Definition of "hydromancy"

Hydromancy is the supposed art of divining by water. The Persians, according to Varro, invented it; Pythagoras and Numa Pompilius made use of it; and we still admire the like wonderful prognosticators.

1797, Encyclopædia Britannica, 3rd edition

Hydromancy was extensively practised by the Egyptian priests and sorcerers[.]

1936, Rollo Ahmed, The Black Art, London: Long, page 39

Hydromancy, like many other methods of divination, seems to have originated in Babylonia and reached the Greco-Roman world via Egypt, in the first century b.c. or earlier.

1985, Georg Luck, Arcana Mundi