The dropping well at Knaresborough, in Yorkshire, is particularly celebrated for them. An overhanging rock, several yards in depth, has been gradually formed of the calcareous matter which the water holds in solution; and, from this rock, it incessantly drops into the basin below. The persons who have the care of the place constantly keep these petrified articles for sale. Even old wigs and hair brooms are subjected to the powers of the water, to furnish subjects for attraction to the visitors. There are other springs of this description in Oxfordshire and Somersetshire, and particularly at Matlock, in Derbyshire. We are informed that at Dalton, on the south side of Mendip, the workmen not unfrequently discover large pieces of oak enveloped in blocks of stone which are four or five tons in weight. Blocks of tufa are, in some countries, cut and used for building stones; and this substance, when burned, becomes an excellent lime. Pieces of it are sometimes hollowed, and used as filtering stones. In the British Museum there is a human skull completely incrusted with stone, which was found in the river Tiber. The warm baths of Hungary are often so thickly coated at the sides and bottom with tufa, that, during certain intervals, it actually fills up the tubes and canals through which they are supplied. The _fur in teakettles_ is a somewhat similar deposit from water in boiling. 187. _PORTLAND STONE, BATH STONE, KETTON STONE, are different kinds of limestone; and, of a texture so hard and compact as to be used in building._ _They have their names from the places where they are respectively found, in Portland Island, near Bath, and at Ketton, in the county of Rutland._ Of Ketton stone several of the colleges in Cambridge are built. Its grain has a singular resemblance to the petrified roe of a fish, whence also it is sometimes called _roestone_. The bridges, St. Paul’s Cathedral, the Monument, and nearly all the buildings of late date in London, are constructed of Portland stone. Some of these kinds of stone, when first dug out of the quarry, are so soft that they are readily worked into any form which use or ornament may require. This is owing to the moisture with which they are naturally impregnated; but when they once become hardened, by exposure to the sun and air, they are extremely firm and solid. On the contrary, other kinds of limestone that are used for buildings imbibe and retain the moisture of the atmosphere, in consequence of which they burst or are crumbled by frost. We are informed that Portland stone was first used in London in the reign of James the First, that monarch, by the advice of his architects, having employed it in the construction of the banquetting house at Whitehall. After the great fire in London, it was brought into general use by Sir Christopher Wren.