5. BORING OF ROCKS, for the purpose of split. ting them by means of gunpowder. We have al ready treated this subject under BLASTING, and shall only add here the mode of boring for this pur pose practised in the mines of Germany.
A boring bar of steel is applied to the stone by its lower end, whilst its upper extremity is struck with a hammer of two pounds in weight. The form of the lower end of the boring-bar is various; some were formed like a swallow's tail, ending in two points ; this form is no longer in use. Another kind has the end formed by the intersection of two wedge shaped edges, with a point at each corner, and one in the middle. A third kind has the end composed of four pyramidal points, with cavities between them. A fourth kind, and which is that most fre quently used, has the end in form of a wedge. See Woman, Bergwerks Lexicon. Stockholm, 1789, Tab. IL Three sizes of boring bars are employed to make one hole ; the first is the shortest and thickest, the second is longer and less in diameter, the third is the longest, and the least in diameter. When a hole is to be made, a small opening is first formed with a pick in the place where the boring-iron is to be applied ; and all pieces of the rock are removed that might impede the action of the powder. Then the workman uses the first boring-iron, which he drives with blows of the hammer till this can reach no farther; he then employs the second and third boring-bars in like manner ; after each stroke of the hammer, the boring-bar is turned round a portion of the circumference. The stone, pulveriz ed by the action of the boring-bar, as it hinders the progress of the operation, must be removed from time to time by means of an iron rod, terminated at right angles by a small round plate. From the dif
ferent diameter of the boring-bars it follows that the end of the hole is of a smaller diameter than the beginning. The depth to which the hole is bored is proportioned to the nature of the rock. It varies from 10 to 15 and 20 inches. When the rock is solid a great way round, a deep hole is not used, because the resistance at a considerable depth, in such a situation, is too great ; so that the explosion does not split the rock round the powder chamber, but acts upwards against the ramming, where it meets with less resistance. But if the rock be laid bare on one side, a deep hole is advantageous. Water is poured into the hole during the operation, to facili tate the action of the boring-iron. When the hole is perpendicularly downwards, it is kept full of water; when the hole is driven from below upwards, no water can be used. The water must be taken out, and the hole dried, before the cartridge is intro duced. The most frequent case is, that one man performs the work, holding the boring-iron in his left hand, and striking on it with the two pound hammer in his right. Sometimes two men are set to do the work, one holding the boring-iron, whilst the second strikes it with a hammer of 4 or 5 pounds; this is done where it is required to make the hole SO or 36 inches deep. When a still deeper hole is want ed, two men strike alternately with heavier ham mers.