igures, as it developed 84 per cent. of work done with only 16 per
cent. loss in friction.

A brief description of this remarkable machine will probably interest
the reader. In the two end pieces of a heavy iron frame were set three
rolls, or cylinders--one in the centre, another below, and the other
above--all three being in a vertical line. These rolls were of cast
iron three feet in diameter, having chilled-iron smooth face-plates of
considerable thickness. The lowest roll was set in a fixed bearing at
the bottom of the frame, and, therefore, could only turn around on its
axis. The middle and top rolls were free to move up or down from and
toward the lower roll, and the shafts of the middle and upper rolls were
set in a loose bearing which could slip up and down in the iron frame.
It will be apparent, therefore, that any material which passed in
between the top and the middle rolls, and the middle and bottom rolls,
could be ground as fine as might be desired, depending entirely upon the
amount of pressure applied to the loose rolls. In operation the material
passed first through the upper and middle rolls, and then between the
middle and lowest rolls.

This pressure was applied in a most ingenious manner. On the ends of the
shafts of the bottom and top rolls there were cylindrical sleeves, or
bearings, having seven sheaves, in which was run a half-inch endless
wire rope. This rope was wound seven times over the sheaves as above,
and led upward and over a single-groove sheave which was operated by the
piston of an air cylinder, and in this manner the pressure was applied
to the rolls. It will be seen, therefore, that the system consisted in a
single rope passed over sheaves and so arranged that it could be varied
in length, thus providing for elasticity in exerting pressure and
regulating it as desired. The efficiency of this system was incomparably
greater than that of any other known crusher or grinder, for while a
pressure of one hundred and twenty-five thousand pounds could be exerted
by these rolls, friction was almost entirely eliminated because the
upper and lower roll bearings turned with the rolls and revolved in the
wire rope, which constituted the bearing proper.

The same cautious foresight exercised by Edison in providing a safety
device--the fuse--to prevent fires in his electric-light system, was
again displayed in this concentrating plant, where, to save possible
injury to its expensive operating par