cal action, we move one of the bodies
with respect to the other, we shall produce, according as the action
overcomes cohesion, more or less disintegration of the bodies. The
resulting wear in each of them will evidently be inversely proportional
to its hardness and the nature of its surface; and it will vary, besides,
with the pressure exerted between the surfaces and the velocity of the
mechanical action. We may say, then, that the wear resulting from rubbing
two bodies against each other is a function of their degree of hardness,
of the extent and state of their surface, of the pressure, of the
velocity, and of the time.

[Illustration: FIGS. 1, 2 and 3.--APPARATUS FOR SAWING STONE.]

According as these factors are varied in a sense favorable or unfavorable
to their proper action, we obtain variations in the final erosion. Thus,
in rubbing together two bodies of different hardness and nature of
surface, we obtain a wear inversely proportional to the hardness and
state of polish of their surfaces. Through the interposition of a
pulverized hard body we can still further accelerate such wear, as a
consequence of the rapid renewal of the disintegrating element.

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, 71/2 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