Indeed, the Anio Vetus was rarely free from discoloration, but as it flowed on a low level it could not affect the other waters, but the Anio Novtis

being of a lofty level and unfailing in its stream, was occasionally used, when the other aqueducts were falling off in their supply, to make good the deficiency, and hence all were more or less contaminated even the Marcia, which was the delight of the Romans for its limpidity and coolness.

The Emperor Nerva undertook to correct this evil. He began by classing the aqueducts according to the goodness of their waters. The Marcia was reserved solely for

drinking others according to their qualities, to other uses, and the Anio Vetus, as the most impure, was reserved for the irrigation of gardens and the cleaning of the streets and sewers. He then caused the water from the Anio Novus to be drawn from the lake itself, instead of the river ; and either because the stream supplying this lake ran over a rocky bed, or that from the depth of the lake, the water was purified by depositing an earthy matter, it issued thence cool and bright, so as to equal in appearance and taste the

Marcia, which it greatly surpassed in quantity. Nerva caused numerous wells to be constructed in the courses of the other aqueducts, in which the waters deposited their sediment. He also framed regulations for the general by means whereof a more vigilant supervision was exdistribution of the water,