BLASTING has long been practised as the most efficient mode of removing or detaching heavy masses of rock in mining operations, and, by reason of recent improvements, has become one of the most important resources of the civil engineer. The old method of blasting rocks consisted in drilling or boring a hole to a considerable depth with suitable instruments, depositing a charge of gunpowder at the lower or farther end of the hole, and then filling up, or 'tamping,' the remainder of the hole with clay, or some other soft mineral substance, well rammed, to make it as tight as possible. A wire laid in the hole during this operation was subsequently withdrawn, and a train of gunpowder inserted in its place; and this train, and consequently the blast it self, was fired by a slow match (often consist ing simply of brown paper smeared with grease), intended to burn long enough to allow the person who fired it to reach a place of safety. Many accidents have arisen from the uncertainty of this process, the risk of which has however been lessened by the sub stitution of copper for iron in the `needle' by which a passage for the train is formed. Beck ford's safety fuse,' too, is a great improve.: ment ; it consists of a small train of powder inserted in a water-proof cord, which burns at so steady and uniform a rate, that by cutting it to a suitable length any desired interval may be secured between the lighting and the plosion.
The great improvement of modern times however consists in the employment of a gal vanic current to ignite the powder,—an ar. rangement which renders premature explosion next to impossible. A galvanic current, so long as it passes along an uninterrupted wire, is perfectly harmless ; but if its course be in terrupted by breaking the continuity of the wire, intense heat, sufficient to ignite powder, is produced. In addition to the superior safety and certainty of this mode of firing, it has the advantage of being applicable under water as well as on land, and, by its perfectly instantaneous action, of enabling the engineer to fire as many blagts as he may desire at one operation, so as to accomplish, by their joint action, effects otherwise unattainable.
Colonel (now General) Pasley first employed galvanism in submarine blasting in 1839, in his successful operations on the wreck of the Royal George at Spithead. Shortly afterwards
galvanic blasting, both on land and under water, was practised both in America and in Scotland ; but it was in January 1843 that Mr. William Cubitt commenced, on the works of the South-Eastern Railway, the stupendous operations which established its capabilities on a a ale never before attempted. He began by thrSwing down, by three simultaneous blasts, consuming together about 18,000 lbs. or more than eight tons of gunpowder, a bulky pro montory called the Round Down Cliff,between the Abbot's Cliff and Shakspere tunnels, near Dover. By this operation, which was attended with very little noise, a cliff nearly 400 feet high was thrown down, and no less than 400,000 cubic yards of chalk were distributed over the beach, covering an area of 18 acres to an average depth of 14 feet; and it was computed that 70001. and six months' time were saved to the company. Several smaller operations followed this great experiment, in one of which twenty-eight blasts were arranged to explode simultaneously, although from some derangement of the wires a few failed. The same agency has since been extensively employed elsewhere, especially at Seaford in 1850.
The latest invention relating to blasting which requires notice is the substitution of gun-cotton for gunpowder, which bids fair still further to diminish danger and to increase the certainty of the operation. From experiments tried early in 1817, at the Standege tunnel, on the Huddersfield and Manchester Railway, and on the works of the Birmingham, Wol verhampton, and Stour Valley Railway at Birmingham, it appears that the effects of the gun cotton is, weight for weight, from three to five times as great as that of gunpowder ; so that a much smaller bore will suffice for the blast, and it may be, with equal labour, carried down much deeper into the rock. A still more important difference is, that the gun-cotton produces no smoke, so that the workmen may resume their labours immedi ately after the explosion, instead of having to wait several hours, and sometimes awhole day, as when gunpowder is used. The cotton also produces much less noise and vibration, and is considered far less liable to accident in removal from place to place than gunpowder.