placed against the dam, and against the sheet pile last driven, a man in armor should go down and drive the spikes so as to hold it in position. This being completed, the next operation will be to fill up the inside of the dam so as to make it water-tight. I have supposed that we will use concrete placed in bags to fill it up to the low-water level, though it is altogether likely some cheaper material may be found to answer. If bags of concrete are used, they should be about half filled, and of such a size as not to weigh more than fifty or seventy-five pounds when immersed in the water. When ready to be put in place, a man in armor would go down into one of the compartments of the dam and place them in the crevices of the rock, taking care to fill up solidly the spaces under the timbers, particularly below the timbers which connect the inside with the outside wall of the dam, under which there would otherwise be leakage. To give room for this operation it might be better to frame these lower timbers, say six inches above the bottom of the outer and inner timbers, though they are not so represented in the drawing. Having these spaces under the timbers securely filled, all the men down there would have to do would be to kick the concrete bags into place and walk over them as they were lowered down until the compartment was filled. The concrete, if made of good cement, would soon harden and the cement would come through the bags so as to unite them and make a water-tight mass. I would recommend that this concrete be placed in layers of about two feet in thickness, carrying it up a layer of about that thickness each day. The concrete being all in position, the upper portion of the dam is to be filled up with clay or some other water-tight material. The middle compartment of the dam is to be kept open, and it is supposed when the dam has been finished in the manner described, that this compartment will be practically water-tight. All that is necessary therefore to reach the surface of the rock within this space, will be to pump the water out of it. If the dam is found not to be entirely water tight, it may easily be made so by calking between the inside sheeting piles, and between their lower ends and the surface of the rock. We are now prepared to sink a shaft into the rock. I have supposed it to be four feet by nine feet in the clear, leaving a ledge of twelve inches between its edges and the face of the sheeting piles on the inside. I suppose that this shaft will be sunk so that its bottom will be in the reference of thirty-six feet, and at this level small tunnels will be run under the rock, as shown in plan in Fig. 1, and in section in Figs. 2 and 3. At the ends of, and in these tunnels, fifty-five separate chambers for powder will be placed, as shown in plan, and connected by insulated wires with the battery or machine for exploding them. Water will then be admitted through the sluice for tamping, and the whole space filled, pumping water into the middle compartment until it is filled to the top or reference of ten feet. The mines may be fired simultaneously by the use of about three of Beardslee's machines. In consequence of the great depth of the charges, I would recommend that the explosion take place at low water. I propose so to adjust the charges in the chambers, that all the rock above the reference of twenty-four feet will be blown up, shattered, and broken to pieces. Much of it will fall back again, but the currents will rapidly remove a great part of it, and the remainder may then be scraped off into deep water, sending down men in armor when necessary to fasten to pieces that may prove too heavy to be scraped off. The quantity of powder that would be necessary to lift the rock and the water above the mines would be about thirteen thousand pounds, but as it will be desirable not only to lift the rock, but to break it up into small pieces, and blow it away as much as possible, I would recommend that about double this quantity of powder be used, say twenty-six thousand pounds; the quantity of powder in each chamber being proportioned to the weight it will have to lift, or to the work it will have to do. ESTIMATED COST. Blasting off and removing top of rock to receive the cofferdam, 150 cubic yards, at $40.. Building coffer-dam, 61,500 feet of timber, at $40 per M, including workmanship and materials.. Cost of placing frame-work in position, say Cost of sheeting piles, 20,300 feet, board measure, at $40, Cost of placing the same, say $1 per piece.. Cost of planking over the dam, joists and covering, 5,000 Cost of shaft with enlargement at bottom, 40 cubic yards, at $10.... Cost of tunnels and powder chambers, 778 running feet, at $15.. Cost of powder, say 26,000 pounds, at 10 cents. Cost of 55 packages, and placing the same, at $20. Cost of pump and fixtures.. Cost of derrick and rigging Add for contingencies, including barrack and smith's shop over the dam, use of boats, superintendence and unforeseen expenses, say 20 per cent. Total, (in coin)... I do not make any estimate of the cost of removing the broken rock after it has been blown to pieces, because it is impossible beforehand to tell what operations will be necessary in order to accomplish this object If the rock is anything like that at Lime Point, and if it is shattered like the blasts there have shattered that rock, the currents in the course of a year would remove the greater portion of it. But if it should be a stronger rock, and come out in larger masses, it would have to be removed by mechanical operations and at considerable cost, which, however, would be far less by having the whole rock broken to pieces, down to the required depth, than would be the case if the rock had to be blown up by piece-meal, involving a removal of the debris after each successive series of blasts. B. L. ALEXANDER, Lieut. Col. Engineers, Bvt. Brig. Gen. U. S. A. SAN FRANCISCO, November 20, 1868. SIR: I have the honor herewith to submit to you a plan and descriptive specifications for the removal of Blossom Rock, in the Bay of San Francisco, California. Blossom Rock, so called, is a sunken rock so well known that I consider a description of its locality unnecessary for these specifications, and shall, therefore, proceed with explaining my plan for removing the same; reference being had to the annexed plan, in which Plan A represents the mode and manner of constructing the necessary machinery for working out the interior of the rock. No. 1 represents the longitudinal section through c d. No. 2 represents transverse sections of rock through K L as represented on the plat of survey made by Edward Cordell, assistant United States Coast Survey, February, 1867. The first object in view is a thorough removal of the rock, so that vessels drawing twenty-four feet of water will be able to pass over the same at low tide. To accomplish this, the mere blasting and breaking up of the rock would not, in my opinion, accomplish the desired object, as the rock in broken masses would still form an obstruction to the navigation of the harbor. I have, therefore, matured a plan by which the entire rock itself shall be excavated in chambers, as shown by the accompanying plan marked A, Nos. 1 and 2. The rock taken out of the interior copartments will be removed through a shaft and discharged into deep water alongside the rock; and when the whole inside of the rock shall have been removed I propose, finally, to blast the crust over the chamber, and drop the same to the bottom of the excavation. To accomplish this, the following mechanical arrangements will be necessary. I make my lodgment on the highest point of the rock, which is nearly or quite level at this point, being five feet under low-water line, and having a sufficient area for the works necessary to be constructed. I moor a scow for working purposes in the position required; I then place a boiler-iron case, nine feet in diameter and thirteen feet high, with flanges on the lower end, on the top of the rock. This flange has a canvas apron three feet wide running entirely around it, and lying on the rock. I then place a lot of sand bags on the top of the canvas apron, and around the case; and to secure the case I use one and a half inch round iron rods, the ends of which are firmly secured into the rock by "Lewis holes," turnbuckles are also placed on each rod for the purpose of tightening them up from all sides. After this is done I pump out what water there is in this case, and make an excavation into the rock downwards, for the purpose of erecting the main case, which is of boileriron, six feet in diameter, and seventeen feet in height. This is set within the outside casing, and also firmly set into the rock and secured with anchor bolts, in the same manner as with the first case. After this is done I fill all cracks between the casing and the rock with Roman cement, excluding the water from the interior iron casing. When it is found that the leakage has been thoroughly stopped, and the platform erected as represented in the plan, I commence sinking a shaft down through and into the main body of the rock to the depth required; at which point I place a pump for removing such water as may be encountered in carrying on the work. From the bottom of the shaft I commence tunneling the rock in all directions. As fast as the rock is worked it is brought to the center shaft through the different tunnels and placed in a tub, when it is hoisted by steam and discharged alongside the rock into deep water by means of the swinging derrick. The rock that will remain between the excavation and the water will be about six feet in thickness, supported by pillars four feet square and ten feet apart from center to center. It is proposed to remove most of these pillars when the rock shall have been tunnelled and to set up in their place wooden supports as often as may be required to sustain the weight over head. When this entire mass of rock shall have been excavated the works will be in readiness to receive the several packages of powder, in such quantities as shall be thought necessary, the wires being laid to the several torpedoes and connecting with a magnetic battery placed in a vessel near by. The chamber is to be filled with water and the torpedoes fired by the battery simultaneously; when it is supposed that the entire shell of rock will be broken into small pieces and be precipitated to the bottom, the timber supports floating out, and the work completed in all its details. All of which is most respectfully submitted, A. W. VÓN SCHMIDT, Lieut. Col. W. R. WILLIAMSON, U. S. A., Civil Engineer. SAN FRANCISCO, November 20, 1868. SIR: I have the honor herewith to submit to you a plan for the removal of Blossom Rock, together with a description of doing the work. I will remove the said rock, in accordance with the plan as proposed by me, in the space of eighteen months from the time of signing the contract, for the sum of seventy-five thousand dollars, currency of the United States, giving such bonds as shall be required for the faithful performance of the work. Yours, respectfully, A. W. VON. SCHMIDT. Lieut. Col. W. R. WILLIAMSON, U. S. A., The SAN FRANCISCO, November 23, 1868. GENERAL: In my letter to you of September 22, I had the honor to express the opinion that the only way of removing Blossom Rock was by drilling deep holes in it, in which to place explosive materials. recent experiments on John Day Rock, in the Upper Columbia, showed the practicability of that method, and the frame devised by Mr. Heuer was successfully used; nevertheless, the method of drilling was tedious and expensive, and, as stated in that letter, the rock should be removed to more than eighteen feet from low water mark. The recent duty to which I have been assigned on the Pacific railroad, and the usual heavy rains in December and January, have decided me to defer the commencement of operations for two or three months; and recently other methods of removing the rock have been suggested, which I consider worthy of serious attention. General Alexander has sent me a project with drawings, for removing the rock, which I inclose. It consists essentially of placing a cofferdam of timber over the rock, pumping out the inclosed water and sinking a vertical shaft, from the bottom of which horizontal radial chambers are excavated for the reception of powder. Very heavy charges then being placed under the rock, it is presumed their explosion will shatter and break it up, and the fragments are to be removed by dredg ing or otherwise. His estimate for the excavations aud blasting is $41,132 in coin, equivalent to $56,345 in United States legal tender notes at 73 cents, their present price in this market, but it does not include the cost of removing the debris, for which no data was at hand. A second project has also been submitted to me by Mr. A. W. Von Schmidt, a prominent civil engineer of this city, which is also inclosed with this. It consists of placing on the rock a cylinder of boiler iron to serve the same purpose as the coffer-dam of General Alexander, and then excavating the rock in such a way that, when completed, there will remain only a shell; then, by means of heavy charges of powder, the shell can be broken up, and the debris falling down will be so small that twenty-four feet of water will be over the rock after the explosions. Mr. Von Schmidt offers to remove the rock, according to his plan, for $75,000 in currency, and give satisfactory and sufficient bonds for the performance of his contract. I have reason to believe that other parties with other plans, not materially different from those above mentioned, would send in proposals, if invited, but I doubt if any one could come within the limit of the amount appropriated. I am now impressed with the idea, from a careful investigation of all the plans submitted, that the most satisfactory way of removing this rock is by a plan similar in design to those of General Alexander and Mr. Von Schmidt, rather than by the method of drilling and blasting first recommended. I have not the authority to make a contract to remove the rock for a specified sum, and the amount appropriated is not sufficient, I therefore refer the whole subject to you, and respectfully ask for instructions. I also request that an appropriation of $30,000, in addition to the former one, be asked for at the next session of Congress, to secure the removal of this rock, the estimate being based upon the amount of the proposal made by Mr. Von Schmidt, increased by $5,000 for surveys and contingencies. I have the honor to be, very respectfully, your obedient servant, Maj. Gen. A. A. HUMPHREYS, Chief of Engineers U. S. A., Washington, D. C. APPENDIX W. Report of Brevet Brigadier General N. Michler, in charge of public buildings, grounds, and works in the City of Washington, D. C. OFFICE OF PUBLIC BUILDINGS, GROUNDS, AND WORKS, U. S. Capitol, Washington City, D. C., September 30, 1869. GENERAL: The following statement in regard to the different duties which have been assigned me, and the progress which has been made in their execution during the fiscal year ending the 30th of June, is respectfully submitted for your information: RIVER CHANNEL IMPROVEMENTS. The system of improvements proposed for the channels of the Potomac in a special report upon the subject, made after a most careful survey and examination, has not yet received the consideration of the general government. |