Popular Science Monthly/Volume 28/February 1886/The Improvement of East River and Hell Gate
By General JOHN NEWTON,
CHIEF OF ENGINEERS, UNITED STATES ARMY.
THE East River is a most important factor in the commercial prosperity of the cities of New York and Brooklyn. Its shores form a large portion of the water-front of both cities, and afford space for many miles of docks. Its channel is scoured by strong tides, which keep it permanently free from shoals of sand and mud. It connects the waters of New York Bay with those of Long Island Sound, and is the most important avenue of coastwise trade in the United States. Since the partial removal of the obstructions in Hell Gate, a very heavy foreign trade has taken this route, notably the petroleum traffic, and, with the completion of the designed improvements, the East River will afford a convenient access for transatlantic steamers. The value of this entrance to New York Harbor will be still further enhanced by the opening of the Harlem River to the Hudson, the preparations for which are now in progress.
The unobstructed navigation of the East River would also have an important bearing on the question of national defense in case of war with a foreign state. It would render the blockading of the port of New York a task of doubled difficulty, and would open the shores of Long Island to our larger war-vessels and to ironclads.
For these reasons the problem of removing the obstructions to the free and safe passage of vessels through the East River has engaged an increasing degree of attention for a considerable time. These obstructions were all accumulated within a short distance of one another, in the narrow strait called Hell Gate, and were occasioned by numerous reefs of rocks encroaching on the channel and the violent currents to which they give rise, making navigation extremely hazardous for all kinds of vessels.
A clear idea of the nature of these obstructions and dangers in the original condition of the strait is given in the report of Lieutenant Commanding Davis, who, with Lieutenant-Commanding David Porter, made a survey of the place in 1848. "The strength of the current," he says, "is such that sailing-vessels can only stem its force or escape from it by a commanding breeze; but, as the main course of the flood-tide keeps the middle of the Eastern Channel, it is most secure for vessels which are coming from the westward with the tide to place themselves in the middle of the stream and follow its direction. This plan, however, is inadvisable for any but small vessels, on account of the rocks, the Pot and Frying-Pan, which lie in or very near the mid-channel, are in the way both going to the eastward and westward, and have but little water on them at low tide. There is also a reef called Way's Reef, which lies in the course followed by steamboats principally when coming from the eastward against a strong flood. It is their custom to keep close round Pot Cove, and run up under Hallet's Point, by which they avoid the strength of the flood. In this part they find an eddy-current in their favor. But on the ebb the greatest danger arises from the divergence of the current, when the ebb branches off into three directions to take the course of the three channels—the main Ship-Channel, the Middle Channel, and the Eastern Channel. The safe navigation depends here upon deciding sufficiently soon at the point of separation which channel shall be taken; and the neglect to do this, or a loss of control over the vessel for any reason, frequently results in being carried on the Gridiron. When a vessel that has attempted the Eastern Channel finds herself carried toward the Gridiron, her only chance for safety is to run for the Middle Channel, which is narrow and made precarious by the middle reef, the outer rock of which is the Negro Head. The Gridiron is, owing to the strong set of the tide upon it, the most dangerous reef in the passage. The reef known as Bread and Cheese, on the eastern end of Blackwell's Island, is also very dangerous. Vessels are liable to go on it in the flood, when it is covered, by getting into the eddy near it with a light wind. The chief danger is on the ebb, and from the same reason that makes the Gridiron dangerous—i. e., the strong set of the tide in that direction."
The reefs (see map) known as the Gridiron, Flood Rock, Hen and Chickens, and Negro Head, are all essentially parts of one reef, which is designated as the Middle Reef. Between this reef and the reef which is marked by the projections of the Great and Little Mill Rocks, is situated the Middle Channel. The Eastern Channel is included between the Middle Reef and Hallet's Point. The South or Main Ship Channel lies to the west of Great and Little Mill Rocks, and between them and Rhinelander's Reef. The two surveying-officers, while agreeing as to the desirability of removing or mitigating the obstructions, made different recommendations respecting the manner in which they should be dealt with.
Lieutenant-Commanding Davis recommended that Pot Rock, the Frying-Pan, and Way's Reef be blasted and scattered, and that the Middle Channel be improved by blasting, so as to make a clear channel of sufficient depth for common vessels and steamboats. As the removal of the larger reefs seemed at that time impracticable, he advised that they be faced with sea-walls or piers rising four feet above high water, and provided with spring fenders such as are used at the ferry-docks, so adjusted as to guide vessels coming in contact with them into the channel-ways. Lieutenant-Commanding David Porter, not regarding the deepening of the Middle Channel as practicable, advised that it be filled in with docks, and recommended the removal of a part of the reef at Hallet's Point.
No efficient method was suggested for removing the rocks it was proposed to take away, further than to blast them and leave the fragments to lie where they might fall, or to be washed away by the currents. This method would obviously make but a slight impression on the larger reefs.
A process of surface-blasting was first applied by M. Maillefert in 1851 His method consisted simply in placing upon the rock a charge of gunpowder, usually of 125 pounds, contained in a tin canister, and exploding it by means of the voltaic current. The weight of the water resting upon the charge served to increase the effects of the explosion. No means were provided for removing the broken rock except by breaking it up by successive explosions till it was fine enough to be carried away by the currents. M. Maillefert's operations resulted, by the use of 620 charges containing 74,192 pounds of powder, at a cost of $13,861, furnished through Mr. Merriam, in removing from the rocks to which they were applied the projecting prominences of small area, but were of little effect when, after reaching the main rock, a considerable extent of surface had to be dealt with. The depth of water was increased, on Pot Rock, from 8 feet to 18·3 feet; on Frying-Pan, from 9 to 16 feet; on Way's Reef, from 5 to 14 feet; and on Shell Drake, from 8 to 16 feet. Bald-Headed Billy and Hoyt's Rocks were blown into deep water. The depth on Diamond Reef was but slightly if at all affected, and no effect was produced on Hallet's Point Reef. In 1852, Congress having appropriated $20,000 for the removal of obstructions, Major Fraser, by Maillefert's method, increased the depth over Pot Rock to 206 feet, adding 23 feet of water to the 18·3 Maillefert had gained, with the expenditure of $6,837. The decrease in the ratio of returns for expenditure was occasioned by the increased surface of the rock, due to increase of depth.
In 1856 an advisory council to a commission on the removal of obstructions in Hell Gate recommended a plan which, as to its general features, was substantially the same as that which Lieutenant-Commanding Davis had proposed in 1848. Instead of Maillefert's process of surface-blasting, they recommended that of drilling. This would have had to be done from within a diving-bell, which was impracticable because of the certain disturbance of the apparatus by currents, and the liability to collisions from passing vessels. All of the plans 80 far tried or proposed seemed to have been limited to removing the smaller rocks lying in the channel; the possibility of clearing away the larger reefs was not yet conceived.
In 1866 the department instructed me to make an examination of Hell Gate, and to present a plan and estimate of the operations necessary to improve its navigation. A report was submitted in January, 1867, with an estimate for removing the reefs by blasting, after drilling the surface from a fixed platform above the water. In this plan, or in whatever plan might be decided upon, it was considered essential that the drills, which, to avoid interference from currents, were to be worked within iron tubes reaching from the platform to the rock, should be attached to a framing kept absolutely fixed while the drilling was going on; and that the divers or the machinery necessary to handle and remove from the bottom the rock blasted should be protected from violent currents. It was proposed to accomplish the latter object by means of 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 in constructing the machine to have it adaptable to all the rocks on which it was intended to operate, whatever their size and the shape of their surface, and that it be perfectly stable against the action of the currents without being unwieldy in size. 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, and certain to destroy it, with a loss of life and much property, if it were not protected.These conditions were fulfilled in the steam-drilling cupola-scow (Figs. 1 and 2). The scow is built very heavy and strong; is provided with an overhanging guard, faced with iron, surrounding it as a protection against collisions; and has a well-hole 32 feet in diameter. The caisson or dome is a hemisphere 30 feet in diameter, composed of a strong iron frame covered with boiler-iron. It is open at the bottom
Fig. 1.—Sectional Elevation.
and top, and is 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 the drill-tubes, twenty-one in number, through which the drills operate. It is connected with the scow by four chains, communicating with four hoisting-engines, by
Fig. 2.—Deck Plan.
which it is lowered or raised. The scow, having the dome swung by chains, is anchored over the rock to be operated upon, by as many anchors as are required to hold it against all the currents. 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 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 which connect it with the scow are unslung, and flexible connections are made between the tops of the drill-rods and the piston-rods of the driving-engines. These connections must be flexible, because, the dome being fixed on the bottom, while the scow holding the drill-engines is certain to swing for short distances back and forth, the connections, if rigid, would be broken. When the drill-holes are completed and ready to be charged, the dome is lifted up, and the scow, carrying the suspended dome, is swung off from the spot to a safe distance (the length of which depends on the amount of the charge), without casting loose the moorings. The charges of explosives, in tin cases of different lengths to suit the varying depths of the drill-holes, are carried to the spot upon a small scow, whence the diver descends to insert them into the holes; the cartridges are handed down to him, already attached to leading wires by the men on the scow; and he is guided from hole to hole by lines connecting the stoppers or plugs inserted in the holes. The scow is withdrawn, the leading wires are connected with the battery, and the explosion is made with invariably certain effects.
To secure satisfactory results in the breaking up of the rock, the drill-holes should be six or eight feet apart, five to six inches in diameter, and should reach to about four feet below the level to which it is desired to break the rock. The broken rock is removed by means 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 171 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 16,7923 pounds of nitro-glycerine and 381 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. Operations were begun here in July, 1869, for the construction of a coffer-dam 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 123 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.
Plan of Excavations Scale
Longitudinal Section through Tunnel No. 7:
Fig. 3.—Hallet's Point.
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 280 two-inch holes; the total length of holes drilled being 50,548 feet of three-inch 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 13,590, 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 3,080 mines and 23 batteries. Each battery assigned to 100 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.
Fig. 5.—Section through Heading F.
Fig. 6.—Section through Gallery 12.
The now 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.
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 Hock. 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 soundings could distinguish between the solid rock and 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 by the inrush of the water, and all the work and probably many lives would have been lost. Occasionally small seams were met and had to be dealt with. One seam was ten inches wide and a hundred feet long; another one, from one to four inches wide and 400 feet long, extending clear across the reef, carried 850 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 21,070 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, with which the rock was to be finally blown up. The holes were slanted upward at angles varying from 75° to 45°, and were made from eight to ten feet deep—except where the existence of seams open to the river made it impossible to obtain the depth wanted—and of sufficient capacity 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—a substance formed by mixing 79 parts of finely-ground chlorate of potash and 21 parts of di-nitrobenzole. It is one of the safest explosives to handle, and the ingredients are absolutely inert when kept separate, and they need not be mixed till just before loading the cartridge; it has 1093 per cent the strength of No. 1 dynamite, when fired under water, and costs but a little more than half as much. The mixing was done in small batches on Great Mill Rock, in a lead-lined trough, and the explosive was packed at once into cartridge-cases 21 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 protected by being dipped in melted resin, beeswax, and tallow. The cartridges, after being loaded, were soldered with a steam-heated soldering-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 drill-holes, 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 represented in Fig. 7, and is 15 inches long and 21 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, represented in Fig. 8, are 24 inches long by 21⁄4 inches in diameter, and are provided at their forward ends with a fulminate primer which is inserted after they are tilled. This primer is shown in half size in Fig. 9, and consists of a fulminate exploder similar to that shown in Fig. 7, in a copper tube containing an ounce of No. 1 dynamite. The
cartridges are secured in the holes by the wire springs shown at their lower ends; and the dynamite cartridge is also wedged in with wooden wedges. Fig. 10 shows the mine-exploder, the position of which in the mine is illustrated in Fig. 12. It consists of a brass cylinder, eight inches long by two in diameter, filled with dynamite. Inclosed within the dynamite is a fuse, shown half size in Fig. 10, 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-wires, they being held firmly in place by the sulphur. The inner ends of the wires are united by a small platinum wire. The ends of the wires are then surrounded with fulminate, and the two parts of the tube arc put together, that containing the wires slipping within the other. The entire fuse is then covered with gutta-percha. The passage of an electric current through the wires heats the platinum bridge to redness, and causes an explosion of the fulminate.
The number of pounds of rack-a-rock put into drill-holes was 240,399; of dynamite, 42,331; total, 282,730 pounds. There were 11,789 drill-holes in the roof and 772 in the pillars, and their total length was 113,102 feet, or more than twenty miles. The whole amount of rock to be broken by the final blast was 270,717 cubic yards, covering an area of about nine acres.
The primary charges, the office of which was by their detonation to produce the explosion of the charges in the drill-holes, were placed along the galleries at intervals of twenty-five feet, and arranged as shown in Fig. 12. They were placed on timbers extending from wall to wall in each of the galleries, and consisted of two twenty-four-inch dynamite cartridges like those already described lashed to the timber, with one of the "mine-exploders," also already described, bound upon them. The entire mine was divided into twenty-four independent circuits. Within each of twenty-one of these circuits were twenty-five fuses or mine-exploders, while three circuits contained twenty-two fuses each. A wire from the battery on the surface of the rock at the mouth of the shaft led from one fuse to the next, until the twenty-five fuses were in the same electrical circuit, and thence back to the battery. So far as was practicable, adjacent charges were put on different circuits, so that if any circuit failed through any fault in the connections, an explosion of its charges would still be insured through the sympathetic action of the adjoining charges. The whole number of these primary charges was 591. Some of the circuits were nearly a mile long.
The fuses prepared for this work had a resistance of 1·73 ohms cold, and 2·76 ohms at explosion. To fire a single fuse, 0·205 ampères were required; to fire a series, 0·615 ampères. A factor of safety of two was used, and double this current was sent through every fuse at the final blast. The battery consisted of sixty cells, all coupled in one series, each of which had an electro-motive force of 1·95 volts and an internal resistance of 0·01 ohms. The plates were six inches by nine inches—four carbon and three zinc plates in each cell, separated by a quarter of an inch. The ordinary bichromate solution was used. The poles were constituted of two large mercury-cups, into one of which were dipped the twenty-four lead wires, while the twenty-four return wires terminated in a third cup. Between this third cup and the remaining pole of the battery stood the apparatus for closing the circuit. It consisted of a stout iron cup containing mercury, in which eat a thin glass tumbler also partly filled with mercury. Two large strips of copper connected the mercury in the iron cup with one pole of the battery, and that in the glass with the cup containing the return wires. To close the circuit through the fuses it was only necessary to break the tumbler so as to let the mercury in it mix with that in the iron cup. To do this at the proper moment, a one-quarter-inch iron rod four feet long, terminating at the top in a small round disk, stood with its point in the bottom of the glass. It was long enough to pass through the roof of the battery-house. A thirty-grain platinum fuse, connected with a small battery at Astoria, was laid on the disk and stuck on with a lump of wax. It had been previously determined by experiment that the blow struck by this fuse on exploding, and transmitted by the iron rod, would be so sharp as to completely pulverize the tumbler and yet not splash the mercury.
The mine was flooded by two siphons of twelve and sixteen inches respectively, in fifteen hours and a half, ending at 3.30 a. m., October 10th. The explosion was set for 11 a. m., October 10th, but the interests at stake were so great, and the details to be looked after to avoid every chance of miscarriage so numerous, that, in spite of the most energetic effort, everything could not be made ready and tested in time to fire at the appointed moment. The explosion did not actually take place till 11.13. This delay caused some confusion in the seismoscopic observations.
The whole area of the reef was shattered. The plan of making the excavations large enough to swallow all the débris of the reef and leave a channel deep enough, without further operations, already abandoned at Hallet's Point as more expensive than dredging up the broken rock, was never entertained at Flood Hock. Hence the sensational view, which many persons expected to witness, of a sudden disappearance of the rock, was not seen. Though the charges all exploded at the same instant, the time and the appearance of the effect above the water-surface varied according to the strength of the rock and the depth of the water. There was no loud report and no dangerous shock. The breaking of some panes of glass and the shaking down of a few bricks and loose ceilings constituted all the damage that was done.
Pending the awarding of a contract for dredging, the work of removing the rock was begun with a scow belonging to the Government as soon after the explosion as possible. From fifteen to thirty tons of rock were removed daily by being hoisted out after having been slung by divers on chains. A contract has been let for the removal of 30,000 tons of the rock at $3.19 per ton, the contractor to do his own surface-blasting. This is less than the price for which rock was removed on similar terms at Hallet's Point. The contractor has two grapples at work, and is removing an average of about 120 tons a day. As a whole, the cost of mining a cubic yard of rock has been reduced 342 per cent from the cost of doing the same work at Hallet's Point. The total cost of the work done on Flood Rock, including the final blast, amounts to $2.99 per cubic yard of the whole amount of rock broken, or $5.66 less than the cost of breaking Hallet's Point. A considerable part of this gain will, however, be expended on the proportionately larger amount of dredging to be done. The net result, however, will show an improvement of not less than 30 per cent, and probably more. The total cost of the final blast at
Hallet's Point was $81,092.24; at Flood Rock it was only $106,509.93, though the blast was 5· 6 times as large. These results together indicate that a great advance has been gained in the economy with which the whole work was carried on; and the progress with the dredging gives promise that an 18-foot channel, 400 feet wide, over the worst part of the reef, may be cleared out by spring. If the funds are supplied as needed, the dredging can all be completed in three years.
The accounts of the observations of the shock of the explosion, have been tabulated by General Abbott, as follows:
It seems that the speed of the oscillation-wave is the greatest through an uninterrupted rock formation.
Fig. 13 is a bird's-eye view of the reefs of Hell Gate, taken from a model constructed from the preceding map and from surveys of the Engineer Department. The white margin around the shores and reefs represents the parts between mean low water and twenty-six feet below it, the level to which all reefs obstructing navigation are to be reduced.
Negro Point (1): on this reef no work has been done. It is to be undermined and cut off on a line with the Sound-entrance wharf. Holmes Rock (2) and Hog-Back (3) are simply to be finished with a sea-wall. Frying-Pan (4) has been reduced to the level of twenty-four feet below mean low water. Way's Reef (6), Shell Drake (7), finished to the full depth of twenty-six feet. Hallet's Point (8), the rock shattered by the explosion has been entirely removed to the depth of twenty-six feet. The projecting point has been cut off, as shown by Fig. 3. Heel-Tap (9), broken to twenty-six feet, dredged to twenty-two feet, to be cleaned to full depth. Great Mill Rock (10), Little Mill Rock (11), connected by a dike; nothing further to be done here. The Gridiron (12), Flood Rock (13), Hen and Chickens (14), Negro Heads (15), broken to thirty feet; 15 is now being removed to open the Middle Channel at once, and the balance afterward to full depth. Rylander's Reef (16) to be embanked. Bread and Cheese (17) has already been embanked. Scaly Rock (18) has been removed.
It will be readily understood from the above description what has been contemplated by these improvements, how far they have already been carried out and what purpose they have served, and how adequate will be the channel after they are completed.