"A temple is constructed such that as soon as a fire is lit on the altar before it, and the sacrifice is made, the doors of the temple open automatically; and as soon as the fire is quenched, they close again" (Heron, Theorem 38, “Pneumatika”).

Automatic opening of temple doors

Another device designed by Heron, allows the doors of a temple to open when a fire is lit at the altar. The doors are connected though a set of axles, pulleys, and ropes to a largebucket. Initially, the system is statically neutral and the weight of the bucketis balanced with counterweights.

When a fire is lit, the air in the altar is heatedand, as it expands, it enters a hollow sphere full of water. Due to the risingpressure in the sphere, some of the water is displaced into the bucket. As the bucket becomes more heavy, it is lowered, opening the doors of the temple.

When the fire at the altar was put out, the pressure inside thealtar would drop, and water would go back to the hollow sphere, pushed by atmospheric pressure. Then, the counterweights would force the doors of thetemple to close.

In this mechanism, Heron exploited the expansion of air heated beneath the altar.