Pig. 16 is a section of the drill hole. The charge of powder is shown at C, the air space at B, the tamping at A.
Let us assume that we have a blue-stone quarry in which we may illustrate the simplest application of the Knox system. The sheet of stone which we wish to shear from place has a bed running longitudinally at a depth of, say, 10 ft. One face is front, and a natural seam divides the bed at each end at the walls of the quarry. We now have a block of stone, say 50 ft. long, with all of its faces free. except one—that opposite and corresponding with the bench. One or more Knox holes are put in of such depth r.nd distance apart, and from the bench, as may be regulated by the thickness, strength, and character of the rock. No man is so good a judge of this as the quarry foreman, who has used and studied the effect of the Knox system in his quarry. Weal care should be taken to drill the holes round and in a straight line. In sandstone of medium hard ness these holes may be situated 10 ft., 12 ft., or 15 ft. apart. If the bed is a tight one—that is. where it is not entirely free at the bottom—the hole should be run entirely through the sheet and to the bed. but with an open free bed holes of less depth will suffice.
The reamer should now be used and driven by hand. Several devices have been applied to rock drills for reaming the hole by machinery while drilling—that is, efforts have been made to combine the drill and the reamer. Such efforts have met with only partial success. The perfect alignment of the reamer is so important that where power is used this point is apt to be neglected. It is also a well-known fact that the process of reaming by band is not a difficult or a slow one. The drilling of the hole requires the greatest amount of work. After this has been done it is a simple matter to cut the V-shaped grooves. The reamer should be applied at the center of the hole—that is, the grooves should be cut on the axis or full diameter of the hole. The gauge of the reamer should be at least I. times the diameter of the hole. While driving the reamer great care should be taken that it does not twist, as the break may thereby be deflected. Beam until von can do so no further—that is, ream to the full depth of the hole. The hole is now ready for charging. First insert the powder, which should be a low grade of explo sive. Do not use dynamite. Black powder, Judson powder, or other explo sives which act slowly, are preferable. No definite rule can be laid down as to the amount of powder to be used, but it is well to bear in mind that as little powder should he used as possible. The powder must, of course, be provided with a fuse or, preferably. a fulminating cap. It is well to insert the cap about the middle of the cartridge.
After the charge the usual thing to do is to insert tamping, but in the Knox hole the tamping should not be put directly upon the powder, but an air space should be left, as shown at B, Fig. 16. The best way to tamp, leaving an air space, is, first to insert a wad, which may be of oakum, hay, grass, paper, or other similar material. The tamping should be placed from 6 to 12 in.
below the mouth of the hole. In some kinds of stone a less distance will suffice, and it is well to bear in mind that as much air space as practicable should intervene between the explosive and the tamping. Care should be observed in tamping, not to destroy the wires which connect with the explo sive, but the tamping should be made secure so that it will not blow out. The hole is now ready for blasting. If several holes are used on a line they should be connected in series and blasted simultaneously. The effect of the blast is to make a vertical seam connecting the holes, and the entire mass of rock is sheared several inches or more.
The Philosophy of the Knox blast is simple, though a matter of some dispute. :Ur. Knox gives the following explanation : "The two surfaces, a and b, Fig. 14, being of equal area, must receive an equal amount of the force generated by the conversion of the explosive into gas. These surfaces being smooth, and presenting no angle between the points. 21 and B, furnish no starting point for but at these points the lines meet at a sharp angle, including between them a wedge-shaped space. The gas acting equally in all directions from the center is forced into the two oppo site wedge-shaped spaces, and the impact being instantaneous, the effect is precisely similar to that of two solid wedges driven from the center by a force equally prompt and energetic. All rocks possess the property of elasticity in a greater or less degree, and this principle being excited to the point of rupture at the points .4 and B, the gas enters the crack and the i rock is split in a straight line, simply because under the circumstances it cannot split any other way." It is doubtless true that. notwithstanding the greater area of pressure in a Knox hole, the break would not invariably follow the prescribed line but for the V-shaped groove, which virtually starts it. A bolt, when strained, will break in the thread, whether this be the smallest section or not, because the thread is a starting point for the break. A rod of glass is broken with a slight jar, provided a groove has been filed in its surface. Numerous other instances might be cited to prove the value of the groove. Elasticity in rock is a pronounced feature, which varies to a greater or less extent, but it is always more or less present. A sandstone has recently been found which possesses the property of elasticity to such an ex tent that it may be bent like a piece of steel. When a blast is made in the Knox hole the stone is under high tension, and, being elastic, it will naturally pull apart on such lines of weakness as grooves, especially when they are made, as is usually the case in the Knox sys tem, in a direction at right angles with t'he lines of least resistance.