from any
convenient electric circuit. The ends of the fuze wires in the
detonating cap are connected by a fine platinum filament (fig. 11),
embedded in a guncotton priming on top of the fulminating mixture, and
explosion results from the heat generated by the resistance opposed to
the passage of the current through the filament. Blasting machines are
made in several sizes, the smaller ones being capable of firing
simultaneously from ten to twenty holes. The fuzes must obviously be
of uniform electrical resistance, to ensure that all the connected
charges will explode simultaneously. The premature explosion of any
one of the fuzes would break the circuit.

In the actual operations of blasting, definite rules for the
proportioning of the charges are rarely observed, and although the
blasts made by a skilful miner seldom fail to do their work, it is a
common fault that too much, rather than too little, explosive is used.
The high explosives are specially liable to be wasted, probably
through lack of appreciation of their power as compared with that of
black powder. Among the indications of excessive charges are the
production of much finely broken rock or of crushed and splintered
rock around the bottom of the hole, and excessive displacement or
projection of the rock broken by the blast. In beginning any new piece
of work, such waste may be avoided or reduced by making trial shots
with different charges and depths of hole, and noting the results;
also by letting contracts under which the workmen pay for the
explosive. In surface rock excavation the location and determination
of the depth of the holes and the quantity of explosive used, are
occasionally put in charge of one or more skilled men, who direct the
work and are responsible for the results obtained.

Blasting in surface excavations and quarries is sometimes done on an
immense scale--called "mammoth blasting." Shafts are sunk, or tunnels
driven, in the mass of rock to be blasted, and, connected with them, a
number of chambers are excavated to receive the charges of explosive.
The preparation for such blasts may occupy months, and many tons of
gunpowder or dynamite are at times exploded simultaneously, breaking
or dislodging thousands, or even hundreds of thousands, of tons of
rock. This method is adopted for getting stone cheaply, as for
building macadamized roads, dams and breakwaters, obta