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. probable that the regulating vane or It is