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 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 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 174 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 26feet 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 16,7923 pounds of nitro-glycerine and 38 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 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

tions were begun here in July, 1869, for the construction of a cofferdam between high and low water marks; and in the following October the excavation of a shaft, conforming in shape to that of the dam, and 32 feet in depth, was begun. Thence tunnels radiating through the rock, with transverse galleries, 25 feet apart, to connect them, were excavated till thirty-five tunnels and ten galleries were constructed, having an aggregate length of 7,426 feet. The tunnels were from 17 to 22 feet high and from 9 to 12 feet wide at the shaft, and tapered off in both dimensions as they went out; and the galleries were from 12 feet high by 9 feet wide down to smaller dimensions.

The work of excavation was commenced in the latter part of October, 1869, and terminated in June, 1875. Deducting the time lost by suspension of work due to the exhaustion of current appropriations, the actual period consumed in this work was four years and four months. The appropriations were, under the law, devoted to many reefs in the East River and Hell Gate besides the one at Hallet's Point; the result being that the work was rarely prosecuted in full force at the latter place. With a more generous grant of money the time consumed until the explosion, which amounted in all to six years and ten months, could have been reduced to four years.

As soon as the excavation was finished, the work of drilling holes in the roof and piers, to be afterward charged with explosives, was begun. At the completion, March 25, 1876, there had been drilled in the roof 5,375 three-inch, in the piers 1,080 three-inch, and 286 twoinch holes; the total length of holes drilled being 56,548 feet of threeinch and 1,897 feet of two-inch holes.

The proximity of the reef to habitations at Astoria, Ward's Island, and Blackwell's Island, made it necessary to devise a system of explosion which, effecting the work of demolition, would at the same time do no damage to life and property. The atmosphere and the rock being the mediums through which the shock would be transmitted, it was essential that the waves propagated through these should be as small as possible.

It was evident, in the first place that, if to each charge its full capacity of useful work in breaking up the rock was assigned, regard being likewise had to the superincumbent weight of water, no external effect of moment would be perceived in the atmosphere. In the second place, it was evident that the magnitude of the rock-wave would depend greatly upon the quantity contained in individual charges, that is, if eighty pounds were required for the individual charge, the vibration of the rock would be much greater than if these charges did not exceed twenty pounds. It was known that eighty-pound charges of nitro-glycerine, fired in numbers of twelve to twenty, did not cause a destructive wave. Again, the reef, after the excavation, being connected with the rock only through the piers and outer edge of the roof, it was inferred that the shock propagated in the rock would be due mainly to the charges necessary to disrupt the piers and roof from their connection with the bed-rock, and not to the charges to break up the roof and piers. The cubic contents of the roof and piers were 63,135 cubic yards, and the amount of explosives

Being at the rate of 0.79 pound to each cubic yard.

The explosives were placed in tin cartridge-cases. The number used was 13,596, 87 per cent being 22 inches and the remainder 11 inches in length. The number of holes charged was 4,427.

The system consisted of 3,680 mines and 23 batteries. Each battery assigned to 160 mines, which were divided into eight groups of twenty each. The mines of each group were connected in continuous series, and a lead and return wire to the battery closed the circuit.

The mines were fired at two hours fifty minutes P. M., September 24, 1876, and there were no injurious shocks in the atmosphere, in the water or underground.

The new facts obtained by this experience are:

1. That an unlimited amount of explosives distributed in blast-holes in moderate charges, proportioned to the work to be done, thoroughly confined in the rock, and tamped with water, may be fired without damage to surrounding objects.

2. That an unlimited number of mines may be simultaneously fired by passing electric currents through the platinum-wire bridges of detonators.

FIG. 5.-SECTION THROUGH HEADING F.

ούτ

SCALE

FIG. 6.-SECTION THROUGH GALLERY 12.

Substantially the same

methods as those which had proved efficient upon the Hallet's Point Reef were applied to the larger and more formidable Flood Rock. Two shafts were sunk from the ridge of the rock (Fig. 4), whence the whole nine acres of the reef-extending 1,200 feet in length and 602 feet in width-was undermined by two sets of parallel galleries, running at right angles to one another. The piers of rock left between these galleries to support the roof of the mine were about fifteen feet square and twenty-five feet apart from center to center. The roof of the cross-galleries, which ran at right angles to the lines of stratification, was blasted down as thin as it would be safe to leave it (Figs. 5 and 6). Considerable risk was incurred in this part of the work, from the danger of the rock crumbling, and from the uneven and uncertain thickness of the roof. The average thickness was 18.8 thick, and the minimum thickness ten feet. The exact thickness could not be ascertained beforehand, for no