this form has suggested the idea that the pipes of 8 inches diameter, when they reached half way down the plane, separated, each one into two of 6 inches diameter which crossed the bridge, and converged into one again half way up the opposite plane. But it may be sup- posed that they continued of the same interior form through- out their length, and that this extra width was made for the purpose of giving an opportunity to fortify the pipes at the place where the pressure to which they were subjected was the greatest. Construction. They commenced the construction by making a trench in the ground of sufficient dimensions for the masonry of the 23 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 thick- ness 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 channel- way 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 1 foot in 640, or about 8£ feet per mile. 24 This Aqueduct supplied about 1,200,000 gallons of water in 24 hours. The velocity of the water was about five times that of the water in the Aqueducts of Rome. This work was constructed at an immense expense, and in substituting the " inverted syphon" for high structures across valleys, there is evidence of the intelligence and skill of those who had charge of the construction. A fragment of a pipe forming part of this reversed syphon, is still preserved in the museum at Lyons, and an instance of the Romans having laid pipes across the beds of rivers, is given by M. Gautier, Architect, Engineer, &c, in his work called " Traite de la Construction des Chemins," published in 1778. About 70 or 80 years ago, he was directed by Mr. Pont- chartrain, Minister of State, to repair to Rochefort, to con- duct spring water to the port from the fountains of the city, which were supplied from a source, though quite insufficient for the city, in the neighborhood. In his researches he dis- covered a good and copious source, at less than half a league, but on the other side of the river, the Charente. Many difficulties were presented, because at low water vessels might ground upon the pipes and injure them. However, Mr. Gautier proposed to lay down two leaden pipes, to preserve a supply in case of accident to one, and to protect them by wooden frames in an effectual way against injury, should vessels lay upon the defence frames 25 during low water. Mr. Begon, intendant of the Marine, approved the plan, but it was finally rejected. " Some years after," says Mr. Gautier, " when I had charge of the roads on the Rhone, and of many other works in the Province of Languedoc, and while at Aries, I heard that a vessel had cast anchor in the Rhone, opposite the city, to take some loading ; but when the commander wanted to sail again he could not raise the anchor. This fact attracted much attention, and many people went to witness the singu- lar circumstance. The Captain, unwilling to lose his anchor, sent down a man, to find what was the matter. The diver reported that the anchor was hooked under something round, but he could not tell what it was. A capstan was applied to raise it, which succeeded. It brought up a leaden conduit pipe from the bottom of