HYDROMANCY, the act or art of predicting the future by the medium of water. See Divination. The word is Greek and composed of ὕδωρ, water , and μαντεία, divination . 

Hydromancy is one of the four general types of divination; the other three are each associated with one of the elements, fire, air, earth; and they are named Pyromancy, Aeromancy, Geomancy . 

Varro says that Hydromancy was invented by the Persians, and that Numa Pompilius and Pythagoras made great use of it. [1] See Hydatoscopy.

Those who have written on Optics have given us the description of several machines used in this science. 

To construct a hydromantic machine, by means of which one will make an image or an object disappear from the spectator’s view, and then will make it reappear for him without altering the position of either one: take two vessels A B F , and C G M K   (Plate [III]: Hydraulics, Figure 31), one of which is higher than the other; fill the first with water, and support it on three small pillars, one of which must be hollow and equipped with a faucet B ; divide the lowest vessel C M into two parts with a partition H I , and fit a faucet to the lower one so as to be able to open and close it at will.

Place an object on the partition that the spectator, placed in O, will not be able to see by the direct ray N L.

If faucet B is opened, with water falling into the cavity C I , the ray N L will be refracted from the perpendicular, and will reflect towards O , and the spectator will see the object by the broken ray N O . If faucet B is closed and the one marked by the letter P is opened, the water will descend into the lowest cavity H I ; the refraction will cease, and no rays will come from the object to the eye. But by closing the faucet P again, and opening the other B , the cavity will refill, and the object will become visible as it was before. See Refraction. 

To construct a hydromantic vessel that represents external objects as though they were swimming in water, take a cylindrical vase A B C D (Plate [III]: Hydraulics Figure 32), divided into two by a glass E F , that is not perfectly polished: apply to point G a lens convex on both sides, and in H incline an elliptical mirror at an angle of 45 degrees; I H and H G should be a little less than the distance of the focus of the lens G ; so that the image of the object can pass through the cavity of the upper vessel; blacken the interior cavity, and fill the upper one with clear water. [2]

These machines belong to hydromancy , considered a branch of natural history; but, to return to its divination aspect, we add, according to Delrio, that there are several types of hydromancy , of which here are the principal ones. [3]

1. When, following invocations and other magic ceremonies, one saw written on the water the names of people or events that one desired to know, ordinarily these names were found written backwards, at least that was the case in the event Delrio cites, according to Niketas Choniates. Annales Book II . [4]
2. A vase full of water was used, and a ring suspended on a thread, with which one hit the sides of the vase a certain number of times.
3. Three small stones were thrown in quick succession into tranquil, dormant water, and from the circles formed on the surface of this water, as well as their intersection, predictions about the future were made.
4. The diverse movements and the agitation of the sea’s waves were examined; the Sicilians and the Euboeans were strongly devoted to this superstition, and some oriental Christians thus baptized the sea every year, as if it were an animated and reasonable being; but it isn’t superstitious to examine the state of the sea, to speculate if the calm will continue, or if a storm will come. Nor should we add to the number of superstitions that fall under the title of hydromancy , the ceremony the doge of Venice performs each year to marry the Adriatic Sea.
5. Predictions were also made based on the color of the water, and from the figures seen or believed to be represented there. It is thus, according to Varro, that Rome was informed what would be the outcome of the war against Mithridate; certain rivers or fountains were considered by the ancients more suitable than others in these operations. See Pegomancy.
6. It was also by a type of hydromancy that the ancient Germans, when they had some suspicion about the fidelity of their wives, claimed to clarify the truth: they threw the babies to whom [the wives] had given birth into the Rhine; and if they floated, they were considered legitimate, whereas bastards sank to the bottom; this is what Claudius alluded to in this verse,

   Et quos nascentes explorat gurgite Rhenus.

   Would it not be this ancient usage by which, in the same country, those accused of sorcery were forced to submit to the ordeal of cold water? See Ordeal.

7. A glass or another vase was filled with water, and after having certain words pronounced above it, it was examined to see if the water would bubble and spill over the edge. 
8. Water was placed in a glass or crystal basin, a drop of oil was added, and it was thought that the things one wanted to learn about became visible in the water, as in a mirror.
9. The wives of the ancient Germans practiced yet another sort of hydromancy , by examining the twists and turns, and the sound made by the water of rivers in the chasms or whirlpools that they formed, to predict the future. Clement of Alexandria Stromata Book I. [5]
10. Finally, we can associate hydromancy with a superstition that was in use in Italy, and that Delrio assures they still practiced in his time. When some people were suspected of a theft, the names of three of these people were written on as many small stones, that were thrown into the water, and he adds that some used holy water for this operation; but he does not add what they discovered by this medium. Delrio, Disquisitionum Magicarum, Book IV, Question VI, Section 3, p. 543 & 544 .

1. Marcus Terentius Varro (116 BCE - 27 BCE) was a prominent Roman scholar and prolific writer.

2. Much of the above description was translated in the original French directly from the entry “Hydromancy” in Chambers’s Cyclopaedia.

3. Martin Delrio (1551-1608) was a Jesuit theologian who wrote classic commentaries and works on witchcraft and demonology, including Disquisitionum magicarum, or, in English, Magical Investigations (Venice, 1640).

4. In his History , Greek historian Niketas (or Nicetas) Choniates chronicled twelfth and early thirteenth century Byzantium via his eyewitness accounts.

5. Clement of Alexandria (c. 150 – c. 215), born as Titus Flavius Clemens, was a Greek theologian and philosopher. Stromata (“ Patchwork ”) is one of Clement’s three major works.