was but 148.5 feet, while that of the interior of the crown of the conduit itself, if produced to the gatehouse, is 151 feet; consequently at this stage the surface of the river was 2.5 feet below the crown of the 9-foot conduit produced, and the capacity of the conduit was much decreased thereby.

It was therefore proposed to raise the height of the dam from a reference of 148 feet to 150.5 feet, so that at its lowest stage the river would fill the upper part of the 9-foot section of the conduit.

The approved project was to remove the coping stones, raise the height of the dam the desired amount, replace the coping stones, securing them in place by iron bolts and clamps, and strengthen the conduit to withstand the increased pressure caused by raising the dam. The dam was slightly out of level, and it was proposed to correct this as much as practicable while raising the dam, so that when completed its crest would be practically at a uniform height throughout. The project contemplated obtaining all of the stone from the Government quarry at Seneca, Md., which was reopened on March 21, 1895. Quarrying was continued there until December 14, 1895, when it was decided that the depth of "stripping" then necessary (over 30 feet) precluded all idea of obtaining stone from that source economically. Six hundred and fifty-one cubic yards of dimension stone and 17 cubic yards of coping had been quarried at Seneca.

On January 21, 1896, authority was granted by the Chief of Engineers to obtain stone by contract, and after due advertisement the bid of Mr. Asa B. Cook, of Petersburg, Va., for furnishing about 1,100 cubic yards of dimension stone (granite) at $18 per cubic yard, and about 100 cubic yards of coping (granite) at $27 per cubic yard, was accepted, and the contract approved by the Chief of Engineers on March 28, 1896. Time of commencement of contract, April 10, 1896; time of completion, July 1, 1896, extended by authority of the Chief of Engineers to September 15, 1896, and further extended to November 15, 1896. Completed November 7, 1896.

The total length of dam raised was 2,751.1 linear feet, the plan and cross section being as shown on the accompanying tracing, Sheet A.

The first stone was laid on September 30, 1895, and the last stone on November 25, 1896, work having been suspended on account of cold weather from December 14, 1895, to March 23, 1896.

During the working period there were, excluding only Sundays and national holidays, 283 working days, out of which there were lost,

On account of high water in the Potomac River.
For want of stone from contractor...

Days.

84

23

107

Leaving 176 days on which it was possible to work when weather overhead permitted.

The velocity of the water on the crest of the dam was very great, and it was known in advance that whatever the form of cofferdam used, it would be exposed on the upstream side to the pressure due to a head of from 3 to 5 feet. Moreover, the Potomac is subject to large and sudden rises during the working season, and on such occasions carries much driftwood.

After a study of the various conditions affecting the problem, it seemed probable that the best results would be obtained by a cofferdam which would protect the workmen and inclose only so much of the dam as could be raised in a single working day, and, after the concrete and mortar had set, could be taken up and used elsewhere.

The proposed plan, with slight modifications to suit the three different cases, was adopted, and proved very efficient and economical. Against a head of from 3 to 5 feet practically no leakage was developed through any of the joints of the three 15-inch courses of masonry laid with the use of the cofferdam. In view of its small cost and great efficiency, it is believed that a detailed description of the cofferdam, the method of using it, and a statement of its cost will not be without interest.

Description of the cofferdam used in raising the height of the United States dam at Great Falls, Md. (See tracing, Sheet B.)-This cofferdam consists of two horizontal movable wooden bulkheads A B C D), each 30 feet long; the 14-inch round iron bars E E, for holding them in position and resisting the pressure of the water, and the sandbag dams F G and H K, for closing the ends. The bulkheads were made by fastening together five 2 by 12-inch by 30-foot Georgia pine planks with three 2 by 12-inch cleats L L L and 40-penny nails. The joints between the boards were then carefully calked with jute. Each bulkhead was also provided with 2 ringbolts M M, so that it could be handled either with the derrick or suspension cable way. After the riprap stone N N N had been placed to shut off a portion of the water, and the old coping had been removed, holes were drilled in the old masonry for the 14-inch iron bars at the angle shown in the section. These holes were put in to a depth of 24 inches, and just inside the upstream building line, so that the bulkheads would rest on the old masonry-which is the same width as the new. The bulkheads were then put in position, and the joint CD between them and the old masonry having been calked with pieces of old bags, and the sandbag dams F G and H K having been built, the cofferdam was ready for use. By the use of an additional row of sandbags on the inside of the cofferdam the headers and stretchers in the bottom course of the downstream face of the new masonry were laid on a bed free from leakage. These sandbags were then removed and the headers and stretchers in the bottom course of the upstream face of the new masonry were laid in water an inch or two deep, but having practically no current. All the new masoury above the bottom course was laid on a bed entirely free from leakage. This style of cofferdam was used in raising the height of the dam in the Maryland channel for a distance of 460 feet after the weather became too cold for the laborers to go into the water. It does not allow the use of any stones in the upstream face of the bottom course that have quarry faces extending beyond the building line, and it requires the spawling of stones at each 14-inch iron bar so as to get the proper width of dam.

In the Maryland channel, until the water became too cold for laborers to enter it with impunity, the bottom of the bulkhead was kept about 6 inches from the upstream face of the dam by means of two figure-8shaped iron braces, through one of the loops of which passed the iron bars E E, while through the other loop passed a similar iron bar, against which the bulkhead rested.

A row of sandbags was carefully placed so that they would be a little below the top of the old masonry and would make a tight joint with it. On top of this row of sandbags the wooden bulkheads were placed, so as to leave between them and the new masonry sufficient space for the passage of the slight leakage through the cofferdam.

This allowed the use, in the upstream face of the bottom course, of stones having quarry faces projecting several inches beyond the building line. Back of the bulkheads, and partly covering the row of sandbags on which they rested, was placed another row of sandbags. The cofferdam being fixed, the other operations were similar to those described in the first case.

In the Virginia channel there was a 6-inch offset below the third course of masonry (see cross section, Sheet A), and the holes for the round iron bars E E were drilled in this offset 4 inches above the upstream face of the new masonry and to a depth of about 12 inches. The bottom of the bulkhead then rested on the comparatively smooth upper surface of the offset, and the space between the bulkhead and the upstream face of the masonry provided a passage for the escape of the slight leakage through the cofferdam, as in the second case. After the cofferdam was fixed, the other operations were identical with those described in the first case.

The masonry in the portion of the dam across the Virginia channel was much superior to that in the part across the Maryland channel (see cross sections, Sheet A). Owing to this fact and to the existence of the offset already described, operations on this part of the dam were much simplified.

The cofferdam just described was successfully used for raising 2,095 linear feet of dam in the Virginia and Maryland channels, the 656 linear feet across Conns Island requiring no cofferdam.

The entire cost of the cofferdam (wooden bulkheads and sand-bag dams), including all operations connected with setting and calking it, was as follows:

Two thousand three hundred and fifty-two cubic yards of stone and concrete were laid behind the cofferdam, which therefore added 88 cents per cubic yard to the cost of these items. As the cofferdam was used for a distance of 2,095 feet, its average cost per linear foot was 984 cents. To strengthen the conduit against the increased pressure which might be developed on account of raising the dam, the embankments in which the conduit rests were widened, as shown in the following table:

This completed the work of strengthening the embankments and makes a total of 9,870 cubic yards of materials used for this purpose. Of this amount, 5,147 cubic yards were put in place during the fiscal year ending June 30, 1896, of which 3,480 cubic yards were charged to the annual appropriation for preservation, maintenance, and repair of

the Washington Aqueduct, and 1,667 cubic yards to the appropriation for raising the dam and strengthening the conduit.

On July 29, 1895, Mr. John K. Cowen, president of the trustees of the Chesapeake and Ohio Canal, addressed a letter to the Secretary of War, asking that the trustees of the Chesapeake and Ohio Canal be given a hearing before any final instructions were given in regard to raising the dam at Great Falls.

This communication was referred to the Chief of Engineers, who, on August 2, recommended that "an officer of the Corps of Engineers be designated to meet an engineer to be selected by the canal authorities, to form a commission to report on this question between the United States and the canal at once. If this recommendation be approved Maj. H.M.Adams, Corps of Engineers, is suggested to act for the United States."

This recommendation of the Chief of Engineers was approved by the Secretary of War on August 5, 1895, and Mr. Cowen was notified thereof by letter from the Secretary of War, dated August 6, 1895.

By letter of August 7, 1895, to the Secretary of War, Mr. Cowen stated that the trustees of the Chesapeake and Ohio Canal designated Mr. G. L. Nicolson, general manager, to meet Maj. H. M. Adams, as suggested in the recommendation of the Chief of Engineers.

The two commissioners after several meetings recommended "that the canal authorities release the United States from all claims for damages to the canal embankment and towpath by reason of raising the dam 24 feet as authorized by act of March 2, 1895, and that the United States pay the estimated cost of the protection, $15,000; the work of raising the dam to be proceeded with at once."

By indorsement of the Secretary of War, dated September 7, 1895, the Attorney-General of the United States was requested to give his opinion as to whether the amount could be paid as recommended by the commission in their report of August 16, 1895.

By letter dated September 14, 1895, the Attorney-General of the United States stated that he was of the opinion that the Department was authorized to pay the $15,000 as recommended by the commission and the Chief of Engineers from the money appropriated by the act of March 2, 1895.

In view of this opinion, the payment of this sum as recommended was authorized by the Secretary of War by indorsement of September 23, 1895.

On September 26, 1895, the officer in charge of the Washington Aqueduct notified the general manager of the Chesapeake and Ohio Canal Company that he was authorized to pay the company $15,000, upon the condition that the canal authorities should release the United States from all claims for damages to the canal embankment and tow. path by reason of raising the dam 2 feet, as authorized by act of March 2, 1895, and requested that the trustees of the company execute an inclosed release to this effect and formally authorize one of their number or the general manager to receive the sum of $15,000, and that such release and written authority be transmitted to this office, accompanied by evidence of the authority of the trustees of the canal company to act in matters relating to the canal.

On September 27, 1895, the general manager of the canal company stated that the matter had been referred to the trustees.

On February 13, 1897, the proper release and vouchers having been signed by the trustees, the award of $15,000 was paid to the canal company.

The following table embraces the total expenditures from the appro