The mills of Pompeii, with slight variations, are all of one type; if there were watermills on the Sarno, no trace of them has been found. The millstones are of lava (p. 15). The lower stone, _meta_, has the shape of a cone resting on the end of a cylinder, but the cylindrical part is in most cases partially concealed by a thick hoop of masonry, the top of which was formed into a trough to receive the flour, and was covered with sheet lead (Fig. 220). A square hole, five or six inches across, was cut in the top of the cone, in which was inserted a wooden standard; this supported a vertical iron pivot on which the frame of the upper millstone turned. [Illustration: Fig. 221.--Section of a mill, restored.] The shape of the upper millstone, _catillus_, may best be seen in Fig. 221. It was like a double funnel, the lower cavity being fitted to the cone of the lower millstone, while that in the upper part answered the purpose of a hopper. The two cavities were connected at the centre by an opening similar to that of an hourglass, which left room for the standard and allowed the grain to run down slowly, when the _catillus_ was turned, to be ground between the two stones. The flour ran out at the base of the cone and fell into the trough, ready to be sifted and made into bread. The upper millstone was nicely balanced over the lower, the surface of which it touched but lightly; it could not have rested on the under stone with full weight, for in that case the strength of a draft animal would not have sufficed to move it. The stones could be set for finer or coarser grinding by changing the length of the standard. The arrangement for turning the mill was simple. In shaping the upper millstone, strong shoulders were left in the narrowest part (Fig. 220), on opposite sides. In these square sockets were cut, in which the ends of shafts were inserted and firmly fastened by round bolts passing through the shoulders (Fig. 221). The shafts were tied to the ends of the crossbeam above by curved vertical pieces of wood, or by straps of iron, which were let into grooves in the stone and so made firm. The crosspiece above, which turned on the pivot in the end of the standard, was sometimes of iron, sometimes of wood with an iron socket fitting the pivot. The framework must necessarily have been exceedingly strong. One of the mills at Pompeii (IX. iii. 10) has lately been set up with new woodwork, and grinds very well. [Illustration: Fig. 222.--A mill in operation. Relief in the Vatican Museum.]