A branch of crystalomancy (divination by water). One method was to throw objects into a full container of water and to interpret the images they formed in the water, or the sounds made when the objects struck the water.
Another, more elaborate method was to place a silver vase filled with water on a moonlit night . Then let the light from a candle be reflected from a blade of a knife onto the water and the inquirer comments on the images formed on the water. A.G.H.