The working vessel having been quickly hauled by the mooring-lines to a safe distance by means of capstans worked, whenever practicable, by the steam-engine, the wires are attached to the machine, and at the signal “all ready” the charge is fired.
The working vessel is then hauled back to her position, and as soon as the water becomes sufficiently cleared of the dark muddy matter stirred up by the blast, to enable the diver to see in it, he descends and examines the result.
If the blast has been effective, he signals for the stone chains to be lowered to him; which being done, he proceeds to sling the large pieces of broken rock, one after another, as they are hoisted up and deposited on deck. All the pieces large enough to sling having been thus removed, he signals for the tub and shovel, and upon their being lowered to him, proceeds to shovel into the tub the small fragments, and to have them hoisted up and piled on deck, until the surface of the rock is sufficiently cleared to place the drill for a new blast.
Submarine Rocks.
—The following brief account of the removal of the “Tower” and the “Corwin” Rocks from the Narrows, at the entrance of Boston Harbour, U.S., from the pen of J. G. Foster, is instructive as illustrating the method of procedure in submarine blasting, and as showing the unfitness, for work of that character, of the slow-burning explosives:—
“Tower Rock,” being the smaller of the two, was selected as the one to be first removed. Its horizontal dimensions being only 50 by 26 feet, it was estimated that one large central charge surrounded by five or six others, all in large and deep drill-holes, would be able to rend the rock into pieces.
The working vessel, the sloop “Hamilton,” of 70 tons, was moored over this rock on the 30th of July, 1867, and the new submarine drilling machine, designed for this work, by Mr. Townsend, the contractor, was placed in position and tried.
Several imperfections were found at the first trial, which prevented its efficient working. While these were being remedied, a trial was made of surface blasts, placed in and around the rock in the positions most favourable to their action. These proved to be entirely without effect. No seams or breaks were made by them in the smooth surface of the rock.
As soon as the submarine drilling machine was perfected, it was put in operation, and successfully worked. The central and the surrounding holes were drilled to depths varying from 2 to 8 feet, each hole being 31⁄2 inches in diameter. These were well charged with black blasting powder, and tamped with sand. In some holes, the charges produced no visible effect, the tamping being blown out like the charge from a cannon. In others, a crater was formed, but with a radius only about one-half the line of least resistance. The holes that were intact were then deepened, and new ones drilled; these were charged with Dupont’s sporting powder. The result was much better, but not what was desired. The pressure of the water, from 23 to 33 feet in depth, seemed to diminish largely the ordinary explosive effect of gunpowder upon rock, as seen in blasts in the open air.
Trial was then made of the patent safety blasting powder, manufactured by the Oriental Company of Boston, the proportions of the ingredients having been modified to increase its strength for this especial use. This produced the desired effect. The rock was rent in pieces; and by drilling additional holes and continuing large charges of the powder, the rock was finally reduced to the required depth.