across this valley by iron pipes, why construct this expensive bridge ? The reply is, that a plan was prepared, and the work put under contract, to construct a low bridge with one arch for water way ; but a supposed value which was attached to the future navigation of the river, was so pressed upon the Legislature, as to induce them to pass a law, requiring that the under side of the arches should be 100 feet above ordinary high tide in the river. The law, therefore, and not the otherwise necessity of the case, has controlled the plan for crossing this valley. MANHATTAN VALLEY. The water is conveyed across this valley by means of iron pipes. A gate cham- ber is placed on each side, by which a connection is formed between the conduit of masonry and the iron pipes, and gates prepared for regulating the flow of water in the same manner as before described for Harlem River. The width of the valley is 0.7917 miles from gate chamber to gate chamber, and the depth at which the pipes are laid in the centre is 102 feet. Two cast iron pipes, each three feet in diameter, are laid down, and provision made for two more, to be laid when they shall be required for the wants of the city. In order to give the pipes sufficient capacity to discharge the quantity of water re- quired, an extra head of three feet is given in crossing the valley. CLENDENING BRIDGE This bridge is constructed over a valley of the same name, that is situated about equally distant between Manhattan valley and the receiving reservoir. The greatest depression is 50 feet, below top of aqueduct, and the valley is 1900 feet across. The line of aqueduct runs 100 feet westerly of, and parallel to, the 9th Avenue. In the plan of the city, streets are laid out, but not yet opened, that cross the aqueduct at right angles. For three of the streets archways are constructed under the aqueduct for their accommodation when they may be opened for use. The archways for each street are, one for carriage-way of 30 feet span, and an arch on each side for side walks of 10j feet span. The style of masonry is similar to that before described for the Sing Sing bridge, and the same method of open walls arid cast iron lining is also adopted. That part of the bridge which has no provision for street arches, is composed of a continuous wall of masonry, carried up on a bevel of one-twelfth its rise to the grade line of the aque- duct, where it is 30 feet wide ; the outside or face of this wall for one foot in breadth, is laid in hydraulic mortar, and the remainder is laid dry. The method of laying up this dry masonry was to lay a course of large stone in such a manner as to give them the great- est solidity, and within a few inches of each other; after the course was laid over the work, the interstices were thoroughly filled with small broken stone, well pounded in, and levelled up to receive the succeeding course. (The same plan was adopted in all the dry foundations which were laid up to support the conduit over low grounds.) After the foundation was carried up to the proper height for the conduit masonry, parapets of hy- draulic masonry were carried up to the level of the roofing arch of conduit, between which the conduit masonry of the usual form (excepting the side walls, which were 12 inches ex* 212 MEMOIR OF THE tra thickness) has been constructed, and covered with earth to protect the sides and top from frost ; the earth covering being secured by turfing carried over from parapet to para- pet. The work presents a very substantial and finished appearance. RECEIVING RESERVOIR. — This reservoir is 1826 feet long and 836 feet wide, and in- cluding its embankments contains 35.05 acres, and its area at top water line, 31 acres, di- vided into two divisions ; the northern division is designed to contain 20 feet depth of water, and the southern 30 feet depth. But they are not fully excavated in some parts, where rock occurs, it not being deemed sufficiently important to incur the expense of ex- cavation in rock for the increased capacity that would be obtained. The reservoir has a capacity of 150,000,000 imperial gallons, as it now stands. The reservoir is formed with earth banks, the interior having regular puddled walls to render them impervious to water ; the outside protected by a stone wall, laid up on a slope of one horizontal to three vertical, the face laid in cement mortar, and the inside