39tii Co-vukkss; ( HOLSE OF REPRESENTATIVES. ( Ex. Doc 2d S ess inv t ' \ No. 90. SURVEY AT HELL GATE, EAST RIVER. f N c/^ i / %LilrO^ LETTEK FROM THE SECRETARY 0E WAR, TRANSMITTING A report of the survey at Hell Gate, Neiv York. 1 run laky 14, 1S67. — Referred to the Committee on Commerce and ordered to be printed. War Department. Washington City, February 11, 1S67. Sir : I have the honor to transmit herewith a communication from the Chief of Engineers, of February 9, 1867, covering a report by General Newton, of the corps of engineers, on the survey at Hell Gate, New York, made pursuant to the act of Congress of June 23, 1866. Very respectfully, sir, your obedient servant, EDWIN 3VT. STANTON, Secretary of War. Hon. Schuyler Colfax, Speaker %f the House of Representatives. Engineer Department, Washington, February 9, 1867. Sir : In obedience to the provisions of the act approved June 23, 1866, making appropriations for the repair, preservation, and completion of certain public works, &c, I have the honor to transmit herewith a copy of the report of Brevet Major General John Newton, United States army, lieutenant colonel of engineers, on the survey of Hell Gate, New York. General Newton submits estimates for three separate projects for the improve- ment, with channels of twenty-six and twenty-four feet in depth at mean low water. JThe first includes the removal of all the rocks enumerated in his report ; the construction of sea-walls ; improvement of the middle channel, and the re- moval of Negro Head. The second includes the cost of removing that part of the middle reef known as Negro Head, Hen and Chickens, Flood Rock, and Gridiron, thus throwing the eastern and middle channels into one. The third includes the removal of the smaller rocks and the construction of the sea-walls, but makes no provision for improving the middle channel, or otherwise inter- fering with the middle reef. The third project, involving the least expenditure of money and greatly ameliorating the danger to navigation, essentially that recommended by the ad- visory council to the " commissioners relative to the encroachments and preser- 2 SURVEY AT HELL GATE, EAST RIVER. vation of the harbor of New York," consisting of General Totten, chief engineer United States army, A. D. Bache, superintendent United States Coast Survey, and Commander (now Admiral) Charles Henry Davis, United State- navy, dated September ;J0, 1856, is recommended as the best to be undertaken at present. The estimate for this project, giving a channel of twenty-six feet at mean low water, is *2, 877,000, to be completed in four years. The amount required for the first year's operations, including purchase of machinery, will be 8900,000. The entire completion of the improvement to be undertaken as the demands of increasing commerce shall warrant. General Newton enters minutely into the method of conducting the work, the nature of the requisite machinery, &c, and his conclusions, derived from a close and careful study of the questions involved, are commended to the attention of the War Department. Very respectfully, your obedient servant, A. A. HUMPHREYS, Ch ief of En gin errs. Hon. Edwin M. Stanton, Secretary of War. United States Engineer Office, lyo. 7 Boirfing Green, New York, January 21, 1S67. General : Having been assigned by department letter of July 21, 1SG6, to the duty of examining Hell Gate, and having been further instructed by letter of July 25 to make such examination with the view of its improvement for the purposes of navigation, and sufficiently in detail to present a plan and estimate for the necessary operations, I have the honor to present the following report : Lieutenant Commanding C. H. Davis, assistant in the coast survey, made an examination of Hell Gate, and a report, dated February 15, 1S48. He states: " Of the sailing vessels which enter Hell Gate passage, it is estimated that one in fifty sustains more or less injury by being forced by the violence of the cur- rents on the rocks and shoals. * * * The damages in this channel arise from the great strength of" the currents, and the number and positions of the rocks and reefs. The strength of the current is such, that sail- ing 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 easiern chan- nel, it is most secure for vessels coming from the westwarc^with the tide to place themselves in the. middle of the stream and follow its direction. * * This plan, however, is inadmissible for any but small vessels, on account of two rocks, the Pot and the Frying Pan, which lie in or very near the mid-channel, and in the way, going to the eastward and westward. * * There is a reef called the 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 Fot cove, and run up under Hallet's Toint, 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 di- vergence of the current at a point marked A on the sketch, where the ebb tide branches off into three directions, to take the course of the three channels, and the main south (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 towards the Gridiron, her only chance for safety is to run for the middle chan- nel, which is narrow and made precarious by the m'ddle reef, the outer rock of SURVEY AT HKLL GaTE, EAST RIVER. 3 which is the Xcgro Head. The Gridiron is, owing to the strong set of the tide on it, the most dangerous reef in the passage. The reef known as the 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 cause as that which makes Gridiron dangerous, that is, the strong set of the tide in that direction." Lieutenant Commander Davis then recommends the removal by blasting of Pot Rock, the Frying Pan, and Way's Reef, and also that the middle reef be blasted so as to make a clear channel of sufficient depth for common vessels and steamboats between Flood and Little Mill rocks. That piers be built upon Hog's Back, the Gridiron, and Bread and Cheese, of stone, sufficiently high to show above the surface of the water at least four t'eet ; and that these piers be faced with wood, and provided with spring fenders. The piers should conform to the natural shape of the reef. Bylander's Reef, by the same report, is dangerously situated on account of the deep waters around it, but no recommendation, except to mark its position, is made. Lieutenant Commanding D. D. Porter, United States navy, under date of October 30, 1S48, reports in favor of the plan of improvement of Lieu- tenant Davis, except the blasting of the middle channel reef, which is too diffi- cult, and suggests it be entirely filled in with docks, provided with spring fenders. He recommends, likewise, the removal of certain small rocks not before mentioned: Bald-headed Billy, a rock opposite Gibbs's Point, and within fifty yards of Blackwell's island; a small and dangerous rock southward of Wool- sey's bath-house ; Scaley Rock, and close to it Shell Rock. Hallet's Point, he states, should by all meaus be removed. " At tide it causes the eddies in Pot cove, and on the ebb tide the eddies in east channel " * * Towards the close of his report, " No one can form an idea of the number of vessels that go on shore during the course of a month. Fifty went on shore during the period I was occupied there, (two months,) and many of them were much injured." He suggests, likewise, " A close examination of the shore at low water. Many rocks will be found to exist of a dangerous character, and they have at different times clone much damage to vessels of a lighter draught." Operations at Hell Gate to improve, the navigation by blasting rocks, from August, 1851, to Recembcr, 1853, under the superintendence of the late E. Merriam, esq., and subsequently of the late Major Fraser, corps of engineers. Mr. Merriam, in his letter of the 22d of December, 1852, to General Totten, chief engineer United States army, states : "When M. Maillefert commenced tiring on Pot Rock, the 19th August, 1851, there was but eight feet of water upon it. On the 3d of September, Lieutenant Bartlett made an examination of that rock, by which it appeared that the rock had been reduced four and a half feet by sixteen charges. On the 18th October, Lieutenant Bartlett addressed a letter, the original draught of which was by him (i. e. Maillefert) handed over to me, in which he stated that M. Maillefert had reduced Pot Rock to sixteen feet, mean low water. At that date M. Maillefert had fired eighty-five charges on Pot Rock. " On the 2d and 3d of January, 1852, he (i. e. Lieutenant Bartlett) again sur- \ yed Pot Rock, and stated (officially) that there was nineteen feet of water on Pot Rock. Up to that date two hundred and thirty-four charges had been fired on that rock. On the 27th March, 1852, Lieutenant Bartlett addressed to me a letter from Washington, in which he stated that he had made a survey of Pot Rock on the 15th of that month, and found net less than twenty and a half feet of water upon it at the shoalest part; and that it had been reduced from two hundred and twenty-five feet length, to one hundred r,nd seventy- two feet length, and from sixty feet width to forty feet width. 4 SURVEY AT HELL GATE, EAST RIVER. " From the recommencement of operations in February to the date of this sur- vey there had been fifty charges fired on Pot Rock, making two hundred and eighty-four charges from the commencement. " It seemed reasonable to suppose, if the water on Pot Rock had been increased from eight to twenty and a half feet, and the length reduced from two hundred and twenty-five to one hundred and seventy-two feet, and its width reduced from sixty to forty feet, by two hundred and eighty -four charges, that two hun- dred additional charges would complete -its reduction to the depth of twenty- four feet; hence my estimate of September 17, 1852." Mr. Merriam, in his statement of operations on the rocks of Hell Gate and East river, submitted to the Committee on Naval Affairs of the United States Senate, August, 1£52, gives the following : RECAPITULATION. Name. Charges. Pounds of powder. Cost. Rock removed. Pot Rock Frying- Pan Way's Reef Shelldrake Bald-headed Billy Hoyt's Rocks Diamond Reef Hallet's Point Total . 284 105 135 1 8 78 3 34, 231 12, 387 15,549 750 • ]25 1,000 9,750 400 $6,837 30 2,116 81 2, 543 66 110 34 500 00 250 00 1,434 42 69 06 m feet. 94 feet. 9l feet. 8£ feet. Entirely. Entirely. 2 feet. No result. 620 74, 192 13,861 59 In the same paper he gives the following record of the depth of water upon the rock, prior to the commencement of his operations, and at the close tnereof : Name. Pot Rock Frying Pan. . . Way's Reef... Shelldrake Diamond Reef Original depth. Depth at close of operations. 8 feet. 20i feet. 9 feet. 18* feet. # 5 feet. IU feet. 8 feet. 16* feet. 16 feet. 18 feet. This reported increase of depth, due to blasting, will be found exaggerated, though without fault of Mr. Merriam, who was deceived by the official reports of the surveys. He is, on the contrary, entitled to great credit for the results actually attained, and for the arduous and efficieut services which he gratuitously rendered to further an important public improvement. Congress in 1S52 having made an appropriation of $20,000 for the removal of rocks at Hell Gate, this operation fell under the charge of the engineer department, and Major Fraser, corps of engineers, was assigned to the duty. Major Fraser presented his preliminary estimates, based upon the depths given above. But immediately after having surveyed Pot Rock,- he reports, October 22, that instead of there being twenty and one-half feet, there was actually but a little over eighteen feet of water upon the rock. This discovery showed a much diminished quantity of work done, and indicated a large increase of his previous estimate. He recommended the removal of the following rocks : Pot Rock, Frying Pan, Way's Reef, the rock near Woolsey's bath-house, certain loose rocks near the shore, a part of Hallet's Point, and certain sharp points at Negro Head, in the SURVEY AT HELL GATE, EAST RIVER. 5 middle channel. On November 10, 1S52, Major Eraser reports Pot Rock to be, at the twenty-four-foot curve, two hundred and twenty feet in length andsixtv- three feet across at the widest part. His survey of June 28 and 29, 18o3, shows a minimum depth on the rock of twenty-one feet four inches, at low water, as the result of firing eight hun- dred charges at a cost of $14,963 O'J. It is quite likely that one hundred charges were subsequently fired, though no certain record can be found, and that -SIS, 000 nearly was devoted to blasting on Pot Hock. . In July, 1S53, Mr. Fraser estimated the cost of removal of one cubic foot of rock at SI 14 Several of his reports during the fall of 1853, and the sketch of his survey in October of that year, appear to have been lost. The method pursued under the superintendence of Mr. Merriam and of Major Fraser excluded the employment of diving-bells or .submarine armor, and the charges of powder were consequently placed in position without that examina- tion of the bottom which would have developed in each charge its full useful effect. The work was finally suspended for want of funds, and since 1S52 no appro- priation has been made by Congress for the removal of these obstructions. The advisory council to the " commissioners relative to the encroachments and preservation of the harbor of New York," consisting of General Totten, chief engineer United States army; A. D. Bache, superintendent United States Coast Survey, and Commander Charles Henry Davis, United States navy, made to that body, September 30, 1S56, an important report upon the improvement of the navigation of Hell Gate passage, which is to be found on page 197 of the Reports of the New York Harbor Commission of 1857, republished by order of the Chamber of Commerce. A thorough survey of this channel was made by Lieutenant Commanding Craven, assistant, Coast Survey, and a current-chart prepared to illustrate their report. The advisory council recommends the removal, by blasting, of Pot Rock, Fry- ing Pan, Way's ^feef, a portion of Hallet's Point, and other rocks of minor conse- quence, Bald-headed Billy, Blackwell's Rock, a rock near Negro Point, two east of Woolsey's bath-house, and one to the southward of that building. The council likewise recommends the closing of the small channel between the Bread and Cheese and the head of Blackwell's island; the building of stone piers, with spring fenders, on Hog's Back, the Gridiron, and Bread and Cheese, and the erection of a beacon on Rylander's Reef. The report of Lieutenants Commanding C. H. Davis and D. D. Porter, and of the advisory council, together with the notes and extracts taken therefrom, furnish a full catalogue of the dangers and obstructions to this navigation, and it is not thought necessary to dwell longer upon this branch of the subject. During the present season surveys and examinations of Hallet's Point, Scaly Rock, the Frying Pan, and Pot Rock have been made, and the results given in the sketches herewith. An examination was likewise made by divers of the surfaces of Pot Rock and Frying Pan, principally with the view of ascertaining the feasibility of blasting by placing the charges upon the rock, and although nothing special has been elicited the indications are that some of the blasting necessary to be done can be effected by simply placing the charge in contact with the rock. WORK PROJECTED FOR THE IMPROVEMENT OF THE CHANNEL OK HELL (.ATE. This portion of the subject has so frequently been di-cussed by officers of distinguished ability, whose reports are published, that little further remains to be said. It is agreed that certain rocks in the channel, viz., Pot Rock, Frying 6 SURVEY AT HELL GATE, EAST RIVER . Pan, "Way's Reef, Shelldrake, and others nearer shore, viz., the rock of Negro Point,- those near Woolsey's bath-house, Blackwell's Rock, together with por- tions of Hallet's Point and of Scaly Rock, should be removed. Sea-walls have been likewise recommended to be built on the Gridiron, Hog's Back, and on the Bread and Cheese, and a stone beacon to be placed on Rylander's reef. To the rocks to be removed may be added the Heeltap Rocks, situated off the end of Great Mill Rocks, which, by their position, compel vessels to make such a circuit as to endanger their safe passage past Rylander's Reef. To the sea- walls may be added one upon Rylander's reef and another along the seventeen- foot curve of Great Mill Rock, on the side of the middle channel, to mark the passage and to sheer off vessels. The great reefs in the middle of pass — Flood Rock, Negro Head, the Giidiron, and Great and Little Mill Rocks — have been little noticed in any scheme of im- provement, notwithstanding they form the greatest obstacle to the pass of Hell Gate being converted into a highway for commerce. In the existing state of the art of removing rock under water, the imagination was appalled at the notion even of meddling with such means. The middle channel is narrow and is rendered dangerous by the projection of Nego Head into its lower outlet. The least improvement that can be recom- mended in this report should include Negro Head, and the increase of the width of this channel to about three hundred and forty feel, as marked on the map sent herewith. Estimates arc submitted for three projects. The first includes the removal of all rocks mentioned at the beginning of this division of the report, with the construction of the sea-walls, and likewise pro- vides for the improvement of the middle channel and the removal of Negro Head. The second includes the cost likewise of the removal of that part of the mid- dle reef known as Negro Head, Hen and Chicken, Plood Rock, and Gridiron, thus throwing the eastern and middle channels into one. The third includes the smaller rocks and the sea-walls, and makes no provision for improving the middle channel, or otherwise interfering with the middle reef. With every improvement of Hell Gate, the number of vessels passing through will increase and the necessity for further room will become more apparent. Hell Gate now washes with its waters the corporate limits of the city; before many years it will form a portion of the busy harbor of New York, and then will there arise a demand for improvement to an extent which has not yet in idea even been entertained. The uncertainty and slowness attending 'blasting under water has had the effect of diverting public attention from that considera- tion of Hell Gate to which the importance of the improvement is really entitled, and it is believed, as soon as the work can be pushed in a regular manner, subject to fixed laws of calculation, like blasting on land, that the extent of improvement needed for the interests of commerce will be the sole limit to public effort in this direction. What extent of work is really demanded now for the future is a problem which more properly belongs for solution to those representing the interests of com- merce in this city, owing to their more perfect acquaintance with its wants and capacity. This report does not prescribe these limits, but the estimates, tables, and other information here furnished will enable any one to calculate the expense of projects different from those formally set forth. Estimates for twenty-four and twenty-six feet depth at mean low water are separately made on each project. To W. S. Edwards, esq., assistant in the Coast Survey, 1 am indebted for sur- veys of Pot Rock, Prying Pan, and the Heeltap Rocks, made under circum- stances of difficulty ; and to G. W. Blunt, commissioner of pilotage, for reliable information in this and other cases for the benefit of the harbor. SURVEY AT HELL GATE, EAST RIVER. 7 MODE OF CONDUCTING OPERATIONS. Tin; quantity of work to be done and the consequent eost, as .shown in the estimates, are bo great as to demand, before embarking largely into it, that a project for the operations, not only reasonable in itself, but subjected to fixed laws of calculations, should be presented; but, before entering upon this subject, it would be well to discuss the process already employed, and ascertain, if possi- ble, what the results have been. The improvements at Hell Gate were made by exploding charges ol powder placed upon the rock, no advantage being taken of submarine apparatus of any kind to establish these charges where the greatest effects might result. So long as the rock was found in isolated or separate pinnacles, this process, imperfect as it was, produced great effects. But when the projecting points were knocked away, and the solid bed of rock reached, progress became slow and doubtful, and the cost great. The small amount of appropriation granted by Congress having been exhausted, the work has since been left in this unsatisfactory state. The rock blasted was supposed to have been blown into deep water by the force of the charge, since there was no provision made tor its removal by other means, but it is evident that such results became very problematical when the area of the rock is increased and the distance to which the fragments should be moved increased. Xo reliable estimate could be framed upon such a process, even if it were sure to be attended with final success, which is more than doubtful. The employment of divers, which the small amount appropriated for this im- provement did not justify, would have had a result more satisfactory, and perhaps have furnished data for calculation. As it is, for this particular locality no ad- ditions to our previous knowledge have been gained. The project presented in this report consists in first preparing the surface for ulterior operations by blowing off the sharp points and projections, by charges placed in position by divers, until the rock is reduced to a more uniform surface, then to blast the rock by introducing charges into drill-holes made for the purpose, and to re- move the debris by divers. The current, which has a maximum velocity of S .30 miles, limits the time of working under water to a little over two hours per day, during and about the time of slack water ; and hence the time and expense, unless some means of shielding the divers and operators from the force of the current can be devised, would prove a serious objection. Even if the persons using the drill were protected from the current, the de- lay incident to this mode of operations would be considerable ; but if the drill-* ing were conducted from the surface of a platform above the water, the same results would be reached as on dry land. A platform of suitable size, with vertical sliding supports, capable of being raised or lowered through a convenient height, is prepared and floated to its position, supported on the decks of two scows or other floats, one on each side. Arrived at the place, the floats are moored, the vertical supports of the plat- form let down to the bottom, securely fastened to the platform, and braced to each other if necessary. This operation being done at the top of the tide, the SCOWS are floated away, when the water falls. The supports to the platform may be placed within ten feet of each other, and the weight of the platform, to insure steadiness and stability, fixed at any desirable standard. The supports may be lashed above and below the platform to the uprights placed for that purpose, and may be wedged also to prevent working. To avoid the interference of the currents with the drills, these may be made to work in hollow iron cylinders, reaching from the platform to the rock. A steam engine or machine worked by hand can then be applied to the drills and the work pushed regardless of the currents. 8 SURVEY AT HELL GATE, EAST RIVER. To remove the rock blasted, it will likewise be necessary to protect the per- sons of tlie divers from the force of the currents, and it is proposed to do this in the following or similar mode : Two cylindrical floats, of sufficient bouyancy, made of boiler iron, are placed say thirty feet from centre to centre. Towards their extremities they are in- clined towards each other until they meet, forming one float, say one hundred and sixty feet long and forty wide. Curtains of iron, extending along the whole outer line of the floats, are permanently fastened at their upper lines to these floats, the lower end of the curtain being attached by falls to davits or small derricks upon the floats, so as to be lowered or raised at will. These curtains are separated into lengths of about twenty feet, that each may be managed in- dependently of the other. The curtains should be flexible, and may be made of plates of iron say one foot wide and twenty long, connected with those in contact by hinge joints. The floats being placed in position and moored securely, the curtains; by sec- tions, are let down to the bottom, their extra depth allowing an extent of several plates to rest upon the rock. An interior space of 160 feet by 40 on the bottom is thus protected, where the divers can fill the buckets, whioh are afterwards raised and emptied into scows, with valve bottoms, lying alongside. The debris is then towed away and de- posited where desirable. There is nothing in these expedients which suggest practical difficulties which skill could not surmount, and confidence is felt that these or similar contrivances can be relied upon to solve this problem. Before work is really undertaken trials should be made and prosecuted until a good result is obtained. There is no doubt felt that the obstacle in the way, viz., the force of the cur- rent, can be neutralized, and it remains only to decide whether the object is worth the money it will cost, a question which it is the privilege of others to de- termine. DATA FOR THE ESTIMATE. The rock in the samples obtained contains an excess of mica, making it soft to the drill. The drill is supposed to bore seven feet a day, though it may av- erage ten feet. The estimated cost of drilling includes mooring and arranging the platform, services of men attending, of blacksmiths sharpening tools, steel expended, services of engineer and firemen, coal used in the smithery and for the engine, services of divers, in arranging the supports of the platform and placing the drilling tubes, labor of unmooring platform, &c. Estimated cost j)cr lineal foot, $4. The cost of each charge includes the estimated cost of drilling the rock, of the canister and charge, labor of divers in placing the charge, services of the operator and boat's crew in firing, expense of wires, &c. The small charges are first placed in the holes and split up the rock ; the larger charges are intro- duced into the crevices and seams thus formed, and complete the work of break- ing up the mass. Estimate for 50 pound charge of gunpowder, $45 50, drilling included. 150 .'" " 41 75. 5 " nitro-glycerine, 43 64, drilling included. 15 " m 35 35. S\\ " " 41 63, drilling included. 11 T V " " 29 29. The sea-wall on the Gridiron is intended to be of large cut blocks, regularly coursed. The top is to be 4 feet above highest water. The general height is 23 feet, at certain points 28 feet, and its thickness is ten feet. Some of the rock SURVEY AT HELL GATE, EAST RIVER. 9 taken from the channel should be deposited behind the wall to the level of low water, to form a backing. This can be done without extra expense. The por- tion of wall to be laid under water is first enclosed by a coffer-work of wood, not water-tight, intended' solely to protect the divers from the force of the cur- rent. This expense is included in the estimate, as well as that of forming a bed for the foundation. The supposed cost will be $80 per cubic yard. The expense of this construction should limit its employment to the most important cases. The rubble sea-walls are composed of stone thrown in loosely to the level of low water, but above that to the top, 4 feet above highest water, a dry wall of split uncut blocks is supposed to be laid by hand. The loose rubble foundation is 12 feet wide at the top, and the slope to the bottom is supposed to be one to one and a half or two. The superstructure is 10 feet thick and 11 feet high. The estimated cost is $4 50 per cubic yard for the rubble, and $24 for the superstructure. A large estimate is necessary for machinery, including scows, boats, anchors, chains, cordage, and liberal allowance for losses, wear and tear, arising from the peculiar nature of the service required. Accidents from collisions, and conse- quent repairs, incident to the strong currents and the number of passing vessels, will probably be frequent, and should be kept in view. In this estimate every machine, scow, or boat is intended for a clearly defined service. The cost from this estimate of blasting and removing each cubic foot of rock is $1 64. Major Fraser in 1853, from the results of the process of removal used by him, found the cost then to be $1 14 per foot, which at present rates would be at least SI 71. But Major Fraser expended nothing for removing the rock by hand, the charge of powder being supposed to do that. The estimate is therefore not in excess, and is kept to this low limit by the contemplated use of the proper machinery. Table. To a depth of 26 To a depth of 24 Subject. ft. at mean Ioav ft. at mean low water. water. Estimated cost of first project s5, 723. 129 15 $5, 008 579 95 Amount saved by using drill, boring into rock at the rate of one inch per minute, including cost of one drilling platform, which might .then be dispensed with. 51,014 00 48,530 00 or less than one per cent. Amount saved by using nitro-glycerine, instead of gun- powder, when its explosive force is estimated at ten times that of the latter 76,430 B0 65,502 00 Amount sated by using nitro-gly -'eriue, instead of gun- powder, when its explosive force is estimated at thirteen times that of the latter 141,745 37 122, 075 30 or T] 3 o per cent. 45 37 ] 12 or less than 2| per cent. The small proportion which an improved process of drilling the rock, or the use of a more powerful explosive agent, will save in the total cost, may surprise mauy ; but the cause is evident, viz., the small proportion of the cost of the whole blasting process +o that of the other items of the project. The table likewise shows that as a question of cost this improvement is inde- pendent of any particular form of improved machinery for drilling, or special explosive agent, leaving to the government full liberty of choice in these re- spects. For the sake of the men employed upon such works no compound 10 SURVEY AT HELL GATE, EAST RIVER. should be used which is liable to explode spontaneously, or from causes beyond the control of ordinary carefulness. Certain conclusions may here be concisely stated : 1. Any reasonable hope of effecting this improvement depends upon the re- jection of misty and fanciful schemes, which cannot be brought within the rules of ordinary calculation. This report has attempted one method of solution, and it is hoped many better may be found. 2. It was necessary in framing the estimate to make just and liberal allow- ances, and to provide for contingencies and accidents attending operations, not only difficult in themselves, but also little assisted by data derived from any source. On the other hand, there is no doubt that a serious and well-considered attempt to push the work will be the means of stimulating mechanical inven- tions suited to this end, which, with the experience gained from day to day, will not only expedite the progress, but also materially diminish the cost, especially in the large item of removing the rock. 3. The precise depth to be attained will be dependent not only upon the present, but upon the future draught of vessels. On this point, it must be con- sidered that this is the only channel leading to New York where additional depth could be gained ; and a liberal standard in fixing this would be best, since if error be committed in the opposite extreme, the cost of remedy would nearly, if not quite, equal that of the first operation. 4. A resurvey of the rocks obstructing the passage, to iix their precise out- lines, is very necessary. The Coast Survey office intend to begin this, it is un- derstood, in the spring. *A marked difference in the character of Prying Pan and the Heeltap Rocks may be seen by comparing the general map sent here- with with the last partial surveys of Mr. W. 8. Edwards, assistant Coast Survey. A modification of the position of the wall on Hog's Back and Holmes Rock may result from a new examination. Respectfully submitted : JOHN NEWTON, Lieut. Col. Engineers and Brevet Major General U. S.A. Brevet Major General A. A. Humphreys, Chief of Eng ineers U. S. Army, Washington, D. C. Tabic showing quantities of rod; to be blasted and remored from certain localities, fcc. Depth increased to 26 feet mean low water. Locality. Pot Rock Frying Pan Ways' Reef and Shelldrake Heel Tap Negro Head and Middle Channel Rock off Negro Point Scaly Rock Hallet's Point Hen and Chicken, Flood Uoek. Gridiron, Negro Head, and Middle Channel 152 180 101 285 :i, 248 21 121 1,391) 6, 808 064 240 707 i 896 736 111 847 | 790 47, G.-iU 41, 48, 27, 74, 893, 5- 33, 850 450 850 250 200 1 550 ! 350 950 1,914,000 387 443 253 685 8, 120 51 303 3, 399 17, 296 320 t 372 I 212 569 6, 821 43 254 2, 937 14, 296 12, 125 12,475 6, 200 23,000 276, 800 1,400 8, 500 ! 73,800 33, 500 51, 675 44, 200 53, 000 1, 456, 400 7, 700 49, 300 841, 030 426,800 4,272,000 SURVEY AT HELL GATE, EAST RIVER. 11 Table showing the quantities of rocl;, 8fe. — Continued. Locality. Pot Rock Frying Pan Wav's Reef and ShelWrake Heel Tap Negro Head and Middle Channel Rock off Negro Point* Scaly Rockt Hallet'a Point , Hen and Chicken, Flood Rock, Gridiron, Negro Head, and Middle Channel Depth increased to 24 feet mean low water. •£ 5 1 1 & I* i w u o S V o < 2 5« o rjl 'Z s 1 .a St u - 152 120 101 * 114 2, 240 1,0G4 840 707 800 15, 080 40, 000 30, 000 27, 850 28, 200 724, 000 368 280 253 264 6, 320 10, 000 9, 000 6, 200 9, 000 196, 000 320 252 212 240 4,704 10,971 30, 000 31,800 29, 500 994, 000 1, 399 9, 790 369, 650 3, 399 73, 000 2, 937 775, 330 5, 800 40, GOO 1,595, 200 14, 500 322, 000 12,180 3, 515, 600 * To obtain a depth of 20 feet at mean low water. tTo cut off its projection into the channel. Estimate. Designation of work and materials FIRST PROJECT— SIX PEARS OPERATIONS, 5, at $41 75 at $45 80 ., at $41 75 Bt, at §1 , at $1 <, viz.. Negro Head, and tin 1 width of tlic middle at $45 -80 s at $41 To To secure a depth To secure a depth of 26 ft. at mean! of 25 ft. at mean low water. low water. 8103,402 20 194,590 75 1,080,405 00 1 48,758 40 208,406 on Blasting- and removing rock, viz : Pot Rock, Frying-pan, Way's Reef, Shelldrake, Heel Tap, rock off Negro Point, Scaly Pock, and Hallet's Point— 2,259 50-poun