The thirty-eighth milestone on the Via Sublacensis was found by Fabretti _in situ_, and the thirty-eighth milestone in the Via Valeria, now at Arsoli, is said by Gruter[136] to have been formerly at _La Sonnoletta_, or _ad fontem Somnulæ_: so that the two sources of the Claudian water, called Cæruleus and Curtius, were at the lake now called S. Lucia, in the territory of Arsoli; and the source called Albudinus is the _first_ of the four springs now called _Acque Serene_, while the other springs of the same name formed the _Aqua Marcia_. The _Piscina Limaria_, referred to by Frontinus as erected by Claudius for filtering the stream, at the entrance of the Anio Novus, at the forty-_second_ mile[137] on the Via Sublacensis, is visible at the _Parata della Cartiera_ of Subiaco. This _piscina_ is not covered over, but open at the top, and is excavated in the rock of the bed of the river Anio, with a great declivity and with four cascades to throw down the sand, wood, and weeds brought down the river, and so to purify the water; but as it was still liable to become muddy in the time of floods, Trajan, according to Frontinus, excavated another _specus_ in the rock of the mountain near the Ponte di S. Mauro, where the great wall of the lake was situated[138]. The _specus_ of the Claudia seems to have been carried on the right-hand side of the valley as far as Vicovaro, about half-way to Tivoli, and then across the valley on an arcade, and carried on under the Anio Novus, and above the Marcia and Anio Vetus. All of these seem to be at different elevations on the side of the hill to the left of the valley, until they arrive at another valley crossing this, called the Valley of the Arches, about two miles from Tivoli, where they were carried across upon arcades. On these ancient arcades, or out of the materials of them, modern bridges have been made, both at Vicovaro and in the Valley of the Arches. The ruins of these splendid arcades are among the finest and most picturesque ruins of the Roman empire.