The house connections are chiefly of 10-cm. (4-in.) pipes, laid on a minimum gradient of 2-1/2%, from oblique branches on the sewer to siphon intercepting traps near the house, as shown by Fig. 17. From this trap a 10-cm. fire-clay inspection pipe is carried up and capped at the sidewalk level with a cast-iron box having a locked cover. From this inspection pipe a branch is connected to a cast-iron fresh-air inlet, in most cases set in the wall of the house, the inlet being 30 cm. above the level of the pavement. _Effect of the Flood on Sewers._--The flood of August 27th and 28th, 1909, partly destroyed one of the main collectors, which was laid along the banks of the river and encased in concrete. This has now been relaid farther north, and out of the way of any future floods. The total length of the new sewers replacing those damaged amounts to 1200 m., and they vary in internal diameter from 20 to 55.9 cm. (8 to 22 in.). MAIN OUTFALL SEWER. The direction of the main outfall sewer was determined after a thorough study of all the available land lying to the north and northeast of the city, as it was the intention of the Company to utilize for irrigation purposes the sewage and any surplus waters that might be developed. The best available site was found to be about 12 km. north of the city, a little northwest of the village of San Nicolas de los Garzas, as shown on Plate II. The long length of outfall required was justified by the cheap cost of the land and its excellent character for sewage irrigation. The sewer was designed for a capacity of 90,000,000 liters a day (36.76 cu. ft. per sec.) in order to allow for conveying surplus waters as well as sewage. [Illustration: PLATE XXII.--OUTFALL SEWER: PLAN OF GROUND SHOWING SEWER; ALSO DETAILS OF VARIOUS SECTIONS.] The outfall intercepts the two main branches of the city sewers at Calle de Allende and Calle de Tapia, and its total length is approximately 11,900 m. The chief type adopted is shown on Plate XXII. It is formed with an invert of radial bricks laid in 1:2 cement mortar, on a foundation of 1:3:5 concrete approximately 7 cm. thick. As the ground was chiefly in hard sillar, only a little concrete was required to mould the bottom to the correct shape. The arch was formed of special radial bricks, 15 cm. (6 in.) deep, laid in cement mortar. These bricks were adopted in preference to concrete, owing to the heavy cost of sand and rock, due to the long haul, and for the purpose of obtaining rapid work. Plate XXI shows the sewer arch, and one of the ventilating columns and manholes. The bricks were obtained from the local brick plant, and form a very satisfactory material for sewers, being well burnt, thoroughly hard, and absorbing not more than 7-1/2% of their weight of water. The contract prices for the labor on the brickwork were 1.25 pesos per sq. m., and 1.38 pesos for the arch. [Illustration: PLATE XXI, FIG. 1.--VIEW OF ARCH, OUTFALL SEWER.]