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 constructed 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 tomix 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