_Aqueduct._--The construction of the concrete conduit was begun in April, 1908. Fig. 13 shows the general types. Type _A_ was adopted in gravel and conglomerate formation, and Type _B_ where the excavation was in "sillar," the soft nature of this rock permitting it to be excavated exactly to the required external diameter of the concrete lining. The concrete which was without steel reinforcement was a 1:2-1/2:3-1/2 mixture, the sand being from the crusher and the aggregate from the river bed, screened to pass a 25-mm. mesh. Where the conduit crossed the river obliquely, immediately below the gravitation tunnel, it was strengthened with mass boulder concrete of Type _C_. During the great flood this heavy section withstood its effects without damage of any kind, but beyond this point, where the conduit had been laid in compact cemented gravels, the scouring action of the flood on the north bank lowered the level of the gravels from 2 to 3 m.; the only damage, however, was the scouring away of the gravels at the south side of the conduit. To prevent such an occurrence in the future, the conduit at that point was strengthened with additional concrete for a distance of 195 m., as shown on Fig. 13. The extra concrete, amounting to 733 cu. m., was a 1:3:5 mixture, in which was embedded 20% of heavy boulders. The top of this special length now forms a weir for the present river flow. Where the conduit enters the bluff on the north side of the river, at 1,200 m., there is an overflow chamber which has a sluice-gate 76 cm. wide, arranged so that the conduit can overflow at the present time when running 76 cm. deep. To deflect the flow in the conduit, a wrought-iron plate, provided with a balance weight, is dropped into a groove on the lower side. The outlet is a 61 cm. concrete tube, having its invert above ordinary flood level, and arranged to be closed by a gate. At 1,963 m. the conduit is carried over an arroyo on a segmental arch of 8 m. clear span, as shown on Fig. 13. There are 5 ventilating columns and 5 manholes on the aqueduct. [Illustration: PLATE X, FIG. 2.--SETTING FORMS FOR SAN GERONIMO CULVERT.] The aqueduct terminates in the Obispado distributing reservoir valve-house, at a level of 558.50 m. The work in connection with this aqueduct was completed by December, 1908. DISTRIBUTING RESERVOIR AT OBISPADO. The main distributing reservoir for the San Geronimo gravity supply is immediately below the historic Obispado (Bishop's Palace), at the western limits of the city. The general arrangement and lay-out is shown on Plate XIII. [Illustration: PLATE XIII.--GENERAL PLAN AND SECTIONS, OBISPADO RESERVOIR.]