_Valve-House._--The invert of the conduit from San Geronimo, where it enters the valve-house, is 558.50 m. above datum. The valve-house, which is built in the center of the reservoir, is shown on Fig. 2, Plate XVIII. One of its special features is the provision of the main overflow at this point instead of within the reservoir proper. The inlet, main supply tunnel, independent by-pass overflow, scour-out pipes, gate-valves, and screens, are all controlled within the valve-house. [Illustration: PLATE XVIII, FIG. 2.--VIEW OF ROOF OF OBISPADO RESERVOIR, LOOKING NORTH.] _Reservoir._--The reservoir is rectangular, 126 by 81 m. (413.28 by 265.68 ft.) at the top, and has a water depth of 4 m. (13.1 ft.). In the design it was necessary to limit it to the lowest economical depth, so as to increase the static pressure over the low-pressure district as much as possible. _Excavation and Embankment._--The excavation, except for a depth of about 1 m. which was in black soil, was chiefly in a disintegrated "sillar" stratum of a heavy clayey nature, the greater part of which could be handled conveniently with plows and scrapers; the actual foundation on the eastern half required blasting for the final depths. The total excavation amounted to 56,479 cu. m., of which 7,255 cu. m. were placed in the embankment, the remainder being deposited in the immediate neighborhood of the reservoir. The final trimming of the banks, which were left 30 cm. full, was not undertaken until the lining was begun. The work was done under contract with Mr. J. S. Nickerson, of Monterrey. The excavation had only one classification, and the contract prices were 0.50 peso per cu. m. for material carried to spoil banks, and 1.00 peso for material placed in the embankment. The excavation was begun in December, 1907, and completed in April, 1908. The work was then left standing until the end of 1908 to allow the banks to consolidate thoroughly prior to lining, which was begun on January 4th, 1909. _Concrete Lining and Roof._--Plate XIII shows the general plan and sections, the main feature being the simple division of the reservoir into 24 rows of columns longitudinally and 15 rows transversely, making a total of 360 columns, less the four left out at the central tower. All the columns are 5 m. apart both ways. The roof was designed for a live load of 100 lb. and a dead load of 150 lb., the same as at the South Reservoir. With the exception of the floor, all the concrete work was reinforced with twisted steel lug bars. The foundation load on the columns for the eastern half of the reservoir is 0.9 ton per sq. ft.; that on the columns for the western half, where the foundation is of very hard sillar and conglomerate, is 1.95 tons per sq. ft.