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.
12. The power most commonly employed to put heavy machinery in operation, is that supplied by water. This is especially the case with regard to mills for grinding grain; but, when this cannot be had, a substitute is found in steam, or animal strength. The wind is also rendered subservient to this purpose. The wind-mill was invented in the time of Augustus Cæsar. During the reign of this emperor, and probably long before, mules and asses were employed by both the Greeks and Romans in turning their mills. The period at which water-mills began to be used cannot be certainly determined. Some writers place it as far back as the Christian era.
13. Wheat flour is one of the staple commodities of the United States, and there are mills for its manufacture in almost every part of the country, where wheat is extensively cultivated; but our most celebrated flour-mills are on the Brandywine Creek, Del., at Rochester, N. Y., and at Richmond, Va.