d 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.

[Footnote 31: The faces of these rolls were smooth, but as
three-high rolls came into use later in Edison's Portland
cement operations the faces were corrugated so as to fit
into each other, gear-fashion, to provide for a high rate of
feed]

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.

Several other important patents have been issued to Edison for crushing
and grinding rolls, some of them being for elaborations and improvements
of those above described but all covering methods of greater economy and
effectiveness in rock-grinding.

Edison's work on conveyors during the period of his ore-concentrating
labors was distinctively original, ingenious and far in advance of
the times. His conception of the concentrating problem was broad and
embraced an entire system, of which a principal item was the continuous
transfer of enormous quantities of material from place to place at
the lowest possible cost. As he contemplated the concentration of six
thousand t