the one to the other, preserved the same dimension throughout. Delorme says that the water in the emitting reservoir, was higher by one foot than that in the receiving one ; but Mr. Villar, a man of science, resident at Lyons, took the level, and found, as might have been expected, that the water in the receiving reservoir was higher by at least 12 inches than that in the emitting reservoir. To construct these individual aqueducts, says another architect, the Roman architects began by forming a trench five feet wide, and ten feet deep, having a uniform slope of one foot in 600. In this trench they formed the aqueduct or water channel of masonry, keeping the same dimensions in the parts excavated from the rock, as in those which were cut into the clay or ground soil. The bottom of this trench was laid with masonry, a foot thick ; on this, two walls were erected, each 1 1-2 feet thick, and 5 feet high, and having a space between them of 2 feet, which formed the canal for the passage of the water ; this space was enclosed on the top by a semi-circular arch 1 foot thick, and then covered with a layer, 2 feet thick, of earth. The bottom of the canal had a coat of cement 6 inches thick, and a coat of 1 1-2 inches on the sides, which reduced the intervals between the walls to 21 inches. The angles were formed by the sides and bottom, rounded by cement. The walls were con- structed with small rough stones, from 3 to 6 inches in thickness, laid in a bed of mortar, so that no void was left between the pieces. It would appear that the use of stones 36 PRELIMINARY ESSAY. larger than 6 inches, was avoided, as the walls formed of small stones, well bedded iri mortar, formed, in the estimation of the ancient Lyonese architects, masses of greater compactness, than those built with larger pieces. The builders also used gravelly sand for this kind of masonry, in preference to fine sand, which is proper only for the cement ; and when they were obliged to use the finer sand, they took care to mix it with pulverised bricks, a practice adopted also in cases where coarse sand was mixed in abundance ; lime burnt from good stones was used with unsparing profusion. The cement employed for the sides and bottom of the aqueduct, was composed of pieces of brick the size of peas, for the first coats, and somewhat finer for the finishing plaster. That on the bottom of the canal, is made of pieces as large as nuts, and in many places the size of eggs ; the composition was made with lime fresh slaked. That which contributed to make good cement, as well as good mortar, in their opinion, both in the one and the other, was the effectual mixing of the ingredients, so as that the mass should be all of the same temper, which is known when the composition was of the same color throughout. In the aqueducts not built in the earth, the walls are from 22 to 24 inches thick, the exterior faces formed of reticulated work, the lozenges being from 3 to 6 inches square, without any course of bricks. The arch of the water channel, where it was not covered with earth, was somewhat curved, to throw off the rain, but yet so flat as to serve for a road of communication between the reservoirs, and within the aqueduct, and which was entered through iron doors placed in the arch of the reservoir, and also in that of the one or two places of the arch channel aqueduct bridge. The subterranean channels had similar entrances, like square pits, the mouths of which were elevated 2 or 3 feet above the surface of the .ground they were driven into. 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. It is probable that the regulating vane or 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