We now come to a different class, in which heat is employed for obtaining an increase of air pressure whereby certain automatic actions are produced. Here we have a priest and priestess officiating at an altar; and the effect of lighting the fire thereon is to cause the two figures to pour libations onto the sacrifice. In this case the altar consists of an air-tight metallic box in communication, by means of a central tube, with a larger box forming the pedestal. Into this lower reservoir is poured the wine or other liquid through the hole marked M. When the fire is lighted the air in the altar is expanded, and pressing on the surface of the liquid in the pedestal, forces some of it through the tubes which pass through the body and down the right arm of each figure. In the next view (Fig. 6) we see how this principle was employed by Hero for the opening of the doors of a temple, the tradition being that when a sacrifice was offered on her altar the goddess Isis showed her invisible presence by throwing open the doors of her sanctuary. In this case the altar consists of an air-tight metallic box communicating by means of a tube (F G) with a spherical vessel (H) partly filled with water. When the altar becomes hot the contained air is expanded, thereby increasing the pressure on the surface of the water, some of which is therefore forced through the bent tube (L) into the bucket (M), which descends by its increased weight, thereby unwinding the cords from the two spindles that perform the function of hinges to the temple doors, at the same time winding up the counterweight (R) on the left. When the fire goes out the altar cools, assuming its ordinary atmospheric pressure, and the water in the bucket is forced back into the vessel (H), and the weight counterbalancing the empty bucket, closes again the doors. [Illustration: Fig. 6.]