These are principally manufactured in Germany; some are made of clay covered with a glaze and baked as in pottery; others are made of alabaster and marble; but the greater part are made of a hard stone found near Coburg in Saxony. The stone is first broken with the hammer into small cubical fragments, and about 100 to 150 of these are ground at one time in a mill, somewhat like a flour mill. The lower stone, and which remains at rest, has several concentric circular grooves or furrows; the upper stone is of the same diameter as the lower, and is made to revolve by water or other power. Minute streams of water are directed into the furrows of the lower stone. The pressure of the runner on the little pieces rolls them over in all directions, and in about one quarter of an hour the whole of the rough fragments are reduced into nearly accurate spheres. Frequently a thick circular slab of oak or elm is used instead of the upper or revolving stone. - Extracted from Gill's Description. See Tech. Repos. for 1828, p. 219.

The late Mr. Henry Guy's method, by which spheres of metal and other hard substances, are produced with perfect accuracy, will be described in Chap. XXXIII. Section 4, of this volume.