‘They consist of two flat cylindrical masses inclosed within a wooden or sheet metal case, the lower, or _bed-stone_, being permanently fixed, while the upper, or _runner_, is accurately pivoted and balanced over it The average size of millstones is about four feet two inches in diameter, by twelve inches in thickness, and they are made of a hard but cellular siliceous stone, called buhr-stone, the best qualities of which are obtained from La-Ferté-sous-Jouarre, department of Seine et Marne, France. Millstones are generally built up of segments, bound together round the circumference by an iron hoop, and backed with plaster of Paris. The bed-stone is dressed to a perfectly flat plane surface, and a series of grooves, or shallow depressions, are cut in it, generally in the manner shown, which represents the grinding surface of an upper or running stone. The grooves on both are made to correspond exactly, so that when the one is rotated over the other the sharp edges of the grooves, meeting each other, operate like a rough pair of scissors, and thus the effect of the stones on grain submitted to their action is at once that of cutting, squeezing, and crushing. The dressing and grooving of millstones is generally done by hand picking, but sometimes black amorphous diamonds (_carbonado_) are used, and emery wheel dressers have likewise been suggested. The upper stone, or runner, is set in motion by a spindle on which it is mounted, which passes up through the centre of the bed-stone, and there are screws and other appliances for adjusting and balancing the stone. Further provision is made within the stone case for passing through air to prevent too high a heat being developed in the grinding operation, and sweepers for conveying the flour to the meal spout are also provided.