lustration of the giant rolls as
installed in the concentrating plant.

In practice, a small amount of power is applied to run the giant rolls
gradually up to a surface speed of several thousand feet a minute. When
this high speed is attained, masses of rock weighing several tons in one
or more pieces are dumped into a hopper which guides them into the gap
between the rapidly revolving rolls. The effect is to partially arrest
the swift motion of the rolls instantaneously, and thereby develop and
expend an enormous amount of kinetic energy, which with pile-driver
effect cracks the rocks and breaks them into pieces small enough to
pass through the fourteen-inch gap. As the power is applied to the rolls
through slipping friction-clutches, the speed of the driving-pulleys is
not materially reduced; hence the rolls may again be quickly speeded up
to their highest velocity while another load of rock is being hoisted
in position to be dumped into the hopper. It will be obvious from the
foregoing that if it were attempted to supply the great energy necessary
for this operation by direct application of steam-power, an engine of
enormous horse-power would be required, and even then it is doubtful
if one could be constructed of sufficient strength to withstand the
terrific strains that would ensue. But the work is done by the great
momentum and kinetic energy obtained by speeding up these tremendous
masses of metal, and then suddenly opposing their progress, the
engine being relieved of all strain through the medium of the slipping
friction-clutches. Thus, this cyclopean operation may be continuously
conducted with an amount of power prodigiously inferior, in proportion,
to the results accomplished.

The sketch (Fig. 4) showing a large boulder being dumped into the
hopper, or roll-pit, will serve to illustrate the method of feeding
these great masses of rock to the rolls, and will also enable the reader
to form an idea of the rapidity of the breaking operation, when it is
stated that a boulder of the size represented would be reduced by
the giant rolls to pieces a trifle larger than a man's head in a few
seconds.

After leaving the giant rolls the broken rock passed on through other
crushing-rolls of somewhat similar construction. These also were
invented by Edison, but antedated those previously described; being
covered by Patent No. 567,187, issued September 8, 1896. These rolls
were intended for the reducing of "one-man-size