ter object by a system of floats and iron curtains so arranged as to constitute a dam protecting a space of 160 feet long and 40 feet wide, within which the work could go on uninterrupted. Another machine was alternatively suggested, embodying the same principle, to consist of an iron caisson or cylinder, pointed at the ends, open at the top and bottom, and having self-adjustable legs to accommodate themselves to the irregularities of the rock, and to support it and keep it level. The top of this structure was to be above the level of the highest tide, and to be framed over so as to form a platform, on which the machinery could be placed, and from which the operations could be conducted. It was important to have it adaptable to all the rocks on which it was intended to operate, whatever their size and shape, and that it be perfectly stable against the action of the currents without being unwieldy. It was furthermore considered necessary to furnish it with a protection against collisions, which were sure to occur from the fleets of vessels passing daily. These conditions were fulfilled in the steam drilling cupolascow. The scow is. built very heavy and strong; an overhanging shortening the moving chains with capstans. When all is fixed, the dome is lowered close to the bottom, and established by letting the legs go to adjust themselves on the bottom. The chains that connect it with the scow are unslung, and flexible connections are made between the tops of the drill-rods and the pistonrods of the driving-engines. These connections must be flexible, because the dome is fixed on the bottom, while the scow holding the drillengines is certain to swing for short distances back and forth. When the drill-holes are ready to be charged, the dome is lifted, and the scow, carrying the suspended dome, is swung off to a safe distance, without casting loose the moorings. The charges of explosives, in tin cases of different lengths to suit the varying depths of the drill-holes, were carried to the spot upon a small scow, whence the diver descended to insert them into the holes. The leading wires were connected with a battery, and the explosion was made with invariably certain effects. Removal of Small Reefs.-Operations with the steam drilling scow were completed January, 1880, upon Diamond Reef. The rock was covered with a large accumulation of loose mate DRILLING-SCOW-SECTIONAL ELEVATION. guard, faced with iron, surrounding it as a protection against collisions; and has a wellhole 32 feet in diameter. The caisson or dome is a hemisphere 30 feet in diameter, of a strong iron frame covered with boileriron. It is open at the bottom and top, and provided with self-adjustable legs so arranged as to be all let go together after it is lowered. This caisson or dome is simply a framework affording a fixed support to 21 tubes, through which the drills operate. It is connected with the scow by four chains, communicating with four hoisting-engines, by which it is lowered or raised. The scow, having the dome swung by chains, is anchored over the rock to be operated upon. The diver then goes down to examine the bottom and see that the position is favorable. The position of the scow is changed by lengthening and rial 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 7 to 13 feet deep, 4 inches in diameter at the top, and 34 inches at the bottom, and were charged with from 30 to 55 pounds each of nitro-glycerin. Coenties Reef was worked upon in alternation with Diamond Reef in 1871 and completed in 1875. In 1872 work was begun on Frying-Pan, and there is now 22 feet at low water. Work was prosecuted on Pot Rock from Aug. 5 till Dec. 28, 1872, during which period there were 16 collisions. The depth on this rock is now 24 feet. Way's Reef, over which the original depth of water was 5 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 Aug. 4, were excavated, till 35 tunnels and 10 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 begun in 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 HALLET'S POINT-PLAN OF EXCAVATIONS. Coffer Dam Mean High Water ear Low HALLET'S POINT-SECTION THROUGH TUNNEL NO. 7. 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. Operations 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, in full force at the latter place. As soon as the excavation was finished, the work of drilling holes in the roof and piers, to be charged with explosives, was begun. At the completion, March 25, 1876, there had been drilled in the roof 5,375 threeinch, in the piers 1,080 three-inch and 286 two-inch 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 As toria, Ward's Island, and Blackwell's Island, made it necessary to devise a system of explosion which, effecting the work of demolition, would do no damage to life and property. It was evident, if to each charge its full capacity of useful work in breaking up the rock were assigned, regard being likewise had to the superincumbent weight of water, no external effect of moment would be perceived in the atmosphere. It was also evident that the magnitude of the rock-wave would depend greatly upon the quantity contained in individual charges; that is, if 80 pounds were required for the individual charge, the vibration of the rock would be much greater than if these charges did not exceed 20 pounds. It was known that 80 pound charges of nitroglycerin, fired in numbers of 12 to 20, 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 Rend-rock.. Vulcan powder... Dynamite... Total Being at the rate of 0.79 pound to each cubic yard. The explosives were placed in tin cartridge-cases. The number used was 13,597, 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 8 groups of 20 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 2 hours 50 minutes P. M., Sept. 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 blastholes 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 fired simultaneously by passing electric currents through the platinum-wire bridges of detonators. Flood Rock.-Substantially the same methods as those that had proved efficient upon the Hallet's Point Reef were applied to the larger and more formidable Flood Rock. Two shafts EXCAVATIONS OF FLOOD ROCK. were sunk from the ridge of the rock, whence the whole reef-1,200 feet in length and 602 feet in width, about nine acres-was undermined by two sets of parallel galleries, running at right angles to one another. The piers of rock were about 15 feet square and 25 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. Considerable risk was incurred in this part of the work, from the danger of the rock's crumbling, and from the uneven and uncertain thickness of the roof. The average thickness was 18.8 feet, and the minimum thick a concretion of bowlders and shells formed upon it. Had the excavations at any time broken into a large seam, the mine would have been flooded, and all the work and probably many lives would have been lost. Occasionally small seams were met. One seam was 10 inches wide and 100 feet long; another, from 1 to 4 inches wide and 400 feet long, extending clear across the reef, carried 350 gallons of water a minute. The latter was dealt with after protecting the completed part of the work by building across the gallery a door capable of withstanding the pressure of the water. The seams were all walled, as fast as they were opened, with Portland cement. The total length of the galleries was 21,670 feet. The galleries were excavated to depths varying with the uneven surface of the reef. The roof was then drilled with holes for the reception of the explosive cartridges. The holes were slanted upward at angles varying from 75° to 45°, and were made from 8 to 10 feet deep-except where the existence of seams open to the river made it impossible to obtain the depth wanted-and of sufficient capacity FLOOD ROCK-SECTION THROUGH HEADING F. ness 10 feet. The exact thickness could not be ascertained beforehand, for no soundings could distinguish between the solid rock and THE CHARGES IN THE MINES. to receive a rigid two-and-a-half-inch cartridge throughout their entire length. The holes were charged with rack-a-rock as the principal explosive (see page 346). The mixing was done in small batches on Great Mill Rock, in a leadlined trough, and the explosive was packed at once into cartridge-cases 24 inches in diameter and 24 inches long, made of copper 0.005 of an inch thick. To prevent the corrosion of the copper by the chemical action of the sulphureted water running through some of the drill-holes, the cartridges were dipped in melted resin, beeswax, and tallow. After being loaded, they were soldered with a steam heated iron, were removed as fast as they were filled, and were carried to the mine in boxes containing twenty each; so that the amount of mixed explosive above-ground at any one time was never enough to do more than local damage in case of an accident. These cartridges were inserted in the drillholes, one after the other, till the holes were filled, the last cartridge in every case being filled with dynamite, with its end left to project about six inches, so that it might receive the full effect of the shock from the initial charges connected with the battery. This cartridge is 15 inches long and 2 inches in diameter. In its forward or projecting end is inserted a small copper shell filled with fulminate of mercury. The other cartridges, charged with rack-a-rock, are 24 inches long by 24 inches in diameter, and are provided at their forward ends with a fulminate primer which is inserted after they are filled. This primer consists of a fulminate exploder in a copper tube containing an ounce of No. 1 dynamite. The cartridges are secured in the holes by divergent wires at their lower ends; and the dynamite cartridge is also fastened with wooden wedges. The mine-exploder consists of a brass cylinder, eight inches long by two in diameter, filled with dynamite. Inclosed within the dynamite is a fuse, the wires from which pass through a divided cork in the mouth of the brass cylinder. It consists of a copper tube nearly filled with 30 grains of fulminate of mercury. Fitting in the open end of this tube is a second tube containing sulphur, through which pass the two conducting |