A jacketed steel cylinder rotating on a horizontal axis and containing steel balls of varying diameter; the interior walls are usually equipped with baffle bars to impart a rolling and cascading action to the balls. The total weight of the balls may be 2000 lb or more. The grinding efficiency depends on the number of contacts between any two balls; thus the greater the number of balls, the more effective the grinding action. The material is introduced through an opening in the axis of the cylinder, which is then hermetically closed. Discharge is by the same opening after replacement of the cover plate with a grill to retain the balls. Ball mills can be adapted to continuous operation in which the feed enters at one end and is discharged at the other. Products ground are dry chemicals, paint pigments, etc.See Pebble Mill; Jar Mill.