Formerly, indications of mines were taken from very unimportant circum stances ; from thermal waters, the heat of which was gratuitously referred to the decomposition of pyrites ; from mineral waters, whose course is however often from a far distant source • from vapors incumbent over particular mountain groups ; from the snows melting faster in one mineral district than another ; from the different species of forest trees, and from the greater or less vigor of vegetation, &c. In general, all such in dications are equally fallacious with the divining rod, and the compass made of a lump of pyrites suspended by a thread.
Gunpowder is the most valuable agent of excavation ; possessing a power which has no limit, and which can act every where, even under water. Its introduc tion, in 1615, caused a great revolution in the mining art. Gun-cotton is equally valuable.
It is employed in mines in different manners, and in different quantities, ac cording to circumstances. In all cases, however, the process resolves itself into boring a hole, and enclosing a cartridge in it, which is afterwards made to ex plods. The ho_e is always cylindrical, and is usually made by means of the borer, a stern of iron, terminated by a blunt-edged chisel. It sometimes ends in a cross, formed by two chisels set transversely. The workman holds the stem in his left band, and strikes it with an iron mallet held in his right. He is careful to turn the punch a very little round at every stroke. Several punches i are employed in succession, to bore one hole ; the first shorter, the latter ones longer, and somewhat thinner. The rub bish is withdrawn as it accumulates, at the bottom of the hole, by means of a picker, which is a small spoon or disk of iron fixed at the end of a slender iron rod. When holes of a large size are to be made, several men must be employed; one to hold the punch, and one or more to wield the iron mallet. The perfora tions are seldom less than an inch in diameter, and 18 inches deep ; but they are sometimes two inches wide, with a depth of 50 inches.
The gunpowder, when used, is most commonly put up in paper cartridges. Into the side of the cartridge, a small cy lindrical spindle or piercer is pushed. In this state the cartridge is forced down to the bottom of the hole, which is then stuffed, by means of the tamping bar, with bits of dry clay, or friable stones coarsely pounded. The piercer is now
withdrawn, which leaves in its place a channel through which fire may be con veyed to the charge. This is executed either by pouring gunpowder into that passage, or by inserting into it reeds, straw stems, quills, or tubes of paper filled with gunpowder. This is exploded by a long match, which the workmen kin dle, and then retire to a place of safety.
As the piercer must not only be slender but stiff, so as to be easily withdrawn when the hole is stamped, iron spindles are usually employed, though they oc casionally give rise to sparks, and conse quently to dangerous accidents, by their friction against the sides of the hole. Brass piercers have been sometimes tried; but they twist and break too readily.
Each hole bored in a mine, should be so placed in reference to the schistose structure of the rock, and to its natural fissures, as to attack and blow up the least resisting masses. Sometimes the rock is prepared beforehand for splitting in a certain direction, by means of a narrow channel excavated with the small hammer.
The quantity of gunpowder should be proportional to the depth of the hole, and the resistance of the rock, and merely sufficient to split it. Any thing additional would serve no other purpose than to throw the fragments about the mine, without increasing the useful effect. Into the holes of about an inch and a quarter diameter, and 18 inches deep, only two two ounces of gunpowder are put.
It appears that the effect of the gun powder may be augmented by leaving an empty space above, in the middle of or beneath the cartridge. In the mines of Silesia, the consumption of gunpowder has been eventually reduced, without diminishing the product of the blasts, by mixing sawdust with it, in certain pro= portions. The bole has also been filled up with sand in some cases, according to Mr. Jessop's plan, instead of being packed with stones, which has removed the dan ger of the tamping operation. The ex periments made in tins way have given results very advantageous in quarry blasts with great charges of gunpowder ; but less favorable in the small charges employed in mines.
Water does not oppose an insurmount able obstacle to the employment of gun powder ; but when the hole cannot be made dry, a cartridge bag, impermeable to water, must be had recourse to, pro vided with a tube also impermeable, in which the piercer is placed.