In regard to the form and size of the city, we must follow the direction of the seven hills upon which it was built. 1. Of these MONS PALATINUS has always had the preference. It was in this place that Romulus laid the foundation of the city, in a quadrangular form; and here the same king and Tullus Hostilius kept their courts, as did Augustus afterwards, and all the succeeding emperors. This hill was in compass 1200 paces. 2. MONS TARPEIUS, took its name from Tarpeia, a Roman virgin, who in this place betrayed the city to the Sabines. It had afterwards the denomination of Capitolinus, from the head of a man, casually found here in digging for the foundation of the temple of Jupiter. This hill was added to the city by Titus Tatius, king of the Sabines; when, having been first overcome in the field by Romulus, he and his subjects were permitted to incorporate with the Romans. 3. MONS ESQUILINUS was taken in by Servius Tullius, who had here his royal seat. 4. MONS VIMINALIS derived its name from the osiers that grew very plentifully upon it. This hill was taken in by Servius Tullus. 5. MONS CŒLIUS owes its name to Cœlius, or Cœles, a Tuscan general greatly celebrated in his time, who pitched his tents here when he came to the assistance of Romulus against the Sabines. Its having been taken into the city is attributed to Tullus Hostilius, by Livy and Dionysius; but by Strabo, to Ancus Martius. 6. COLLIS QUIRINALIS was so called from the temple of Quirinus, another name of Romulus; or from the Curetes, a people that removed hither from a Sabine city, called Cures. It afterwards changed its name to Caballus, Mons Caballi, and Caballinus, from the two marble horses, with each a man holding him, which are set up here. They are still standing, and, if the inscription on the pilasters be true, were the work of Phidias and Praxiteles; made by those masters to represent Alexander and his horse Bucephalus, and sent to Nero as a present by Tiridates king of Armenia. 7. MONS AVENTINUS derived its name from Aventinus, an Alban king, from the river Avens, or from (ab Avibus) the birds, that used to flock there from the Tiber. Gellius affirms, that this hill was not enclosed within the bounds of the city, till the time of Claudius; but Eutropius expressly states that it was taken into it even so early as that of Ancus Martius. As to the extent of the whole city, the greatest, recorded in history, was in the reign of Valerian, who enlarged the walls to such a degree, as to surround a space of fifty miles. The number of inhabitants, in its flourishing state, is computed by Lipsius at four millions. The present extent of the walls is about thirteen miles. Sir John Hobhouse walked round them in three hours, thirty-three minutes and three quarters; and Dr. Burton did the same in three hours and ten minutes.