The building which first attracts attention, among those now standing, is the _Arch of Janus_, the Sabine god. It has four equal sides and arches, turned to the four points of the compass, and forty-eight niches, probably intended for the reception of small statues. Bas-reliefs on the inverted blocks employed in the lower part of this edifice, show that they must have been removed from earlier buildings. This was probably used as a portico for shelter or business for those who trafficked in the Forum; there were many similar porticoes in ancient Rome. On the left of the arch of Janus is a narrow alley, spanned by low brick arches, which leads first to the beautiful clear spring of the Aqua Argentina, which, according to some authorities, is the place where Castor and Pollux watered their horses after the battle of the Lake Regillus. "Then on rode those strange horsemen, With slow and lordly pace; And none who saw their bearing Durst ask their name or race. On rode they to the Forum, While laurel boughs and flowers From house-tops and from windows, Fell on their crests in showers. "When they drew nigh to Vesta, They vaulted down amain, And washed their horses in the well That springs by Vesta's fane. And straight again they mounted And rode to Vesta's door; Then, like a blast, away they passed, And no man saw them more." _Macaulay's Lays._ The alley is closed by an arch of the celebrated _Cloaca Maxima_, the famous drain formed by Tarquinius Priscus, fifth king of Rome, to dry the marshy land of the Velabrum. "Infima urbis loca circa Forum, aliasque interjectas collibus convalles, quia ex planis locis haud facile evehebant aquas, cloacis a fastigio in Tiberim ductis siccat."--_Livy_, lib. i. c. 38. The Cloaca extended from the Forum to the Tiber, and is still, after 2,400 years, used, during the latter part of its course, for the purpose for which it was originally intended, though Pliny was filled with wonder that, in his time, it had already withstood the earthquakes, inundations, and accidents of seven hundred years. Strabo tells that the tunnel of the Cloaca was of sufficient height to admit a waggon laden with hay, but this probably supposes the water at its lowest. Agrippa, who cleaned out the Cloaca, navigated its whole length in a boat. The mouth of the Cloaca, composed of three concentric courses of blocks of peperino, without cement, is visible on the river a little to the right of the temple of Vesta.