Mine is a term applied to works carried on underground, for obtaining minerals generally, but chiefly for metallic ores. The internal parts of the earth, as far as they have been investigated, consist of various strata or beds of substances, extremely different in their appearances, specific gravities, and chemical qualities, from one another. Neither are these strata similar to one another in different countries; and in one district, the strata varies considerably in its nature, at very short distances apart. Rocks of most kinds are traversed in every direction by cracks or fissures, having, in many instances, the appearance of those formed in clay and mud while gradually becoming dry in hot weather. These fissures are in general filled with substances formed of materials differing from the rocks in which they are situated. When they contain minerals partly composed of any kind of metal, they are called metallic veins, lodes, or courses. Metallic veins are only found in what are called the primitive rocks, as granite and slate; and, in general, their course is from east to west.
A vein rarely consists of metal in a pure and malleable state, but is almost always found in chemical combination with other substances; in this state it is called an ore, the metal of which is separated by the process called smelting, which is, in fact, a melting-out of the metal from its combinations, usually effected by the addition of such foreign substances as will, by their chemical affinities, assist in the separation of the metal. The thickness, extent, and direction of a vein of metal, depends on many circumstances; in general, its course downwards is in a slanting direction, more or Jess inclined; if it continues in a straight line, and of a uniform thickness, it is called a rake; if it occasionally swells out in places, and again contracts, it is termed a pipe-vein, and the wider parts of the vein are called floors; sometimes the vein divides itself into branches, and then it is said to take horse; in other cases a cross grain will interfere with it, and heave or lift it, as it were, from 10 to 20 feet out of its course. At times it will be reduced to a mere thread, and at last become completely obliterated, appearing again at a distance. In many of these cases the difficulty of tracing these precious deposits through their rocky labyrinths must be evident.
In all probability, however, the metals were at first procured from detached fragments of the ores, such as had been separated from the upper parts of the veins in which they were originally deposited; and in this manner is gold yet procured, by washing the sands of certain rivers. The pursuit of these scattered pieces of ore would naturally conduct the persons thus employed to the beds from which they had been detached, and in turning over the soil to procure the loose fragments, the backs of the veins would be laid open and discovered.
The tin of Cornwall was the first metal sought after in Britain of which we have an historical account; but the traces of the most ancient tin-works exhibit no symptoms of their having been pursued but in situations where the soil with which it was mixed could be easily removed, or where the ore could be laid bare by conducting over it streams of water to carry off the lighter parts of the soil. Lead is often found near the surface of the earth, and as the ores generally exhibit a metallic appearance, that metal was probably an early object of pursuit; but it was not until machines were invented to pump away the waters, and until gunpowder had furnished the means of splitting the hardest rocks, that man was enabled to penetrate strata of every description that opposed his progress. These inventions, therefore, form most important epochs in the history of mining. The hammer and wedges were probably the first instruments employed for splitting rocks, and the pick followed, which is used both as a hammer and a wedge. Previously to the use of iron, wedges of dry wood were made use of by driving them into clefts of the rock, and then wetting them, so as to cause them to swell and force the parts asunder.
The means employed for raising up the minerals to the surface were at first extremely rude. The windlass and bucket may be reckoned an improvement which took place in a later stage of mining. This simple mechanism had its origin in Germany; and before it was introduced into this country, the mode adopted here was by making successive stages, upon each of which men were placed, who raised the excavated matter from one to the other until it reached the top, in the same manner as is now commonly practised in digging out the foundations for houses, or for making deep drains. In South America the ores are for the most part carried up by the Indians; and where the situation admits of sloping roads, on the backs of mules. To Germany may also be traced the introduction of hydraulic machines for raising the water constantly collecting in the mines. Pumps were adapted to the shafts, and their constant action secured by giving motion to their pistons by wheels turned by descending streams of water. To England, however, belongs the merit of having greatly improved the pump-work and the water-engines to their present effective condition; and by the subsequent application of the steam engine to this purpose, the mining processes of our countrymen have so far surpassed those of other countries, as to render their adoption indispensable in most situations.
Although copper is now the greatest metallic product of the county of Cornwall, it is comparatively, to the other metals, of modern discovery, not having been worked longer than a century. The reason assigned for its having so long remained concealed, is the assumed fact, that copper generally occurs at a much greater depth than tin; and that, consequently, the ancients, for want of proper machinery to drain off the water, were compelled to relinquish the metallic vein before they reached the copper. It is stated by Pryce in his Mineralogia Cornubiensis, as a general rule, that tin seldom continued rich and worth working lower than 50 fathoms; but of late years the richest tin mines of Cornwall have been much deeper. Trevenen mine was 150; Hewas Downs, 140; Poldice, 120; and Herel Vor is now upwards of 130 fathoms in depth. Upon the first discovery of copper ore, the miner, to whom its nature was entirely unknown, gave it the name of poder; and it will be hardly credited in these times, when it is stated that he regarded it not only as useless, but upon its appearance was actually induced to abandon the mine: the common expression upon such an occasion was, "that the ore came in and spoilt the tin." About the year 1735, Mr. Coster, a mineralogist of Bristol, observed this said poder among the heaps of rubbish; and seeing that the miners were wholly unacquainted with its value, he formed the design of converting it to his own advantage, and accordingly entered into a contract to purchase as much of it as could be supplied.