indicated by the dotted lines.
Another method is the "centre cut," in which the holes are drilled in
parallel rows on each side of the centre line of the tunnel, drift or
shaft. Those in the two rows nearest the middle are directed towards
each other, and enclose a prism of rock, which is first blasted put by
heavy charges, after which the rows of side holes will break with
relatively light charges.

_Explosives._--A great variety of explosives are in use for blasting
purposes. Up to 1864, gunpowder was the only available explosive, but
in that year Alfred Nobel first applied nitroglycerin for blasting,
and in 1867 invented dynamite. This name was originally applied to his
mixture of nitroglycerin with kieselguhr, but now includes also other
mechanical mixtures or chemical compounds which develop a high
explosive force as compared with gunpowder. Besides these there are
the so-called flameless or safety explosives, used in collieries where
inflammable gases are given off from the coal.

Gunpowder, or black powder, is seldom used for rock-blasting, except
in quarrying building-stone, where slow explosives of relatively low
power are desirable to avoid shattering the stone, and in such
collieries as do not require the use of safety explosives. Gunpowder
is exploded by deflagration, by means of a fuze, and exerts a
comparatively slow and rending force. The high explosives, on the
other hand, are exploded by detonation, through the agency of a fuze
and fulminating cap, exerting a quick, shattering, rather than a
rending force. Dynamites and flameless explosives are made in a
variety of strengths, and are packed in waterproofed cartridges of
different sizes. The grades of dynamite most commonly employed contain
from 35 to 60% of nitroglycerin; the stronger are used for tough rock
or deep holes, or for holes unfavourably placed in narrow mine
workings, as sometimes in shaft-sinking or tunnelling. When of good
quality high explosives are safer to handle than gunpowder, as they
cannot be ignited by sparks and are not so easily exploded. The
ordinary dynamites used in mining are about four times as powerful as
gunpowder.

Nitroglycerin in its liquid form is now rarely used for blasting,
partly because its full strength is not often necessary but chiefly
because of the difficulty and danger of transporting, handling and
charging it. If employed at all, i