GRINDING AND CRUSHING. 291 GRINDSTONE. ])ol)l)lcs are substituttil for niptal balls. In the first type of ball mills named the material is fed to the mill so as to be ground between the. traveling balls and the path on which they travel; in the seennd type the material and balls are introduced together into the rotating drum, and the grinding is done by the constant movement and fall of the l)alls. The inside of the drum is provided with a hard lining to withstand wear. Hall mills are used ehielly for grinding cement, clinker, phosphate rock, etc. in use was the invention of James Bogardus, of New York City. Two wheels, having on their faces concentric grooves, have dilVerent axes of revolution, being eccentric, whence the mill derives its name. The wheels are placed hori/ontally, and the lower one is turned by a shaft at the rate of from GOO to 800 revolutions p<'r minute. The ii]iper wheel also takes ou a motion from the ini- jjuise of material brought against it, but being eccentric, the material is brought diagonally against the edges of the groove, making the grind- ^^^^^^^^^ ^^^^^^^ Fia. 6. TRANSVERSE ASD LO.VGITfDIXAL SECTIONS OF BALL MILL FOR FINE GRINDING. Rolls and edge-runner mills are also used for grinding as well as for crushing. A form of edge- runner mill much used in grinding cement is the Griffin mill. The pulverizing roll is attached to the lower end of a shaft which is suspended by a universal joint w-ithin a horizontal pulley. Above the roll on the shaft is a fan and on the bottom of the roll are plows or stirrers, which agitate the material being ground. Four stand- ards support the grinding mechanism proper. The clinker enters a hopper from which it is carried by a screw feed into the pan. which is surmounted bv a ring-sha])ed die or bearing sur- face for the roll. The shaft carrying the roll revolves with the pulley, and being hung so as to swing free, the centrifugal force brings the edge of the roll hard against the die, so that it revolves on the shaft in a reverse direction to the rotation of the shaft. Where a quantity of material to be reduced has been fed into the mill sufficient to fill the pan as high as the plows on the lower side of the roll, they work it in, stir it up, and throw it against the die so that it is acted upon by the roll. The cnffee-mill type of grinding machine is commonly employed for grinding bark, spices, dry paint, and similar easily crushed substances. This form of mill consists of a conicalsliaiied runner rotated by a vertical shaft inside a cone- shaped casting. The grinding surfaces of siich mills are usually of cast iron, and are corrugated or ribbed to facilitate the grinding action. One of the most ingenious, simple, and effective mills ing very efl'ective and preventing clogging. Over 200 barrels of sugar have been ground per hour in a l(j-inch mill. It will grind 5 tons of oil- cake, half a ton of bark, 2 tons of white lead in oil, 4 tons of iron oi'e, or 2 tons of other ores per hour. It is very strong and not easily disoi'dered. Grinding machines for shaping and polishing metals, glass, etc., are referred to under Grinding. See also Flouk. GRIN'DLE. One of the many quaint local names, more fully 'John A. Grindle,' given in the Southern Mississippi Valley to a strange fish, the mudfish (Ainia calva), elsewhere de- scribed under BowFiN. GRINDSTONE. A flat, circular stone made to revolve upnii an axis, and used for grinding steel, glass, and other stones, etc. Grindstones are of various degrees of coarseness, according to the purpose for which they are to be used. Grindstones are usually mounted more or less simply, but the large stones employed in cutlery manufacture are turned by machinery at a speed as great as practicable without bursting the stone by the centrifugal force. Grindstones are com- monly made of a siliceous sandstone, in which the grains are sharp and there is little cement to bind them together. The surface is this pre- vented from wearing smooth. Some of the best grindstones in the United Stales come fri>m Berea. Ohio, and are obtained from certain layers of the Carboniferous sandstone or Herea grit. Artificial grindstones of very uniform and per-
