of a steam-grapple. Provision was made for the attachment of curtains, or dams, of chain-netting, rope-netting, or canvas, to the bottom of the dome and to the steam-grappling apparatus, as a shield against currents, but it was not found necessary to use anything of the kind.
Operations with the steam-drilling scow were completed January, 1880, upon Diamond Reef. The rock was covered with a large accumulation of loose material which bad first to be removed with a dredging-machine, after which all of the ledge that was uncovered was attacked. The holes were drilled from seven to thirteen feet deep, four and a half inches in diameter at the top, and three and a half inches at the bottom, and were charged with from 30 to 55 pounds each of nitro-glycerine.
Coenties Reef was worked upon in alternation with Diamond Reef in 1871 and completed in 1875.
In 1872 work was commenced on Frying-Pan, and there is now twenty-two feet at low water.
Work was prosecuted on Pot Rock in Hell Gate from August 5 till December 28, 1872, during which period the scow was much exposed to collisions, of which sixteen took place. In one of them the colliding vessel was drawn under the scow and carried off the dome, which was afterward recovered, considerably damaged, in eighty feet of water. The depth on this rock is now twenty-four feet.
Way's Reef, over which the original depth of water was five feet, having already been cleared by the application of M. Maillefert's process of surface-blasting in 1851 and again in 1869, to 1712 feet, was operated upon from August 4, 1874, to January 20, 1875, and the depth of water was increased to 26 feet at low water. The rock within the 26-feet curve measured 235 feet in length by 115 feet of maximum width. To accomplish the result two hundred and sixty-two holes were drilled to an aggregate depth of 2,130·4 feet, sixty-five drill-blasts and sixteen surface-blasts were made, and 16,79234 pounds of nitro-glycerine and 3812 pounds of dynamite were consumed.
For the removal of Hallet's Point Reef it was determined to employ a process of undermining the rock by tunnels and galleries, from which mines should be exploded to break up the whole mass of the rock at once. Similar processes had already been suggested by General Alexander, United States Engineers, and A. W. von Schmidt, C. E., for the removal of Blossom Rock, in San Francisco Harbor. The reef in question (Fig. 3) was in the shape of a semi-ellipse, extending 720 feet in length along the shore, and to a distance of 300 feet in breadth into the channel; and the cubic contents necessary to be removed, in order to secure a depth of 20 feet at mean low water, amounted to 53,971 cubic yards. The reef was dangerous, not only in itself, but also on account of the eddies to which the tidal currents gave rise on either side of it, according to their direction. Opera-
