Aqueduct : upon the bottom of this trench was laid a mass of masonry 1 foot thick, upon which two walls were built, each li foot thick and 5i feet high, these walls standing 2 feet apart, and surmounted by a semicircular arch of a thickness of 1 foot and generally covered with earth 2 feet deep. The interior had a coat of cement plastering, 6 inches thick on the bottom and H inch thick on the sides. The walls were constructed with small stones from 3 to 6 inches in

thickness, bedded in mortar so that no spaces could be found

between them. They avoided the use of stones of greater thickness than 6 inches, because the walls built of small stones, well filled with mortar, formed a mass more solid and impervious than with larger stones, on account of the great quantity of mortar used.

No bricks were used in the construction of the channelway of the Aqueduct.

Ventilators were constructed along the course of the Aqueduct 2 feet square, and rising above the ground 2 or 3 feet. The Aqueduct when it was above the ground, was supported upon a wall of masonry, and the side walls of the channel-way had an increased thickness. When it was elevated 6 or 7 feet above the ground, the foundation wall was six feet thick ; but when it had a greater elevation it was supported upon arches and piers, and upon the elevation depended the span of the arch, the thickness and height of the piers. The general declivity in the channel-way, was