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 8¼ feet per mile. 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 “_Traité de la Construction des Chemins_,” published in 1778. About 70 or 80 years ago, he was directed by Mr. Pontchartrain, Minister of State, to repair to Rochefort, to conduct 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 discovered 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 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 Arles, 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 singular 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.