the facing is of reticulated masonry, and being built with black and grey pieces, has much the appearance of a chess-board. However, these open ings having weakened too much the higher piers, the builders were under the necessity of strengthening some of them, by counter-forts of the same sort of masonry. Two arcades in the valley of Bannau fell in 1757, from this precaution not having been taken from these piers, after their fall, it was ;

observed that the arches were divided into compartments in their depth, by voussoirs formed of a double row of great bricks.

The quantity of water, according to Delorme, which was furnished by this tqueduct in twenty-four hours, was 1,323,000 cubic feet. This author, however, did not appear to have made any corrections for the diminution of velocity on account of friction, and other circumstances affecting the motion of fluids after these ,have been made to bear ;

on his calculation, the quantity has been estimated as not being greater than about 172,800 cubic feet, or about 1,209,600 gallons. The receiving reservoir for the valley of Bannau is eighteen feet long, and 6 feet wide. It was pierced, according to Delorme, with twelve openings, for as many syphons, this

PRELIMINARY ESSAY. 39

increase in the number of pipes being made on account of the valley being deeper than that of the Garon, and, as he thinks, as the pressure increases, while the depth is

augmented, that the architects multiplied the number of syphons, to divide this force by diminishing their diameter in proportion a circumstance which, if accurate, would clearly decide that the difference between the weight and the pressure of water, was unknown to the Roman architect. The rest of the syphon bridges are similar to that of the Garon. There is now no vestige of the emitting reservoir of St.