ts, he devised an analogous factor,
providing all the crushing machinery with closely calculated "safety
pins," which, on being overloaded, would shear off and thus stop the
machine at once.

The rocks having thus been reduced to fine powder, the mass was ready
for screening on its way to the magnetic separators. Here again Edison
reversed prior practice by discarding rotary screens and devising a form
of tower screen, which, besides having a very large working capacity
by gravity, eliminated all power except that required to elevate the
material. The screening process allowed the finest part of the crushed
rock to pass on, by conveyor belts, to the magnetic separators, while
the coarser particles were in like manner automatically returned to the
rolls for further reduction.

In a narrative not intended to be strictly technical, it would probably
tire the reader to follow this material in detail through the numerous
steps attending the magnetic separation. These may be seen in a
diagram reproduced from the above-named article in the Iron Age, and
supplemented by the following extract from the Electrical Engineer,
New York, October 28, 1897: "At the start the weakest magnet at the top
frees the purest particles, and the second takes care of others; but the
third catches those to which rock adheres, and will extract particles
of which only one-eighth is iron. This batch of material goes back for
another crushing, so that everything is subjected to an equality of
refining. We are now in sight of the real 'concentrates,' which are
conveyed to dryer No. 2 for drying again, and are then delivered to
the fifty-mesh screens. Whatever is fine enough goes through to the
eight-inch magnets, and the remainder goes back for recrushing.
Below the eight-inch magnets the dust is blown out of the particles
mechanically, and they then go to the four-inch magnets for final
cleansing and separation.... Obviously, at each step the percentage of
felspar and phosphorus is less and less until in the final concentrates
the percentage of iron oxide is 91 to 93 per cent. As intimated at the
outset, the tailings will be 75 per cent. of the rock taken from the
veins of ore, so that every four tons of crude, raw, low-grade ore will
have yielded roughly one ton of high-grade concentrate and three tons of
sand, the latter also having its value in various ways."

This sand was transported automatically by belt conveyors to the rear of
the works to b