Vitruvius has given some minute directions, strongly illustrative of the importance of the subject, for choosing a proper situation for a theatre. "When the Forum is finished, a healthy situation must be sought for, wherein the theatre may be erected to exhibit sports on the festival days of the immortal gods. For the spectators are detained in their seats by the entertainment of the games, and remaining quiet for a long time, their pores are opened, and imbibe the draughts of air, which, if they come from marshy or otherwise unhealthy places, will pour injurious humors into the body. Neither must it front the south; for when the sun fills the concavity, the inclosed air, unable to escape or circulate, is heated, and then extracts and dries up the juices of the body. It is also to be carefully observed that the place be not unfitted to transmit sound, but one in which the voice may expand as clearly as possible." The ancient scene was not, like that of the modern stage, capable of being shifted. It consisted of a solid building (_scena stabilis_), representing the facade of a royal palace, and adorned with the richest architectural ornaments. It was built of stone, or brick cased with marble, and had three doors, of which the middle one, called _porta regia_, larger and handsomer than the others, was supposed to form the entrance to the palace. This was used only in the representation of tragedies, and then only by the principal personages of the drama. The door in the right wing was appropriated to inferior personages, and that on the left to foreigners or persons coming from abroad. In our plan, the five angles of the triangles not yet disposed of determine the disposition of the scene. Opposite the centre one are the regal doors; on each side are those by which the secondary characters entered. Behind the scene, as in the Greek theatre, there were apartments for the actors to retire into; and under it were vaults or cellars, which, as in the modern stage, served for the entrance of ghosts, or the appliance of any needful machinery. The _proscenium_, or space between the orchestra and the scene, answering to our stage, though deeper than the Greek, was of no great depth, which was not required for the performance of ancient dramas, in which only a few personages appeared on the stage at once. Besides, in the absence of any roof, the voice of the performers would have been lost if the stage had been too deep. That of Pompeii is only about twenty-one feet broad, though its length is one hundred and nine. Along the front of the stage, and between it and the orchestra, runs a tolerably deep linear opening, the receptacle for the _aulæum_, or curtain, the fashion of which was just the reverse of ours, as it had to be depressed instead of elevated when the play began. This operation, performed by machinery of which we have no clear account, was called _aulæum premere_, as in the well-known line of Horace:[19]