The altar is hollow, and when a fire is lighted thereon, the air contained in the interior expands and begins to press upon the water with which the globe situated beneath is filled. The water then rises through a bent tube which leads to a species of pot, into which it falls. The pot is suspended upon a cord which passes along a pulley, doubling immediately, in order to enroll itself about two cylinders, which turn upon pivots, said cylinders forming the prolongation of the axes upon which the doors above turn. Around the same cylinders are enrolled in a contrary manner, two other cords, which also unite into one before passing along a pulley, and then hanging vertically for the support of a counterpoise.
It is clear that when the water from the globe enters the pot, the weight of the latter will be augmented and it will sink, pulling upon the cord which has been wound about the cylinders { 8} in such a way as to cause the doors to open, when it is drawn in this direction.
The doors close themselves in the following manner: The bent tube, which places in communication the globe and the pot, forms a siphon, the longest branch of which plunges into the globe. When the fire is extinguished upon the altar, the air contained in the latter and in the globe, cools, and diminishes in volume. The water in the pot is then drawn into the globe, and the siphon, being thus naturally influenced, operates until the water in the pot has passed over into the globe. In measure as the pot lightens, it remounts under the constraint of the counterpoise, and the latter, in its descent, closes the doors through the intermedium of the cords wound around the cylinders.
[Illustration: HOW THE STATUES WERE MADE TO POUR LIBATIONS WHEN A FIRE WAS KINDLED ON THE ALTAR.]
Heron says that mercury was sometimes used in place of water, by reason of its superior weight. { 9}
Certain altars were provided with such mechanism as to afford to the faithful even more astonishing spectacles. Here is another experiment from the learned Heron:
“_To construct an altar so that when one kindles the fire thereon, the statues which are at the sides shall pour out libations_.”
There should be a pedestal, upon which are placed the statues, and an altar closed on all sides. The pedestal should communicate with the altar through a central tube, also with the statues by means of tubes, the ends of the latter terminating in cups held by the statues. Water is poured into the pedestal through a hole, which is stopped up immediately afterward.