That this plan was not generally adopted by them in cases where great expense has been incurred to maintain the uniform declivity of the conduit over valleys, may be accounted for perhaps by the want of proper material for the construction of pipes. In cases where this plan has been adopted leaden pipes were used, and since it is only within the last century that iron pipes have been invented, we may reasonably conclude that considerations of such a nature would have induced them to adopt the more expensive plan of maintaining the general inclination of the conduit by vast structures of masonry.

By substituting inverted syphons instead of maintaining a uniform declivity in the conduit, would not give the requisite discharge of water at the elevation of the terminus of the Aqueduct, and perhaps they preferred, rather than diminish this elevation of the supply of water, to incur the expense of

high structures across valleys. The Roman Emperors, with all their power and the wealth which was at their command, knew how to perpetuate the glory of their reign by the erec-

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tion of Temples, Palaces and other public buildings, and what is more natural than to suppose that in the construction of these Aqueducts, which were considered so essential to the public welfare, they should encourage works of such architectural magnificence? Whatever the reasons might

have been for maintaining the elevation of their Aqueducts over valleys by such expensive structures, we have no right to charge them with the want of that knowledge which the plan of some of their Aqueducts clearly proves them to have possessed.