BLASTING. Before gunpowder was invented, the separation of masses of stone from their native rock could only be effected by means of the hammer and wedge, or by the still slower method of fire and water. In soft and stratified rock, wedges are still used for quarrying stones for building purposes, but in hard rock, or where regularity of fracture is no object, gunpowder is universally employed. There are two kinds of B. —first, the small-shot system; and second, that of large blasts or " mines." The small-shot system consists of boring holes into the rock, of from 1 to 6 in. in diameter, and of various depths, according to circumstances. In hard rock, this is done by a steel pointed drill, struck by a hammer, and turned partly round after each blow, to make the hole cylindrical. The addition of a little water serves to preserve the temper of the boring tool, and makes the rock more easy to cut. In soft rock, when ever the hole is to be vertical, a "jumper" is used; this is a weighted drill, which acts merely by its own weight. when let fall from about a foot in height. The powdered stone is removed at intervals by a "scraper." The rate of progress varies, of course, with the hardness of the rock. At Holyhead, the average work done by three men in bard quartz rock, with 1]4- in. drills, is 14 in. in depth per hour; one man holding the drill, and two striking. After the holc ,is bored, It is cleaned out, and the powder poured dawn. A wad of dry turf or hay is put over the charge, and the rest of the hole " tamped," or tilled with broken stone, clay, or sand. The shot is fired by a length of Bickford's patent fuse. When it is desirable to prevent the stones from flying about, when the shot is fired, a shield of boiler-plate, or of brushwood weighted, may be laid over the hole.
Small shots may be fired, even under water, by inclosing the charge in a tin case, with a tube of powder reaching to the surface; or in a canvas bag, well tarred, tied at the neck round a length of Bickford's fuse, which burns under water. The charge is inserted in the drill-hole; and the weight of the superineumbent water acts as tamping.
In removing the wall between the old and new Shadwell basins of the London docks, shots were fired under water within a few yards of vessels lying in the basin, by using moderate charges, and by keeping a raft of timber floating over the hole, as a shield to prevent anything flying upwards.
The voltaic battery has been used for firing shots, chiefly under water, since 1839, in which year it was employed at the wreck of the Royal George and at' the Skerryvore light-house.
When a large mass of rock has to be removed at once, or where a steady supply has to be daily furnished of irregularly broken stone, for breakwater or other purposes, recourse must be had to large blasts, or " mines." The greatest isolated example of this kind of blasting was the overthrow, in 1843, of the Rounddown cliff at Dover. by 18,500 lbs. of powder, in three separate charges, fired simultaneously by voltaic electricity. But
by far the grandest system of B. by mines is to be seen at the quarries for supplying stone to the breakwater at Holyhead, where small shots having been found inadequate, large mines were introduced in 1830. These large blasts are of two kinds—" shafts" sunk from the top of the rock; and "headings," or galleries driven in from the face.
The shaft-holes are 6 ft. long by 4 ft. wide, of various depths, according to the height of the rock, but seldom much exceeding GO ft. The deal-box, with the charge of pow der, is placed in a chamber cut at one side of the shaft, so that the tamping may not be in the direct upward line of fire. The tamping consists of the stone and debris which have come out of the shaft; and the wires from the battery are protected from injury by being laid in a groove cut in a batten placed up one angle of the shaft.
It is evident that the same point in the rock may be reached as well by a heading or gallery driven in from the face of the rock, as by a shaft from the top, and often by a shorter route. Headings are made 5 ft. high by 3 ft. 6 in. wide, and are driven, if pos sible, alon,ng a natural joint in the rock. The direction of the gallery is changed and sunk at parts, to prevent the tamping from being blown out. Four men can, on the average, drive 5 ft. run of heading per week; but cannot sink above 3 or 4 ft. of shaft, which has a greater sectional area, and is more inconvenient to work in.
The charge of powder may be divided and placed in two or more separate chambers; and it is better thus to spread a heavy charge over a length of face, than to have it in one spot, at a greater distance from the face than about 30 feet.
The charges for these mines vary from GOO lbs. to 13,000, and even more, pounds of powder; and the produce is from 2 to 6 tons of stone to the pound of powder, according to the density of the rock and the position of the mine.
Besides the quarrying of stone, B. is used for military objects, or where total destruc tion is aimed at. and an excess of powder is little or no objection.
Of late years great improvements have been effected in the production and applica tion of explosive agents other than gunpowder, which latter, until lately, may be said to have been exclusively used for the purpose of blasting. Nitro-glycerme (q.v.) and gun cotton (q.v.) were discovered within two years of each other; but while gun cotton eras immediately applied to industrial purposes, nitro-glycerine was destined to remain a chemical curiosity for about 16 years.
Dynamite is a preparation of nitro-glycerine and porous earth, in the form of a pasty mass, which, without impairing its explosive properties, has the effect of rendering it perfectly safe to handle.
One of the most celebrated applications of boring and blasting to modern engineer ing was the driving of the Mont Cenis tunnel. See TUNNEL.