Eos carried away the corpse of Memnon through the air. They also had machines for producing thunder and lightning. We do not know how the lightning was made, and it is difficult to imagine that it could have been produced with any great result in broad daylight. The thunder was caused by rolling bladders full of little stones to and fro on brass plates in the hollow space under the stage. In this hollow space were also probably the “steps of Charon,” a contrivance for bringing the spirits of the dead on to the stage. Nothing certain is known concerning these steps, but it is very probable that they were managed after the fashion of our trap-doors, for undoubtedly the floor of the stage covered a hollow space, and thus a contrivance of this kind was very easily produced. We must next consider the plays which had to be performed here. On the old Greek stage there were three kinds of drama--tragedies, comedies, and satyric dramas. The comedies were acted singly, and each constituted a complete whole; but tragedy, as it developed out of the Dionysus legend and the division of the action into three connected therewith, was so constructed that a large circle of myth was treated in three separate tragedies, whose contents were connected, but which were structurally complete in themselves, and these were called a Trilogy. But about the same time the curious custom originated of following up these three serious pieces, with their deeply pathetic contents, by a merry satyric drama by the same author,--a wild farce, in which a chorus of satyrs was introduced in connection with some mythical action, which of course, only appeared in travesty; and this combination of four dramas was called a Tetralogy. Unfortunately no tetralogy has come down complete to us; the trilogy of Aeschylus alone, which deals with the story of Orestes, gives us some notion of the mode in which the tragic poets arranged their material in the form of a trilogy. The first part, “Agamemnon,” represents the murder of Agamemnon by Clytemnestra and Aegisthus; the second, the “Choephorae,” the vengeance taken by Orestes on the murderers; the third, the “Eumenides,” the absolution for the murder of his mother by the Areopagus. The tragic poets did not very long abide by the custom of presenting complete tetralogies at the Dionysia, in which the trilogy presented one connected subject. It attained its complete development under Aeschylus, but Sophocles already began to depart from it, and in the tetralogies with which he and Euripides competed, the internal connection between the tragedies was wanting. In later times it was customary for tragedies complete in themselves to be acted singly, so that the poets competed with drama against drama; still, the inscriptions show us that even in the fourth century tetralogies were acted, though they may not have been connected. Each of these three kinds of drama underwent several changes during the course of Greek literature.