The gradual wear effected over the entire surface of a body brings about
a polish, while that effected along a line or at some one point
determines a cleavage or an aperture.

The process usually employed in quarries or stone-yards for sawing
consists in slowly moving a stone-saw backward and forward, either by
hand or machinery, and with scarcely any pressure. Mr. P. Gray has,
however, devised a new process, which is based upon the theoretical
considerations given above. His _helicoidal saw_ is, in reality, an
endless cable formed by twisting together three steel wires in such a way
as to give the spirals quite an elongated pitch.

The apparatus in its form for cutting blocks of stone into large slabs
(Figs. 1, 2, and 3) consists of two frames, A A, five feet apart, each
formed of two iron columns, 7½ feet in height and one foot apart, fixed
to cast iron bases resting upon masonry. At the upper part, a frame, B B,
formed of double T-irons cross-braced here and there, supports a
transmission composed of gearwheels, R R, and a pitch-chain, G G. Along
the columns of the frame, which serve as guides, move two kinds of
pulley-carriers, C C. The pulleys, D D, are channeled, and receive the
cable, a a, which serves as a helicoidal saw. The direction of the saw's
motion is indicated by the arrow. The carriages, C C, are traversed by
screws, V V, which are fixed between the columns. The extremity, v, of
the axle of the pulley to the right is threaded, and actuates a
helicoidal wheel, E, which transmits motion to the wheel, R, through the
intermedium of the vertical shaft, F. This transmission, completed by the
wheels, R R, and the pitch-chains, G G, is designed to move the saw
vertically, through the simultaneous shifting of the carriages, C C. A
tension weight, P, through the intermedium of pulleys, D_{1} D_{1},
permits of keeping the saw taut. A reservoir, H, at the upper part of the