Fig. 21.
When the blasting is in stratified rock, the position of the charge will frequently be determined by the natural divisions and fissures; for if these are not duly taken into consideration, the sinker will have the mortification of finding, after his shot has been fired, that the elastic gases have found an easier vent through one of these flaws, and that consequently no useful effect has been produced. The line of least resistance, in this case, will generally be perpendicular to the beds of the strata, so that the hole for the charge may be driven parallel to the strata and in such a position as not to touch the planes which separate them. This hole should never be driven in the direction of the line of least resistance, and when practicable should be at right-angles to it.
The instruments employed in boring the holes for the shot are iron rods having a wedge-shaped piece of steel welded to their lower ends and brought to an edge so as to cut into the rock. These are worked either by striking them on the head with a hammer, or by jumping them up and down and allowing them to penetrate by their own weight. When used in the former manner they are called borers or drills; in the latter case they are of the form [Fig. 21], and are termed jumpers. Recently power jumpers worked by compressed air, and drills actuated in the same manner have been very successfully employed. Holes may be made by these instruments in almost any direction; but when hand labour only is available, the vertical can be most advantageously worked. Hand-jumpers are usually about 4 feet 8 inches in length, and are used by holding in the direction of the required hole, and producing a series of sharp blows through lifting the tool about a foot high and dropping it with an impulsive movement. The bead divides a jumper into two unequal lengths, of which the shorter is used for commencing a bore-hole, and the longer for finishing it. Often the bit on the long length is made a trifle smaller than the other to remove any chance of its not following into the hole which has been commenced.
Drills and jumpers should be made of the best iron, preferably Swedish, for if the material be of an inferior quality it will split and turn over under the repeated blows of the mall, and thus endanger the hands of the workman who turns it, or give off splinters that may cause serious injury to those engaged in the shaft. Frequently they are made entirely of steel, and this material has much to recommend it for this purpose; the length of drills varies from 18 inches to 4 feet, the different lengths being put in successively as the sinking of the hole progresses. The cutting edge of the drills should be well steeled, and for the first, or 18-inch drill, have generally a breadth of 2 inches; the second, or 28-inch drill, may be 13⁄4 inch on the edge; the third, or 3-foot drill, 11⁄2 inch, and the fourth, or 4-foot drill, 11⁄4 inch.
Fig. 22.
The mode of using the drill in the latter case is as follows; The place for the hole having been marked off with the pick, one man sits down holding the drill in both hands between his legs. Another man then strikes the drill with a mall, the former turning the drill partially round between each blow to prevent the cutting edge from falling twice in the same place.