We read in Pausanias (Arcadica, lib. viii., ch. xvi.) that at Jerusalem the tomb of a woman of the country, named Helen, had a door made of marble like the rest of the monument, and that this door opened of itself on a certain day of the year, and at a certain hour, by means of a machine, and closed again some time afterward. “At any other time,” adds he, “had you desired to open it, you had sooner broken it.”
According to Pliny (xxxvi. 14), the gates of the labyrinth of Thebes were so constituted that when they were opened they emitted a noise like that of thunder.
Heron, in his “Pneumatics,” gives us an explanation of some of these prodigies.
Our first [engraving] is sufficiently clear to permit of dispensing with a reproduction of the Greek engineer’s text in this place. It will be seen that when the door is opened, a system of cords, guide-pulleys, and rods pushes into a vessel of water a hemisphere, to the upper part of which a trumpet is fixed. The air compressed by the water escapes through the instrument and causes it to make a sound.
MECHANISM FOR OPENING AND CLOSING THE DOORS WHEN
A FIRE WAS LIGHTED UPON THE ALTAR.
Our [second] and [third] engravings are likewise borrowed from Heron.
The altar is hollow, as shown at E, in second [engraving]. When fire is lighted thereon, the air contained in the interior dilates and presses against the water with which the globe situated beneath is filled. This water then runs through a bent tube into a sort of pail suspended from a cord that passes over a pulley, and afterward separates into two parts, and winds around two cylinders movable upon pivots, and forming a prolongation of the axes around which the doors revolve. Around the same cylinders are wound in opposite directions two other cords, which likewise unite into a single one before passing over a pulley, and then hang vertically in order to hold a counterpoise.