:MATERIAL, SIZE. AND SLOPE OF CONDUITS depend largely upon whether the separate or combined system is adopted. Vitrified clay or terra-cotta sewer pipes (see PIPE) are now almost universal ly used for all sewers up to 21 inches. and some times up to 30 inehes in diameter, whether the system be separate or combined, but in the com bined system there is comparatively little oppor tunity for using the smaller sizes of conduit. Cement pipe is also used in a few cities. Where vitrified clay pipe cannot be used, and iron pipe is not required for its greater strength or tighter joints, brick is the material most commonly em ployed. The size of pipe sewers, in the separate sanitary system, ranges from 6 to 30 inches, hut 8 to 21 inches is a more common range. The 6 or 8 inch pipe is used for laterals and for conduits receiving the sewage from a few laterals. Iron sewers may be used up to 5 feet or more in diameter, but they rarely go above 3 feet, and are not often employed in any size, because of cost. There is practically no limit to the size
of brick sewers. In the large combined sewers stone masonry is occasionally and concrete fre quently substituted for brick, particularly in the lower part, or invert, of the sewer, and on heavy grades, where the *emir due to high velocities and street stand and other dirt is likely to wear the brick. Both wood and steel ha ye been used for large out fall sewers, especially for submerged pipe. Crossings beneath streams are frequently made by means of so-called inverted siphons.
Terra-cotta, iron, and wooden pipe are generally round; brick and concrete are given various shapes, depending largely upon the available grade. Where feasible, all large sewers, other than iron, are smaller at the bottom than the top, in order to concentrate the dry-weather flow and diminish the chanees for stoppage.