The names of each are given under the section of it, and it will be observed that no two are alike; this was no doubt done in order that the workmen might always know which aqueduct each belonged to, at the points where they cross each other, so that if there was any obstacle in one of them it might be readily removed. Some of those which came from a clay soil, as the Appia and the Virgo, were liable to get choked up by the quantity of deposit left by the water, and it was necessary to have them cleared out from time to time, as is still the case with the Virgo, now called the Aqua di Trevi. One of the small streams that were collected to form the Aqua Hadriana, which comes from near Gabii, was a petrifying spring, which quite choked up the _specus_ in the course of a century. It was restored by Alexander Severus, but the same water seems still to have been used. The Aqua Felice comes from the same sources, but the petrifying spring was carefully excluded, and now runs in a ditch, giving a coat of stone to the sticks and the weeds. PLAN OF THE AQUEDUCTS. ON THE CŒLIAN. [Illustration: AQUEDUCTS ON THE CŒLIAN AND ESQUILINE LEADING TO THE COLOSSEUM.] PLAN OF THE AQUEDUCTS. ON THE CŒLIAN[237]. Three branches diverge from the great central reservoir at the Arch of Dolabella,—one goes straight on, nearly due north to the Palatine, the second north-east to the Colosseum, and the third west to the Aventine. AND ON THE ESQUILINE. From the other great reservoir called the Sette Sale, or the Thermæ, and from thence to the Colosseum. A.A.A. The drain under the road between the Cœlian and the Palatine, now called Via di S. Gregorio, leading to the Via Appia. B. Arch of Constantine. C. Meta Sudans. D. Thermæ of Titus on the Esquiline, with the great _piscina_ called Sette Sale, and the aqueduct leading from it to the Colosseum. E. Monastery and Church of S. Gregory. F. Site of the Porta Capena, with the aqueduct over it, leading to the Piscina Publica under the Aventine: and on to the mouth at the Porta Trigemina, in the bank of the Tiber. G. Church and monastery of SS. John and Paul. H. Church and monastery of S. Clement. I. The great reservoir and Piscina of Nero over the Arch of Dolabella, and reservoir of the Aqua Appia to the left of it; this is underground in the garden of the Villa Celimontana, and from thence the _specus_ goes underground to the cliff of the Cœlian, where another _piscina_ is shewn, near the bottom of the plan, to the left. Under that part of the Cœlian Hill on which the Claudium stood are seen four _piscinæ_, two on the western side opposite to the Palatine, and two on the northern side opposite to the Colosseum; of these latter the one at the corner is of the time of Nero, that in the middle of the northern front is of the time of Alexander Severus, when the upper storey of the Colosseum was building of stone. MAP OF THE AQUEDUCTS ON THE EASTERN SIDE OF ROME.