THE HELL-GATE EXPLOSION.
The series of operations which resulted in the blowing up of the great rocky reef at Hallett's Point on Sunday, September 24th, must be regarded as the most brilliant piece of scientific engineering that has yet been accomplished. General Newton formed his plans, and entered upon the work in July, 1869. For over seven years he has been preparing for a grand experiment to occupy but a few seconds, and so accurately did he calculate, and so complete was his command of the irresistible forces to be called into action, that the experiment proved completely successful, and affords an impressive illustration of the prophetic power that is conferred by a knowledge of the elements and forces of Nature. It was the physicists and chemists who long ago worked quietly and obscurely in their laboratories, with little reference to practical ends, and animated only by the desire to acquaint themselves with the laws of the natural world, that paved the way for the great engineer to do this important service for the interests of New York and the commerce of the world.
The reef at Hallett's Point, which has formed such a dangerous obstruction in the Hell-Gate channel as greatly to hinder navigation through Long Island Sound, was of an irregular crescent shape (as shown in the figure), some
700 feet long, and extending out 300 feet into the channel, with an area of about three acres. The rock is a tough, hornblende gneiss, with veins of pure quartz, and lies in strata of various degrees of inclination. The plan of operations was to build a coffer-dam on the rock near the shore to bar out the water, to sink a shaft to the requisite depth, to honey-comb the whole rocky mass by excavation, and then to blow up the shell by charges of dynamite in the roof and supporting columns, to be fired by the agency of galvanic batteries. The shaft was sunk to a depth of 33 feet below the line of low water, and ten tunnels were then opened to distances varying from 31 to 126 feet. The cubic contents of the rocky mass, above the depth of 26 feet, at mean low water, amounted to 51,000 yards. The tunnels radiating from the shaft varied from 7 to 22 feet in height, and from 9 to 12 feet in width, and, as they advanced, the height rapidly decreased, owing to the downward slope of the surface of the reef. As the main tunnels diverged from each other, subsidiary tunnels were introduced, and a system of transverse galleries was excavated (as shown in the figure), and which left 172 supporting pillars of variable dimensions. The total length of tunnels was 4,857 feet, and the length of galleries 2,568 feet, making the entire length of passage excavated 7,425 feet. The excavations being completed, so that the roof of rock above was reduced to a thickness of from 8 to 16 feet, the preparation for the explosion began by drilling the rock for the charges. The whole number of blast-holes drilled into the roof and piers was 4,427, varying from 7 to 10 feet in depth, and from 2 to 3 inches in diameter. Each one of these holes was charged with three kinds of explosives, all compounds of nitroglycerine, viz., dynamite, rendrock, and vulcan-powder, in separate cartridges or canisters. Fifty thousand pounds of these explosives were buried in the apertures. Ninety-six galvanic batteries, of ten cells each, were employed to ignite the charges. The firing-point was 650
