f this operation. The enormous force exerted
during this process may be illustrated from the fact that during its
development, in running one of the early forms of rolls, pieces of rock
weighing more than half a ton would be shot up in the air to a height of
twenty or twenty-five feet.

The giant rolls were two solid cylinders, six feet in diameter and five
feet long, made of cast iron. To the faces of these rolls were bolted a
series of heavy, chilled-iron plates containing a number of projecting
knobs two inches high. Each roll had also two rows of four-inch knobs,
intended to strike a series of hammer-like blows. The rolls were set
face to face fourteen inches apart, in a heavy frame, and the total
weight was one hundred and thirty tons, of which seventy tons were in
moving parts. The space between these two rolls allowed pieces of rock
measuring less than fourteen inches to descend to other smaller rolls
placed below. The giant rolls were belt-driven, in opposite directions,
through friction clutches, although the belt was not depended upon for
the actual crushing. Previous to the dumping of a skip, the rolls were
speeded up to a circumferential velocity of nearly a mile a minute, thus
imparting to them the terrific momentum that would break up easily in a
few seconds boulders weighing five or six tons each. It was as though a
rock of this size had got in the way of two express trains travelling
in opposite directions at nearly sixty miles an hour. In other words,
it was the kinetic energy of the rolls that crumbled up the rocks with
pile-driver effect. This sudden strain might have tended to stop the
engine driving the rolls; but by an ingenious clutch arrangement the
belt was released at the moment of resistance in the rolls by reason of
the rocks falling between them. The act of breaking and crushing would
naturally decrease the tremendous momentum, but after the rock was
reduced and the pieces had passed through, the belt would again come
into play, and once more speed up the rolls for a repetition of their
regular prize-fighter duty.

On leaving the giant rolls the rocks, having been reduced to pieces not
larger than fourteen inches, passed into the series of "Intermediate
Rolls" of similar construction and operation, by which they were still
further reduced, and again passed on to three other sets of rolls
of smaller dimensions. These latter rolls were also face-lined with
chilled-iron plates; but, unlike the