The entrance of the water into the aqueduct was regulated by a vane, or sliding valve, of a certain dimension, as only a certain quantity was allowed to flow into each branch. This does not appear ever to have exceeded 21 inches of elevation, which was sufficient to fill all the syphons ; without this precaution the volume of Avater, which might have risen to 4 feet, would have been too great. probable that the regulating vane or It is

sluice could be raised, or lowered, at pleasure to allow for the increase of head, and that

the superfluous quantity was at times permitted to flow into some other channel.

Where the aqueduct was carried above ground, it was buiH on a footing of masonry 6 feet thick, even where the elevation above the surface did not exceed 6 or 7 feet ; but when it was greater, arches were formed, and also piers, when the elevation was considerable on this elevation depended the span of the arches, the thickness of the piers, and ;

their height. For an opening of 18 feet in height the width is 12 feet, and the piers 6

PRELIMINARY ESSAY. 37

feet, sustaining a semi -circular ; when the inequality of the surface did not allow the piers to have an equal height of 18 feet to each opening, the piers were shortened, and the other parts remained of the same general dimensions. The piers of the arches in some places, are rather less on the face than 6 feet, varying from 5 feet 8 inches to 6 feet ;