For removing vast quantities of rock or blowing up ledges, the best method is mine blasting. For this purpose shafts are sunk dither vertically, or horizontally, or both, into the ledge to be removed; enormous quantities of powder, dynamite or other explosives are placed at the bottom or end of the shafts, which are then closed up by rocks, earth, etc., and the charge is fired either by fuse or by electricity, most generally the latter.
The first blasting on record was in 1627, in a mine in Schemnitz, Hungary, by Caspar Weindl, a Tyrolese. miner. From that locality the practice spread slowly, reaching England in 1670, and being introduced in the Cornwall mines in 1689.
One of the greatest mine blasting operations ever attempted was the removal of Flood Rock in that section of the East River, New York, known as Hell Gate. An entrance shaft was sunk on the Long Island shore, from which the reef projected. From this shaft nearly 20 tunnels were bored in all directions, and con nected by lateral galleries. The rock covered about nine acres and the tunnels were made large enough for the broken roof to fall into them and leave 26 feet of water in the channel. The roof left above the tunnels varied from 10 to 18 feet in thickness. The charge consisted of 240,399 pounds of rackrock powder and 44,331 pounds of dynamite distributed in over 46,000 holes. The explosion was measurably success ful, the dredging necessary afterward being readily accomplished. Other great blasts on record are: the blasting off of the face of the mountain of marble at Colonnata, in the Car rara district of Italy, where eight tons of high explosives distributed in tunnels dislodged 600, 000 cubic yards of marble; a blast in a hill side at Piedra, Cal., where 114,000 pounds of Judson powder and 11,400 pounds of Hercules 60 per cent dynamite threw down 350,000 cubic yards of rock; a blast at Tennino, Wash., to get rock for a government jetty, where 43,100 pounds of blasting powder and 1,200 pounds of 60 per cent dynamite broke out 220,000 cubic yards; and a blast at Tompkins Cove, N. Y.,
where 23,575 pounds of 60 per cent gelatine dynamite and 26,550 pounds of granulated nitro glycerine powder broke out 210,000 cubic yards of bluestone.
Surface blasting is generally used to remove reefs and obstructions to navigation, high ex plosives such as dynamite, gun-cotton or nitro glycerine being the only effective agencies in an unconfined space, as the detonation is so sudden that the shock is instantly trans mitted to the rock with which it is in contact. Numerous important improvements have been made in blasting by the substitution of rock boring machines for hand labor. Of such ma chines, in which the jumper or drill is repeatedly driven against the rock by compressed air or steam, being also made to rotate slightly at each blow, there are many varieties.
In recent years blasting has been used scien tifically in the improvement of farming land. Stumps are blown out, boulders broken up, hardpan shattered, subsoils loosened, drainage crevices opened in low and wet spots, ditches cut, gullies filled, etc. Holes are blasted out for setting young fruit trees, loosening the sub soil advantageously for several feet, and old trees are restored to vigorous growth and bear ing by the judicious firing of blasts in the sub soil between old orchard rows. Worn out land has been revitalized by blasts set 15 feet apart and three feet deep, at a cost of $18 per acre, the good results remaining for several years. See also EXPLOSIVES.
Cosgrove, J. J., 'Rock Ex cavating and Blasting' (Philadelphia 1913) Daw, A. W. and Z. ‘V., 'Blasting of Rock' (London 1898) ; Du Pont de Nemours, E I., Powder Company, 'Blasting Supplies' ; (Farmers' Handbook); and 'Safety Blasting Explosives' (Wilmington, Del., 1915); Gutt mann, O., 'Blasting) (London 1906)i Prelini, C., and Submarine blast ing (New (New York York 1912) ; Weston, E. M., (Rock Drills) 1910).