[Illustration: FIG. 153.] may regard the orchestra as the centre of the whole structure. This was originally only a level dancing place, and its shape was usually an incomplete circle, since part was cut off by the stage, which extended at right angles along the orchestra. Opposite to this the circumference of the orchestra was surrounded in concentric lines by the raised seats of the auditorium, the theatre in the true sense of the word. There is no fixed standard for the shape of the orchestra and the corresponding auditorium; sometimes it is a semicircle with the circumference extended a little way on both sides, sometimes it is lengthened by a tangent, or some other line at right angles to the circle. In the great theatre of Dionysus the orchestra was originally a perfect circle; a complete semicircle, which is common in the Roman theatres, was very unusual in those of Greece. Here, as in the structures used for games, such as the Stadia and Hippodromes, the Greeks tried as far as possible to utilise the natural conditions of the ground for their theatres. If possible, therefore, they placed the auditorium on some natural elevation; thus, the great theatre of Dionysus extends up the southern slope of the Acropolis; and if there was no such elevation they often supplied it by artificial mounds of earth, differing thus from the custom of the Romans, who, in consequence of the greater development of their architectural knowledge, were able to build a theatre on an open space, and to support the auditorium by strong sub-structures. The Greek mode of building had the advantage of greater cheapness and security, and, if the nature of the ground permitted, also enabled them to make exits and entrances for the public besides those below. In the theatre of Dionysus there were side approaches on the high ground also. The auditorium of the Greek theatre was usually situated in some beautiful spot, from which the visitors to the theatre, at any rate those on the higher ground, who were not hindered by the structure of the stage opposite, had an extensive view. Thus the theatre of Syracuse (Fig. 151) had a glorious view over the harbour and town--in fact, nearly all the theatres in the neighbourhood of the sea are usually so built that the auditorium is open towards the sea, and the fresh breeze may refresh the public during the hot hours of the day.