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 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