The word _stagnum_ is commonly translated _pond_, but it does not necessarily mean _only_ a pond; any reservoir of water might be so called; the _castellum aquæ_, or large cistern for the water supplied by the aqueducts, was also a _stagnum_. The “Stagna Neronis[60]” are mentioned by Martial, as well as by Tacitus[61], who also mentions a _stagnum navale_ among the games in the public theatre of Augustus[62]. Suetonius[63] compares the Stagnum of Nero to a “sea” surrounded by the buildings of a city: a strong expression, which shews that there were some buildings immediately round it. The Claudium on the Cœlian, the Porticus Liviæ on the Summa Sacra Via, and Porticus of Nero himself on the Esquiline, would be visible on three sides of it. The representation of this great building on the coin of Titus was evidently taken from the design of the architect. It represents a building of two storeys only, with gigantic statues under each of the arches of the corridors. The _new naumachia_ made by Augustus in the Trastevere were larger reservoirs for the same purpose; this _naumachia_ is also called a _stagnum_ by Tacitus[64], who describes a similar scene in the _stagnum_ of Agrippa[65] (which was in his _thermæ_, near the Pantheon), with the letting in the water suddenly for a naval battle; and then letting it off again as suddenly, and having a supper in the same place; he also mentions the _stagna_ of Nero[66], and the _stagnum navale_ of Augustus[67]. The _stagnum_ of Agrippa was supplied with water by his aqueduct (the Virgo), and it has been mentioned that those of Nero were supplied by three aqueducts, two from the Cœlian and one from the Esquiline. The remains of the _specus_ and of the _piscinæ_ have been already mentioned[68]. There are slight remains of three reservoirs in the gallery, lined with the peculiar cement used only for the aqueducts, called _opus signinum_. From the Esquiline the water was brought to the Amphitheatre in leaden pipes, after serving the Thermæ of Titus; a quantity of these leaden pipes have been found in excavations at different periods[69], as recorded at the time by eye-witnesses, and some of them are still preserved as mementoes in the office of the Municipality. Suetonius[70] mentions that some of the amusements for the people provided by Nero were held in the wooden amphitheatre of Statilius Taurus in the Campus Martius, but he mentions the _naumachia_ separately, and we have no account of any _stagnum_ having been there, nor is it probable, as it could only have been supplied with water by the aqueduct of Agrippa, and it was originally built before that aqueduct was made. The _naumachia_ were an essential part of the amusements of many Roman amphitheatres, and there are considerable remains of the canals for them at Capua and at Tusculum. Suetonius, in his life of Titus, thus writes:—