" rocks to small pieces,
which at the time of their original inception was about the standard
size of similar machines. At the Edison concentrating plant the broken
rock, after passing through these rolls, was further reduced in size by
other rolls, and was then ready to be crushed to a fine powder through
the medium of another remarkable machine devised by Edison to meet his
ever-recurring and well-defined ideas of the utmost economy and
efficiency.

NOTE.--Figs. 3 and 4 are reproduced from similar sketches on pages 84
and 85 of McClure's Magazine for November, 1897, by permission of S. S.
McClure Co.

The best fine grinding-machines that it was then possible to obtain were
so inefficient as to involve a loss of 82 per cent. of the power
applied. The thought of such an enormous loss was unbearable, and he did
not rest until he had invented and put into use an entirely new
grinding-machine, which was called the "three-high" rolls. The device
was covered by a patent issued to him on November 21, 1899, No. 637,327.
It was a most noteworthy invention, for it brought into the art not only
a greater efficiency of grinding than had ever been dreamed of before,
but also a tremendous economy by the saving of power; for whereas the
previous efficiency had been 18 per cent. and the loss 82 per cent.,
Edison reversed these figures, and in his three-high rolls produced a
working efficiency of 84 per cent., thus reducing the loss of power by
friction to 16 per cent. A diagrammatic sketch of this remarkable
machine is shown in Fig. 5, which shows a front elevation with the
casings, hopper, etc., removed, and also shows above the rolls the rope
and pulleys, the supports for which are also removed for the sake of
clearness in the illustration.

For the convenience of the reader, in referring to Fig. 5, we will
repeat the description of the three-high rolls, which is given on pages
487 and 488 of the preceding narrative.

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 about three feet in
diameter, made of cast-iron, and had face-plates of chilled-iron. [31]
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 an