pipes are not to be less than ten Roman feet in length, and were named from the breadth of the lead before it was rounded into a pipe.

The manner of conducting water by pipes is thus regulated ; if the spring head had

a sufficient current to the city, and no higher hills intervened, the interval is, by

walling, raised to a proper level, as mentioned in the description of channels of masonry, or else a circuit round may be taken if not very long but if there be frequent ;

valleys, the courses are to be directed down the declivities, and when arrived at the bottom, a sub-structure is to be built, but not high, that the libramentum, (or level, or counterpoise,) may be as long as possible this will be the venter. "When arrived at the opposite declivity, as on account of the length of the venter, the water swells gently, it is

pressed upward to thetopof the ascent whereas if the venter should not be made in the ;

valley, nor a sub-structure built level, but should be bent, the joints would be destroyed. In the venter, also, columnarics. are to be raised, through which the force of the vapors

may be dissipated. These columnariae are supposed to have been always open at the top, and to reach above the level of the aqueduct. It was not unusual, when the level from the spring head to the city was obtained, to erect a castellum at every 200 actus* distance, that if damage should happen at any place, the whole work needed not to be taken down, and that the defective part might