Page:Popular Science Monthly Volume 29.djvu/859

Rh sufficient to remove all the rock from above. The same project was to be applied also to the Gridiron Reef. This method of excavation was not new, as it had been proposed by General Alexander, of the United States Engineers, for the removal of Blossom Rock, San Francisco Harbor, and afterward carried out by Van Schmidt, a civil engineer; but that reef was a very small one in comparison with those to be removed at Hell-Gate. The general also, in the same report, proposed a plan for a scow to remove the isolated reefs. This scow was constructed in 1870. It consisted of an iron dome thirty feet in diameter, supported centrally in a well, in a scow one hundred and twenty feet long by forty-eight feet beam; this dome could be raised or lowered by derricks on the scow. When resting on the rock, the space inclosed was virtually cut off from all currents, the drills could be worked steadily, and the space was accessible to divers. Above the dome were steam-drills, the drills passing through tubes in the dome. This dome-scow proved a practical success, and was not only used at Hell-Gate but also on other reefs, as at Coenties and Diamond Reefs.

The progress was so satisfactory at Hallet's Point that shafts were sunk at Flood Rock in 1876. General Newton had been at the same time making extensive experiments on all the known explosives, to test their applicability to the breaking up masses of stone, while extensive experiments were also made with batteries, wires, and fuses, with a view to their use in the final blast. The excavation being completed, and the mines charged and wired, the explosion at Hallet's Point took place September 24, 1876.

It was a success. The work at Flood Rock was prosecuted more energetically, but with varying appropriations; essentially in its general features it was like the work at Hallet's Point, but with such modifications as had been developed by experience. In one essential feature both differed from what had been originally designed by General Newton. In his first report he had proposed to make the cavities of the galleries sufficient to receive the superincumbent stone. As he proceeded, he did not consider this necessary; a great deal of stone since the blast at Hallet's Point had been removed by grapples, which was practically found to be cheaper than by mining. In its application to the work at Flood Rock, the spaces were smaller compared with the solid rock than at Hallet's Point; galleries were considered only necessary to give access, so as thoroughly to shatter the rock. Flood Rock was fired on the 10th of October, 1885, and a full description of the work was furnished by General Newton for the February number of this journal.

All the problems which were involved in the several steps leading up to the consummation of that stupendous work were completely and conscientiously studied out; and the accuracy of the studies was fully exemplified in the exact correspondence of results with what was aimed