The hole is now ready for charging.  The powder should be a low explosive, like black or Judson powder or other explosives which act slowly.  No definite rule can be laid down as to the amount of powder to be used, but it should be as small as possible.  Very little powder is required in most rocks.  Hard and fine grained stone requires less powder than soft stone.  Mr. Knox tells of a case which came under his observation, where a block of granite “more than 400 tons weight, split clear in two with 13 oz. of FF powder.”  He compares this with a block of sandstone of less than 100 tons weight “barely started with 21/2 lb. of the same grade of powder, and requiring a second shot to remove it.”

It is obvious that enough powder must be inserted in the hole to produce a force sufficient to move the entire mass of rock on its bed.  In some kinds of stone, notably sandstone, the material is so soft that it will break when acted upon by the force necessary to shear the block.  In cases of this kind a number of holes should be drilled and fired simultaneously by the electric battery.  In such work it is usual to put in the holes only 4 or 5 ft. apart.  The powder must, of course, be provided with a fuse or preferably a fulminating cap.  It is well to insert the cap at or near the bottom of the cartridge, as shown in Figs. 8 and 9.

After the charge the usual thing to do is to insert tamping.  In the improved form of hole the tamping should not he put directly upon the powder, but an air space should be left, as shown at B, Fig. 8.  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 as much air space as practicable should intervene between the explosive and the tamping.  If several holes are used on a line they should be connected in series and blasted by electricity.  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 this new method of blasting is simple, though a matter of some dispute.  The following explanation has been given.  See Fig. 10.

[Illustration:  Fig. 10]

“The two surfaces, a and b, 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, A and B, they furnish no starting point for a fracture, 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 opposite 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, A and B, the gas enters the crack and the rock is split in a straight line simply because under the circumstances it cannot split in any other way.”