Hydromancy (also known as Ydromancy and Hydrascopy) is a method of divination by means of water, including the color, ebb and flow, or ripples produced by pebbles dropped in a pool.
Derived from the Greek hudor ('water') and manteia ('prophecy')
The Persians, according to Varro, invented it; Pythagoras and Numa Pompilius made use of it.
Pausanius (2nd century AD) described the fountain near Epidaurus dedicated to Ino into which loaves were thrown by worshippers hoping to receive an oracle from the goddess. If the loaves were accepted they sank in the water which meant good fortune, but if they were washed up from the fountain it meant bad luck.
A custom of ancient Germanic tribes was to throw newborn children into the Rhine. It was thought if the child was spurious he would drown, but if he was legitimate he would swim. Such a custom appears to be a precursor of the 17th century custom of “swimming witches” perhaps related to the Anglo-Saxon law of trial by water.
The Jesuit M. A. Del Rio (1551–1608) described several methods of hydromancy. The first method described depicts a ring hanging by a string that is dipped into a vessel of water which was shaken. A judgment or prediction is made by the number of times which the ring strikes the sides of the vessel.
A second method is when three pebbles are thrown into standing water and observations are made from the circles formed when the objects strike the water.
The third method described depended upon the agitation of the water, this custom was prevalent among Oriental Christians of annually baptizing that element, at the same time as taking especial care to show that the betrothment of the Adriatic by the Doge of Venice had a wholly different origin.
A fourth method used colors of the water and figures appearing in it by which Varro stated that many prognostications were made concerning the Mithridatic War. This branch of the divination proved so important that it was given a separate name and there arose from it the divination of fountains whose waters were frequently visited.
In a fifth method of hydromancy mysterious words are pronounced over a glass of water, then observations are made of its spontaneous ebullience.
In the sixth method a drop of oil was let drop into a vessel of water, this furnished a mirror through which wondrous things became visible. This, Del Rio said, is the Modus Fessanus.
The seventh method of hydromancy was cited by Clemens Alexandrinus who cited that women of Germany watched the whirls and courses of rivers for prognostic interpretations. The identical fact was mentioned by J. L. Vives in his Commentary upon St. Augustine.
A simpler and safer way is to cast offerings, such as bread, into a pool. If they remained there, it was a good omen; if they drifted ashore before they sank, it boded ill. This may account for the still popular custom of throwing coins into a fountain for good luck, as they are sure to sink and stay there.
An old way of picking somebody's name was to write each name on a separate stone and toss them all in a pool. Later, the stones were fished out and if all the names were washed off, or nearly so, with the exception of one, that name represented the person to whom the question pertained. A modern combination of these methods, which is simpler, easier and more convenient, is the Floating Slip.