The _millstones_ employed in grinding corn require to be made of a peculiar kind of stone. The greater proportion of our millstones are procured from a particular spot in Western Germany. At about ten miles from Coblentz is a small town called Andernach, the chief trade of which is in millstones, procured from the neighbouring quarries of Nieder Mendig. There are several quarries, averaging about fifty feet in depth, each quarry shaped like an inverted cone, down the sides of which the quarrymen descend by a spiral path. The quarrymen have to cut away through a superincumbent layer of soft porous stone, till they come to a layer of hard, blackish, heavy stone, regularly porous, and yielding sparks when struck with iron. This is the millstone, and requires good and well-prepared tools to work it; it is supposed to be a compact lava from some extinct volcano; and as there are fissures or gaps at intervals, these facilitate the separation of the stone into blocks suitable for millstones. All round the bottom of the conical cavities, the stone has been excavated in galleries or horizontal passages. The stones are brought to shape by means of hammers and chisels. A deep socket is cut through the middle of such stones as are intended for runners, or upper stones. The furrows on the surfaces of the stones are produced by means of a double-edged hammer, about 14 lbs. weight. Windmills are of two kinds; in one the wind is made to act upon vanes or sails, generally four, which are disposed so as to revolve by that action in a plane which is nearly vertical; and in the other, the axis of revolution being precisely vertical, any point on the surface of a vane revolves in a horizontal plane. The former is called a _vertical_ windmill, and the latter a _horizontal_ windmill. The building for a vertical windmill is generally a wall of timber or brickwork, in the form of a frustrum of a cone, and terminated above by a wooden dome, which is capable of revolving horizontally upon it. A ring of wood, forming the lower part of the dome, rests upon a ring of the same material at the top of the wall, and the surfaces in contact being made very smooth, the dome may easily be turned round upon the wall; and is prevented from sliding off by a rim which projects from it, and descends over the interior circumference of the lower ring. The dome in turning carries with it the windsails and their axle; and thus the wind sails may be adjusted to agree with the direction of the wind, or the plane in which the radii of the sails turn may be made perpendicular to that direction. The revolution is sometimes accomplished by the force of a man applied to a winch near the ground, but in general the wind itself is made to turn the dome or the mill by means of a set of small vanes, which are situated at the extremity of a long horizontal arm projecting from the dome, in a plane passing through the vertical shaft of the mill, and on the side opposite to the great sails.