HYDROMANCY

, the act or art of divining or foretelling future events by means of water.

This is one of the four general kinds of divination: the other three respecting the other elements, fire, air, and earth, are denominated respectively pyromancy, aeromancy, and geomancy.

Varro mentions the Persians as the first inventors of Hydromancy, adding, that Numa Pompilius and Pythagoras made use of it.

The writers on optics furnish us with divers Hydromantic machines, vessels, &c. For example,

To construct an Hydromantic machine, by means of which an image or object shall be removed out of the sight of the spectator, and restored again at pleasure, without altering the position, either of the one or the other. Provide two vessels, ABF, CGLK (Plate 12, fig. 3), the uppermost filled with water, and supported by three little pillars, one of which BC is hollow, and furnished with a cock B. Let the lower vessel CL be divided by a partition HI into two parts, the lower of which may be opened or closed by means of a cock at P. Upon the partition place an object, or image, which the spectator at O cannot see by a direct ray GL.

If now the cock B be opened, the water descending into the cavity CI, the ray GL will be refracted from the perpendicular GR to O; so that the spectator will now see the object by the refracted ray OG. And again, shutting the cock B, and opening the other P, the water will descend into the lower cavity HL; where, the refraction ceasing, no rays will now come from the object to the eye: but upon shutting the cock P, and opening the other B, the water will fill the cavity again, and bring the object in sight of O afresh.

To make an Hydromantic vessel, which shall exhibit the images of external objects as if swimming in water. Provide a cylindrical vessel ABCD (fig. 4, pl. 12.) divided into two cavities by a glass EF, not perfectly polished: in G apply a lens convex on both sides; and in H incline a plane mirror, of an elliptic figure, to an angle of 45 degrees; and let IH and HG be something less than the distance of the fccus of the lens G; so that the place of the images of an object radiating through the same may fall within the cavity of the upper vessel: let the inner cavity be blacked, and the upper filled with clear water.—If now the vessel be disposed in a dark place, so as the lens be turned towards an object illuminated by the sun, its image will be seen as swimming in the water.