There were 13 bridges of stone to support the Aqueduct across valleys or over rivers, two of which were not built up to the plane of the Aqueduct, but were crossed by leaden pipes which descended on one side of the valley and, crossing the bridge, ascended on the opposite side. In another instance the pipes descended and crossed upon a wall of masonry and reached the opposite side of the valley. One instance, where pipes were used, will give an idea of their general form: the bridge was about 40 feet high and the perpendicular height of the Aqueduct above it was 140 feet. Nine leaden pipes of about 8 inches interior diameter and one inch thick were laid upon the inclined planes and across the level part of the bridge; thus communicating with the opposite crests of the valley. These bridges which were constructed for the support of pipes, were wider in the bottom of the valley and also half way up the inclined plane, than they were for the remainder of the distance; and 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 supposed that they continued of the same interior form throughout 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 Aqueduct: upon the bottom of this trench was laid a mass of masonry 1 foot thick, upon which two walls were built, each 1½ foot thick and 5⅓ 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 1½ 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.