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 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 extent that it may be bent like a thin piece of steel. When a blast
is made in the new form of 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
this system, in a direction at right angles with the lines of least
resistance.
Horizontal holes are frequently put in and artificial beds made by
"lofting." In such cases where the rock has a "rift" parallel with the
bed, one hole about half way through is sufficient for a block about
15 ft.
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