EAST RIVER AND HELL GATE IMPROVEMENT. 439 



of a steam-grapple. Provision was made for the attachment of cur- 

 tains, or dams, of chain-netting, rope-netting, or canvas, to the bot- 

 tom 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 ac- 

 cumulation of loose material which had 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 ex- 

 posed 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 17^ 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 16,792f pounds of nitro-glycerine and 

 38^ pounds of dynamite were consumed. 



For the removal of Hallet's Point Reef it was determined to em- 

 ploy 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 Gen- 

 eral 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, ex- 

 tending 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 26 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- 



