y. 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."

Another theory which is much the same in substance is then given, and
after some general discussion of the theory of the action of the
forces under the several systems, the paper continues:

The new form of hole is, therefore, almost identical in principle with
the old Portland canister, except that it has the greater advantage of
the V-shaped groove in the rock, which serves as a starting point for
the break. It is also more economical than the Portland canister, in
that it requires less drilling and the waste of stone is less. It is,
therefore, not only more economical than any other system of blasting,
but it is more certain, and in this respect it is vastly superior to
any other blasting system, because stone is valuable, and anything
which adds to the certainty of the break also adds to the profit of
the quarryman.

It is doubtless true that, notwithstanding the greater area of
pressure in the new form of 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 the
starting point for the break. A rod of gla