Condition of the water at Great Falls, Dalecarlia Reservoir and distributing reservoir, and height of water over dam at Great Falls for each day during the year-Continued.

According to the scale for recording the condition of the water as regards clearness, the numbers 0 to 7 inclusive, correspond to very turbid; 8 to 14 turbid; 15 to 21 slightly turbid; and 22 to 36 clear. The following table shows the condition of the water at various parts of the system during the year:

CCC 2.

INCREASING THE WATER SUPPLY OF WASHINGTON, DISTRICT OF COLUMBIA.

This work is being prosecuted under an act making appropriations to provide for the expenses of the government of the District of Columbia for the fiscal year ending June 30, 1899, and for other purposes, approved June 30, 1898.

The clause of this act making appropriations for this work is as follows:

Washington Aqueduct Tunnel: The Secretary of War is hereby authorized and directed to resume work on the Washington Aqueduct Tunnel and its accessories and the Howard University Reservoir, authorized by section two of the act approved July fifteenth, eighteen hundred and eighty-two, entitled "An act to increase the water supply of the city of Washington, and for other purposes," and to prosecute and complete the same; the work on the said tunnel and accessories to be carried on in accordance with the plans of the board of experts as set forth in its report dated January seventeenth, eighteen hundred and ninety-six, House Document numbered one hundred and sixty-six, Fifty-fourth Congress, first session, which plans have been approved by the Chief of Engineers and the Secretary of War. And to carry out the provisions of said act and this paragraph, the balance remaining unexpended from the appropriations made by the said act of July fifteenth, eighteen hundred and eighty-two, and by subsequent acts for said purpose, amounting to two hundred and ninety-seven thousand two hundred and ten dollars and fifty cents, is hereby reappropriated, to be advanced out of the revenues of the United States, and not subject to the conditions of the capital account created by the act of July fifteenth, eighteen hundred and eighty-two, and the sum of two hundred and ninety-seven thousand two hundred and ten dollars and fifty cents is also hereby appropriated out of the surplus general revenues of the District of Columbia, to be applied to suen parts of the work and in such order as to time as the Secretary of War may deem necessary to promote as soon as practicable the completion of the entire system of said works: Provided, That the sums herein set apart and appropriated shall be immediately available and shall be expended under the direction of the Secretary of War and the supervision of the Chief of Engineers; and the work shall be carried on by contract or otherwise, as the Secretary of War may deem best for the public interest.

The work is to be carried on, in compliance with the above act, "in accordance with the plans of the Board of experts as set forth in its report, dated January 17, 1896."

That portion of the report referred to which contains the plans of the Board of experts is as follows:

PERCOLATION.

One of the objections against the tunnel that has been urged with much force is the liability of leakage or percolation outward from the tunnel into the ground when it is subjected to the pressure of the water. This is a point to which we have devoted much time and attention. It is evident that should the leakage be sufficiently great the dangers to be apprehended from this source might be considerable. These possible dangers, however, are materially lessened by a consideration of the fact that from all the information we have been able to collect, it appears that for a considerable portion of the tunnel the natural level of the water in the ground is not far different from that of the hydraulic grade line of the water in the tunnel.

Since this tunnel was abandoned the new Croton Aqueduct, supplying the city of New York with water, has been completed. A portion of this aqueduct, embracing a length of about 7 miles, is subjected to internal pressure with a maximum pressure due to a head of about 130 feet. In this case no serious percolation has been observed, although the surface of the ground is at several points much below the hydraulic grade line. An experiment made for the purpose of determining the amount of this leakage outward from the aqueduct showed that the total loss in the whole 7 miles from this cause did not exceed 225,000 gallons in twenty-four hours.

With this practical example before the commission, and guided by our own judg

ment and experience, we believe that if the spaces now existing between the brickwork and the rock are thoroughly filled with rubble masonry and grouted, that the tunnel can be made sufficiently water-tight, and that the leakage under a head of 150 feet will not be large enough to prove an obstacle in the way of the successful completion and operation of the tunnel. In this connection we recommend that the interior surface of the whole tunnel should receive a double coating or wash of pure Portland cement. This treatment will aid materially in making the brickwork more impervious to the passage of water.

Objection has also been urged against the tunnel on the ground of the danger of contamination to the supply from the percolation of ground water into the tunnel when it is empty. This condition can occur but seldom, perhaps but once or twice a year at the most, when the tunnel is pumped out for examination. The amount of this percolation is very slight, even under present conditions, with the lining practically open. While our examinations of the tunnel were under way, samples of water at various points were collected for analysis, and not without some difficulty on account of the small amount of water flowing. These analyses do not indicate that anything of a contaminating nature passes into the tunnel when it is empty.

If the tunnel is completed in accordance with our recommendations, the percolation will be so trifling as to be unimportant, especially when taken in connection with the general absence of organic matter in the wells of those portions of the city which are much more thickly populated than is the ground directly over the tunnel.

SILTING.

An

It is known that at times the Potomac water contains a large amount of silt. idea of the amount which may be expected to deposit in the tunnel may be formed from the results of sedimentation which has taken place in the past in the present aqueduct, where it has not been the source of serious trouble. If these deposits occur they should not be allowed to accumulate, but should be removed from time to time, and this can be readily done by means of the four shafts retained and which will be equipped with the necessary hoisting machinery to accomplish this object.

COMPLETION OF THE MAIN TUNNEL.

A portion of the tunnel, about 6,000 feet in length, now contains no lining, and the remainder is provided with a lining of varying section, as shown on one of the accompanying plates. Our estimate for the completion of the tunnel proper is, therefore, naturally divided into two parts.

We have estimated for completing the unlined portions of the tunnel with brickwork of the same form as that used in the lined portions which are provided with an invert. There are large quantities of loose rock in the tunnel at these points, which can be used for the rubble-masonry backing.

Our examination of those portions of the tunnel already lined, taken in connection with the published testimony and supplemented by the statements of others whom we have called before us, shows that there is a great diversity in the character of the lining. In some portions of the tunnel vertical rubble side walls have been built. Where these occur we have been unable to find any void spaces of magnitude behind the walls. In other cases the side walls are built of brick, generally filled on the outside to the crown of the arch with loose packing. Some portions contain a brick invert. As a general rule, empty spaces exist over the crown of the arch. In the westerly part of the tunnel these spaces are comparatively small, but in the easterly portion they are much larger, in some cases extending to more than 10 feet in height. These cavities must be filled with rubble masonry, laid solidly in cement mortar. In some cases timbering exists in the spaces over the arch. The timbering should be taken out in sections and rubble masonry substituted. Where the spaces will not allow of laying rubble masonry, provision has been made for filling the spaces with stone packed by hand and then grouting the whole mass with cement under pressure. In other words, it is intended to fill every space or void now existing in these portions of the tunnel either with cement, grouted rubble, or with rubble masonry laid in cement, thus securing a solid backing everywhere between the lining and the rock.

Where no invert now exists, we recommend a slight excavation of the rock and the building of an invert of Portland cement concrete, smoothly finished.

REENFORCEMENT OF THE ROCK CREEK SECTION.

Experiments made by Major Knight in 1895, and detailed in his report upon this subject, indicate that the covering of rock over the tunnel in the Rock Creek Valley is not sufficient to provide the necessary resistances required, or to prevent serious