[Illustration: FIG. 64.--LONGITUDINAL SECTION OF BASILICA OF MAXENTIUS.] The principal reason for speaking of basilicas is that by the above cut you may see the great change made in architecture about this time by the use of columns, only half the height of the building, which were united by arches. This was a very important step, and is, in truth, one of the principal features that mark the progress of the change from ancient to Gothic architecture--a change not fully developed until the twelfth century. I shall not say much of the theatres, amphitheatres, and baths of ancient Rome, because it is not easy to treat them in the simple manner suited to this book; they were magnificent and costly, and made an important part of Roman architecture; they were probably copied from the public buildings of the Etruscans. Marcus Scaurus built a theatre in 58 B.C. which held eighty thousand spectators; it had rich columns and statues, and was decorated with gold, silver, and ivory. The first stone theatre in Rome was built in 55 B.C., and was only half the size of that of Marcus Scaurus. Parts of the theatre of Marcellus still remain in the present Orsini Palace in Rome, and serve to give an idea of the architecture of the period immediately before the birth of Christ. The Emperor Augustus boasted that he had found a city of brick and had changed it to one of marble, but after his time architecture suffered a decline, and its second flourishing period may be dated from A.D. 69. To this time belongs the Colosseum, also called the Flavian Amphitheatre; it covers about five acres of ground, and is sufficiently well preserved for a good idea to be formed of what it must have been when in its best estate. The enormous size of these ancient Roman edifices is almost too much for us to imagine, and the most extensive of them all were the _Thermæ_, or public baths. The Baths of Diocletian, built A.D. 303, were the largest of all; they had seats for twenty-four hundred bathers. These baths were in reality a group of spacious halls of varied forms, but all magnificent in size. The great hall of the Baths of Diocletian was three hundred and fifty feet long by eighty feet in width and ninety-six feet high; it was converted into a church by Michael Angelo and is called S. Maria Degli Angeli, or Holy Mary of the Angels. Many splendid pictures which were once in St. Peter's are now in this church, and copies of them made in mosaic fill the places where they were originally hung.