locality. The quantity required for this purpose has somewhat exceeded our expectations, although no definite estimate on this point could have been made, until the character of the rock was shown in detail by the excavations. The puddle trench is in the embankments, for the entire circuit, and, consequently, extends to eight thousand three hundred feet in length. Throughout its extent, it is connected, either immediately, or by the concrete, with the solid rock, which extends over the entire space occupied by the reservoir, and forms the bottom thereof.

The concrete has been so used as to prevent the possibility of any minute threads of water working through the seams of the rock, and coming in contact with the puddle. The material for the puddle has been obtained within the reservoir basin, and is composed of clay and sand, in such proportions as to make a firm clay mortar impervious to water. The puddle has been made in its place, by working the materials in the manner required by the specifications, into a solid mass, extending from the rock bottom throughout the length and height of the embankments.

The embankments have been made as described in the specifications in layers of six inches, compacted by heavy rollers, and each layer connected with the one on which it rests. The materials, consisting of clayey earth, sand and loam, have been selected with great care, and disposed to different parts of the work, according to quality and character.

Whatever has been unsuitable for the construction of the embankments has been placed outside of the work.

Since the commencement of the work on the new reservoir, it has been deemed advisable to add two rows of sheet piling at the foot of the slopes of the central bank, wherever the rock does not approach near the surface.

When completed, the reservoir will be inclosed by a compact earth embankment, connected everywhere with the rock bottom by the puddle bank, and lined on the whole of the inner face by a heavy stone slope wall, capable of resisting any waves to which it may be subjected, and impenetrable by any animals whatever. Every part of the work on this reservoir has been under the daily scrutiny of the engineer of the department, and the constant watch of the inspectors appointed to see that the specifications be strictly adhered to.

Thus far these specifications have been fully carried out, and the Board feel well assured that if the work be prosecuted to completion with equal care and exactness, it will prove all that can be asked for on the score of security and permanence.

The embankments, as compared with those of the existing receiving reservoir, present greater strength for the same and relative depth of water, which is the proper function for comparison of the resistance to be overcome. The receiving reservoir has stood sufficiently long to test its strength, and it has shown no symptom of decay, nor indications of weakness.

The foundations of the gate-houses are in an advanced state of progress, and will be ready for the masonry in the spring. The contract for the masonry for the gate. houses, which are connected with the embankments, was

awarded to competent contractors on the 26th day of October, 1858, and sent to the Board of Aldermen for confirmation on the 28th day of October, 1858. It is deemed of the utmost importance that the contract should be confirmed as soon as possible, in order that the work may proceed. Should it not be soon perfected, the delay will prevent the preparation of the materials during the winter, which is requisite, in order that the masonry may be commenced as early as possible in April next. Such delay will also prevent the contractors for the embankments finishing their work, and necessarily give them reason for demanding an extension of their time for its completion.

Certificates to the Comptroller have been issued by the President in favor of Messrs. Fairchild, Walker & Co., for the following sums, the deduction of twenty per cent, provided for in the contract, having first been made:

Under the head of Croton Water Works' extension, is also included the topographical survey of the Croton valley. This survey, which was commenced during the summer of 1857, and prosecuted up to December of that year, was resumed in May last, and completed in November. The engineer in charge of the party during this season

has been Mr. George A. Cushing, and under his more immediate command the surveys have been prosecuted.

As stated in our last annual report, the object of this survey is to establish suitable points for storage reservoirs, and to obtain further hydraulic information, which will be useful in extending the work when necessary.

The party engaged in this survey have, during the two seasons, run and closely examined lines, amounting in length to 480 miles, have made instrumental examinations of sixteen sites for reservoirs, and obtained all the information necessary to estimate the cost and capacity of each, and to enable the Board to make a proper selection from among them.

The length of Ridge line, inclosing the valley, or defining the water shed of the Croton river, above the point at which our Aqueduct begins, has been found to be one hundred and one miles. Within this circuit there are thirty-one natural lakes and ponds. The length of the main stream of the Croton river, above the Croton dam, is thirty-nine miles, and the aggregate length of all the principal tributaries is one hundred and thirty-six miles.

The total area or water shed of the Croton, above the commencement of the Aqueduct, is ascertained to be three hundred and fifty-two square miles. This is a vast area compared with the supply of water yielded during the dry seasons, and presents an opportunity for an interesting comparison. By the guages of the Croton river, made before the aqueduct was undertaken, the minimum. flow of water during the year was found to be 33,804,000 New York gallons in twenty-four hours, which gives

ninety six thousand and thirty-four New York gallons per square mile of water shed in the driest season.

The water shed of the Brooklyn Water Works is set down in the engineer's report as about thirty-five square miles. From this area the guages at the driest season show a minimum discharge of 20,279,958 New York gallons in twenty-four hours, which gives five hundred and seventynine thousand four hundred and twenty-seven New York gallons per square mile during the driest season.

It will thus be seen that the minimum discharge or supply of water per square mile, on this section of Long Island, is a little more than six times (6.033) as great as it is in the Croton valley. This inequality is not due to any material difference in the rain fall of the two localities.

By a record of the rain and snow fall from the year 1840 to 1853 inclusive, kept by Mr. J. F. Jenkins teacher of the academy at North Salem, Westchester County, we find that the average annual rain-fall for fourteen years was 42.68 inches. By the register of rain and snow guage at Erasmus Hall, Flatbush, L. I., we find the annual average fall for the same period of time to be 41.32 inches. The difference between the two is slight, but if taken into account at all, would still further increase the disproportion in the minimum supply, inasmuch as the annual fall in the Croton Valley is the greater of the two.

The great cause of the difference in the discharge above mentioned is to be found in the geological character of the two localities. Within the Croton basin or water