until the beginning of the next piece, the Prometheus Unbound, in which he was again represented as chained to a cliff. The ‘bronteion’ was a device for imitating the noise of thunder behind the scenes, and was of a very simple character. Pebbles were poured out of a jar into a large brazen vessel; bags were filled with stones and flung against a metal surface; or leaden balls were dropped upon a sheet of leather stretched tight.[664] The ‘keraunoskopeion’ was obviously intended to imitate lightning, though the description in Pollux is unintelligible. But Heron, the mathematician, speaks of a device used in automaton theatres, by which a plank, with a flash of lightning painted on a dark background, was shot out of a box into a receptacle below. Possibly Pollux may be alluding to an arrangement of this kind.[665] The ‘stropheion’ was some sort of revolving machinery, by which heroes were exhibited in heaven, or deaths at sea or in battle were represented. The ‘hemikyklion’ was semicircular in shape, and gave a distant view of a city, or of a person swimming in the sea. The ‘hemistrophion’ is merely mentioned by name, and no description of it is appended.[666] The question whether a drop-scene was used in the Athenian theatre during the great period of the drama is one which has not yet been satisfactorily settled. In Roman theatres a drop-scene was invariably used between the different plays, the mechanism being exactly the reverse of that employed in modern times. When a play was going to begin, the curtain was let down into a narrow crevice in front of the stage, and at the end of the performance was drawn up again.[667] There can be no doubt that similar curtains were used in Greek theatres at a later period; but the question is whether they were used at Athens during the fifth and fourth centuries. There are no references to anything of the kind in the extant Greek dramas, and there are no passages in ancient writers which can be held to prove the existence of a drop-scene in the early Athenian theatre.[668] The question must therefore be discussed on general grounds. To our modern notions a drop-scene appears to be almost a necessity in the case of plays which commence with the actors already in position upon the stage. In the Greek drama such plays are not infrequent. For instance, in the opening scene of the Oedipus Tyrannus the Thebans are discovered kneeling at the altar before the palace of the king. In the Troades, when Poseidon comes forward to speak the prologue, he sees Hecuba stretched upon the ground in an attitude of despair. The Orestes of Euripides opens with Orestes stretched upon a bed in front of the palace, and his sister Electra watching beside him. Many other examples might be cited of plays which begin with the actors already in a fixed position. Unless, therefore, a drop-scene was used between the plays, it would have to be supposed that the actors came on the stage in