[Illustration:  FIG. 21.—­SO-CALLED TEMPLE OF THE SIBYL AT TIVOLI.]

We must not omit to say that, besides temples of this typical rectangular form, there were others of a round shape, encircled by columns, like that graceful structure at Tivoli commonly, though mistakenly, known as the temple of the Sibyl, and that small building which still exists in an impoverished condition near the Tiber, and which used to bear the erroneous title of the temple of Vesta.  Others again were simply round and domed, like the true temple of Vesta in the Forum, or the superb and impressive Pantheon in the Campus Martius.  So far as the bare round was broken in these cases, it was either by a pillared portico, as with the Pantheon, or by engaged columns and ornament, as with the true temple of Vesta.

The mention of the temple of Vesta reminds us that it is time to face about, and, passing behind the temple of Julius, to look in the opposite direction, from V. Before us lies this circular shrine, a form gradually developed from the primitive round hut which once served as house to the prehistoric ancestors of the Roman stock.  As it was the duty of the maiden daughters of that ancient tribe to keep alight the fire upon the domestic hearth, so through all the history of Rome it was the duty of certain chosen virgins to keep perpetually burning the hearth-fire of the city.  The roof of the temple is open in the middle, and you may perhaps see the smoke issuing from it.  But if you are a male, you may not enter.  No man, except the chief Pontifex, may set foot inside the shrine of the virgin goddess, who is attended by virgin priestesses.  Close behind the temple stands the house of these Vestals.  They are in a large measure the ancient prototype of the modern nun, and their house is the prototype of the convent.  Six nobly-born young women, sworn to chastity, and dressed in a ritual garb, live in an edifice of much magnificence under the rule of one who is the chief Vestal, a sort of Mother Superior.  Many pedestals of the statues of such chief priestesses still remain, and we can clearly trace the arrangement of their abode, with its open court—­once containing a garden and cool cisterns of pure water—­its separate room for each Vestal, its baths, and its resources of considerable comfort and even luxury.

[Illustration:  FIG. 22.—­VESTAL VIRGIN]

If, as you face this way, you look up to your right, you will perceive the Palatine Hill rising steeply above you, with its summit crowned by the lofty palaces and gardens constructed by the Caesars.  At the side and corner which look down upon the Forum stands the part built by Caligula, the epileptic who thought himself no less than a god, and who in consequence not only turned the temple of Castor into a lower vestibule to his own house, but also built a bridge across the valley over the temple of Augustus and the Basilica of Julius to the Capitoline Hill, so that he might visit and converse with Jupiter, his