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.
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