The diameter of the common mill-stones is, in general, from five to seven feet, and their thickness varies from 12 to 15 or 18 inches. They usually endure 35 or 40 years ; and. when they have been employed for a long time, so as to be considerably diminished, they are edged, or cut anew, in order to communicate to their surface a figure contrary to that which they-originally bore; afterwards the upper mill-stone is made the lower, or bed-.stone.
These stones are an article of extensive utility, and were formerly imported in great numbers from France: the Burr-stones of that country having been found harder and more durable than any that were dug out of British quarries.— To prevent the national expence incurred by such importation, the patriotic Society for. the Encouragement of Arts, etc. offered a liberal premium for the discovery of a quarry of mill-stones similar to the French burrs ; which desirable object was attained in 1799, by Mr. RichArd Bowes, of Conway, in North Wales, to whose widow the Society, in 1800, voted the reward of 1OOl.
The quarry which Mr. Bowes discovered, extends to a very considerable distance from the town of Conway. The stone dug from it, appears, from its external characters, to consist of quartz and cherts. When first taken out of the soil, it is much softer, and more easily wrought into its proper shape, than after it has been exposed to the air, though only for a day. The vein in the quarry contains every variety of the stone, whether cellular, close, hard, or soft, and runs to such a depth, that the industrious discoverer con-sidered it as inexhaustible.
Numerous certificates from the most respectable persons have been sent to the Society above mentioned, all of whom agree in stating, that the Conway-stone will form a very vauable substitute for, and answer every purpose of, the French burr-stones.
In the year 1796, a patent was granted to Mr. MAJOR Pratt, for his invention of a method of manufacturing a composition-stone, calculated for grinding corn, and various other articles, in the same manner as is effected by the common mill-stones. His artificial compound is stated to consist in mixing certain proportions of siliceous and argillaceous earths (that can only be ascertained by practice), with about one-seventh part of calcareous earth. These are exposed to a fire, heated to the degree usually required in calcining lime, for the space of twenty-four hours, or such farther period as experience alone can determine; after which the composition may be formed into durable stones, that are said to afford proper substitutes for those compounded by Nature.