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

* The actus is 120 feet, according to Columella and Pliny, as quoted in notes to Vetruvius, p. 170. [Eo.]

PRELIMINARY ESSAY. 11

be the more readily found. But those castellums are to be built neither in the decursions, nor in the plane of the venter, nor in the pressures on the part of the aqueduct where the water is raised by the weight or pressure of the descending water, nor in any of the valleys, but always in the even plane. But when it was required to conduct water at less expense, tubes of earthen-ware were made, having a thickness of not less than two inches, and these tubes were so formed that one end being tongued, the

one entered the other then the joints were cemented with quick lime, tempered with oil. In the descents, level with the venter, a stone of the red kind is to be placed at the angles, so perforated that the last tube of the decursions and first on the plane of