It is difficult to say when this old fountain of Fontana’s disappeared. It was probably removed either at the time when Antinori erected the obelisk for Pius VI or in the following pontificate when the same architect suggested to Pius VII the idea of replacing it by the present granite basin. This basin had stood since 1594 in the Campo Vaccino, the mediæval name for the ruins of the Roman Forum. It had been placed there during the pontificate of Clement VIII (Aldobrandini) by the city magistrates on a piece of ground given to them by Cardinal Farnese, near the three columns of Castor and Pollux and the Church of S. Maria Liberatrice. They had provided a high travertine base for it, and it was fed from three jets of the Acqua Felice, which, some eight or nine years previously, had been brought to Rome by Sixtus V. The basin was used as a watering-trough for cattle, and by the time Pius VII rescued it the travertine base had entirely disappeared under the gradually rising level of the Campo Vaccino--that strange composite mass of rubbish, earth, and ruins which, up to the second half of the nineteenth century, covered the old Forum floor to a depth of more than twenty feet. The basin measures twenty-three metres in circumference, and when it was thus sunk in the ground it became a pleasant pool through which the carters walked their horses to refresh them on a warm and dusty day. The removal of this basin was actually accomplished in 1818, when the architect Raphael Stern (who built for Pius VII the Bracchio Nuovo) designed the present fountain of Monte Cavallo. He sank the basin in the pavement between the horse-tamers and erected in the middle of it a second basin which rests upon a travertine base. The water of the fountain rises in a copious jet from the centre of the second basin to a height somewhat below the heads of the horses and, returning on itself, falls in a generous overflow into the lower basin. [Illustration: The Fountain of Monte Cavallo.] To some, the chief interest of this composite group of obelisk, statuary, and fountain centres in this lower basin, for it is none other than the granite tazza into which Marforio once poured the water from his urn, far, far back in the days of Charlemagne, and no one knows for how many years before that. The obelisk which forms the centre of this group of antiquities now clustered together in the Monte Cavallo is one of a pair which flanked the entrance to the Mausoleum of Augustus. Its mate was erected by Sixtus V and Domenico Fontana near the Church of S. Maria Maggiore.