CLAY. Any natural mixture of earths which breaks down or disintegrates in water, and affords a plastic ductile mixture may be called a clay. Pipe-Clay is of a grayish-white colour, has an earthy fracture, and a smooth greasy feel. Potter's Clay disintegrates by ex posure to the air ; when mixed with sand it is made into bricks and tiles. Stourbridge Clay is of a dark colour; it is extensively employed in the manufacture of crucibles. Brick-Clay or loam varies much in texture, and composition; it lies in abundance upon the London clay, and frequently rests upon an interposed bed of sand. Ldndon Clay is a blueish deposit, often including beds of sand.. stone. Plastic Clay consists of a variable number of sand, clay, and pebble beds, irregu larly alternating, lying immediately upon the chalk. Porcelain Clay is of various shades of white ; it is dull and opaque ; occurs friable or compact ; feels soft to the fingers, and ad heres to the tongue.
Clay is an essential component part of all fertile soils. A clay soil consists of a large proportion of alumina united to silica of vari ous degrees of fineness, and frequently also a portion of carbonate of lime. In rich wheat soils the silica is very fine, and intimately mixed with the alumina ; but English clays are not usually of this character, and are not so much approved for wheat crops as lighter soils. Clay land will bear a repetition of the same crops much oftener than lighter lands. The great disadvantage of clay soils in a moist climate like that of Great Britain arises from an excess of water, and the obvious remedy is perfect draining of the subsoil. [Dxsnexsm.] Clay is extensively used in many parts of England to improve light land, by being car ried on the surface in considerable quantities.
Burnt clay is used as manure. Clay by burn ing alters its nature ; it becomes insoluble in water, and loses its attraction for it ; it then resembles silicious sand, and may greatly im prove a very strong retentive clay, tempering it and rendering it more porous.
The Newcastle district will furnish many specimens of clays used in the Arts, for exhi bition at the approaching industrial festival. The manufacture of fire clay into bricks, &c., is carried on very extensively in the neighbour hood of this town ; and among the articles exhibited, formed of clay, will be fire-bricks, vases, pedestals, and ornaments of various kinds. Stourbridge will in like manner send specimens of those clays and clay-manufac tures which have given celebrity to that town.
CLE'PSYDIVi. Before the invention of pendulum clocks, it was not unusual in astron omical observations to measure time by the flowing of water, upon a principle which, in its most simple application, resembled that of the hour-glass, but which was varied by con trivances for accuracy or ornament. Such a contrivance was called a clepsydra or water clock. The Chaldeans divided the day into twelve equal parts by a Clepsydra. Water clocks were used in Egypt, Greece, and Rome; and there is evidence that they were used in India in the twelfth century. Many fanciful forms of Clepsydra; are given in old works on Hydraulics ; but modern clock and watch work has rendered them quite obsolete.