Blasting

Blasting, an operation of much practical importance in mining and civil engineering, for the removal of obstruction by explosives such as gunpowder, guncotton or other special preparations of nitro-glycerine. Thus in tunnelling through hard rocky material, holes of 1 to 1-1/2 inch diameter are bored by hand or machine to the depth of a few feet, a cartridge of the explosive is pushed to the farther extremity of each hole, which is then tamped or blocked up with sand or clay sufficiently firm to prevent the explosion simply acting in the directly outward direction. A fuse leads from without to the embedded cartridge, and takes a known time to carry ignition to it; during this time the workmen retire and wait for the explosion in a sheltered spot, It is often expedient to fire a number of such charges at the same time, in which case electricity lends itself readily for the simultaneous heating of the fuses. Thus in the mine-system of blasting, where it is necessary to remove very large masses such as reefs or islets that obstruct ship-way, the rock is honeycombed with small tunnels, charges of the explosive are placed all over the area to be acted upon, and the fuses are connected by wires which lead to a safe distance, from which the firing may be effected by the passage of the electric current round the circuit. The best instance of this kind is that of the blasting away of a reef at Hell Gate, Long Island Sound, New York, where a charge of 120 tons of rapid explosive, distributed through about 20 miles of drill-holes, was fired in a single operation.