Gastromancy means either:
Divination by looking into large-bellied, bulbous glasses, which is similar to crystal ball gazing.
Divination through mysterious voices that are actually produced by ventriloquism.
From Greek gastēr, belly.
Presumably, the voices came from trees, rivers, deep in the ground, and even the dead. Modern spirit mediums have improved on this by going into pretended trances or working in complete darkness to convey the impression that the voices come from another world. The name of Engastrimythes, given by the Greeks to the Pythiæ (priestesses of Apollo), indicates that they made use of this artifice.
Another method of practicing the ancient gastromancy connects it with crystal-seeing, as vessels of glass, round, and full of clear water, were used, which were placed before several lighted candles. Replies were then obtained from the magical appearances seen in the illuminated glass vessels.