5. The grinding surfaces of the stones have channels, or furrows, cut in them, which proceed obliquely from the centre to the circumference. The furrows are cut slantwise on one side, and perpendicular on the other; so that each of the ridges which they form, has a sharp edge; and, when the upper stone is in motion, these edges pass one another, like the blades of a pair of scissors, and cut the grain the more easily, as it falls upon the furrows.
6. By a careful inspection of the following picture, the whole machinery of a common mill may be understood.
A represents the water-wheel; B, the shaft to which is attached the cog-wheel C, which acts on the trundle-head, D; and this, in turn, acts on the moveable stone. The spindle, trundle-head, and upper stone, all rest entirely on the beam, F, which can be elevated or depressed, at pleasure, by a simple apparatus; so that the distance between the stones can be easily regulated, to grind either fine or coarse. The grain about to be submitted to the action of the mill, is thrown into the hopper, H, whence it passes by the shoe, or spout I, through a hole in the upper stone, and then between them both.
7. The upper stone is a little convex, and the other a little concave. There is a little difference, however, between the convexity and the concavity of the two stones: this difference causes the space between them to become less and less towards their edges; and the grain, being admitted between them, is, consequently, ground finer and finer, as it passes out in that direction, in which it is impelled by the centrifugal power of the moving stone.
8. If the flour, or meal, is not to be separated from the bran, the simple grinding completes the operation; but, when this separation is to be made, the comminuted grain, as it is thrown out from between the stones, is carried, by little leathern buckets fastened to a strap, to the upper end of an octagonal sieve, placed in an inclined position in a large box. The coarse bran passes out at the lower end of the sieve, or bolt, and the flour, or fine particles of bran, through the bolting-cloth, at different places, according to their fineness. At the head of the bolt, the superfine flour passes; in the middle, the fine flour; and at the lower end, the coarse flour and fine bran; which, when mixed, is called canel, or shorts.
9. The best material of which mill-stones are made, is the burr-stone, which is brought from France in small pieces, weighing from ten to one hundred pounds. These are cemented together with plaster of Paris, and closely bound around the circumference with hoops made of bar iron. For grinding corn or rye, those made of sienite, or granite rock, are frequently used.
10. A mill, exclusively employed in grinding grain, consumed by the inhabitants of the neighborhood, is called a grist or custom mill; and a portion of the grist is allowed to the miller, in payment for his services. The proportion is regulated by law; and, in our own country, it varies according to the legislation of the different states.
11. Mills in which flour is manufactured, and packed in barrels for sale, are called merchant mills. Here, the wheat is purchased by the miller, or by the owner of the mill, who relies upon the difference between the original cost of the grain, and the probable amount of its several products, when sold, to remunerate him for the manufacture, and his investments of capital. In Virginia, and, perhaps, in some of the other states, it is a common practice among the farmers, to deliver to the millers their wheat, for which they receive a specified quantity of flour.