, of which more than half is often merely spar and veinstone, are broken into smaller pieces with hammers, an operation commonly performed by boys and young women, when a good deal of the 'parry matter is picked out by hand and at once rejected : the residue is pieces of ore, more or lees mixed with veinstone, and often nearly free from it. From rich veins a large proportion of the ore is obtained in a very Imre state, and in this cam it is only necessary to break down the large irregular masses into small fragments of a pretty equal sire, in order to render it marketable and fit for the furnace, there being no earthy or sparry matter to eeparate from it Thia rich ore In immediately therefore arranged in circular heaps upon the dressing-floors, containing a certain number of tons each, commonly fifteen or twenty, and in this state it much the appearance of fine metallic gravel. The poorer class of ores, after being broken by hamniers, and partially separated from the nuarix by picking, have still a great variety of manipulations to undergo, which depend on the nature of the metal and the quality of the ore itself. Of these processes it will be sufficient here to notice three, " crumbing " or " grinding," " jigging," and " stamping," each of which is performed by an appropriate machine. The crushing-mill, or grinder, consists of one or more pairs of iron rollers, placed within a very short distance apart, an I kept in motion either by the direct action of a water-wheel or by cog-wheels attached to it. Immediately above the rollers is a hopper, into which the lumps of poorer ore are thrown, when, falling through between the rollers, they are completely crushed into small fragments. In some crushing-mills there arc two or three pairs of rollers, those below being placed very near together, so es to reduce the stuff falling from above still finer, and by an inge nious application of sieves, kept in motion by the machine, the stuff can be sorted into two or three different sizes. Although by passing through the crushing-mill tho ore, with its accompanying veinstone, has been reduced to very small fragments, the two substances are still as completely intermixed as ever ; but iu the next process, by the jigging-machine, or " brake-sieve," they are, to a considerable extent, separated. - This machine consists of a wooden frame, open at tho top, and provided with a strong screen, or iron grating, at the bottom : It hangs over a cistern of water, being suspended to a long lever, the motion of which alternately plunges it into the water and raises it out, tvith a peculiar jerk each time. The ores being placed in the sieve, and subjected for a short time to this operation, the heavy metallic pieces settle at tho bottom, while the lighter fragments of spar and veinstone are thrown to the top, and every now and then very dexter ously skimmed off with a piece of board by a man who stands by. In the operation of jigging, a very important separation is thus effected, as three products are obtained by it—the .small rich particles of ore, which pass through the sieve into the cistern below, and aro removed occasionally, as zany be necessary ; the Larger rich fragments, which occupy the bottom of the cistern ; and the poor earthy matter, which forms a layer at the top. This last product, although poor, still con tains too much metal to be lost : it consists of small fragments of rock or veinstone, many of which have small particles of ore either attached to them or intermixed with them, and, to any eye but that of the miner a, would appear quite worthless, no less from the small quantity of the ore than the manifest difficulty of separating it from such a mass of stony matter. To extract the ore from this refuse matter, several processes are used, which are chiefly grinding between rollers placed very close to each other, stamping to a fine powder by the stamping mill, and finally, washing upon an inclined plane. In this operation the fine metallic mud or " slime," being carefully spread over the inclined plane at the upper end, a gentle stream of water is allowed to flow over it, which washes the light earthy particles towards the bottom, leaving tho heavier metallic ones in a very pure state towards tho top. As in this process, and indeed all other operations of dressing in which a stream of water is employed, many of the smallest and most minute particles of the ore are carried away by it, the waste of which, in an extensive mine, would be considerable, it is arranged that all such water shall pass into successive reservoirs, termed " slime-pits," in which the metallic particles fall to the bottom, and are from time to time collected and subjected to such treatment as to obtain them in a tolerably pure state, as a good deal of earthy slime is always deposited with them.