A pair of iron cylinders five feet long and six feet in diameter, covered with steel knobs, were set fifteen inches apart in a massive frame. The rolls weighed about seventy tons. By means of a steam engine these rolls were revolved in opposite directions until they attained a peripheral speed of about a mile a minute. Then the rocks were dumped into a hopper which guided them between the rolls, and in a few seconds, with a thunderous noise, they were reduced to pieces about the size of a man's head. The belts were released by means of slipping friction clutches when the load was thrown on the rolls, the breaking of the rocks being accomplished by momentum and kinetic energy. The broken rock then passed through similar rolls of a lesser size, by means of which it was reduced to much smaller pieces. These in their turn passed through a series of other machines in which they were crushed to fine powder. Here again Edison made another remarkable invention, called the "Three-High Rolls," for reducing the rock to fine powder. The best crushers he had been able to buy had an efficiency of only eighteen per cent, and a loss by friction of eighty-two per cent. By his invention he reversed these figures and obtained a working efficiency of eighty-four per cent, and reduced the loss to sixteen per cent. The problems of drying and screening the broken and crushed material were also solved most ingeniously by Edison's inventive skill and engineering ability, and always with the idea and purpose in mind of accomplishing these results by availing himself to the utmost of one of the great forces of Nature--gravity. The great extent of the concentrating works may be imagined when we state that two hundred and fifty tons of material per hour could be treated. Altogether, there were about four hundred and eighty immense magnetic separators in the plant, through which this crushed rock passed after going through the numerous crushing, drying, and screening processes. [Illustration: EDISON AT THE OFFICE DOOR OF THE ORE-CONCENTRATING PLANT AT EDISON, NEW JERSEY, IN THE 'NINETIES] If it had been necessary to transfer this tremendous quantity of material from place to place by hand the cost would have been too great. Edison, therefore, designed an original and ingenious system of mechanical belt conveyors that would automatically receive and discharge their loads at appointed places in the works, covering about a mile in transit. They went up and down, winding in and out, turning corners, delivering material from one bin to another, making a number of loops in the drying-oven, filling up bins, and passing on to the next one when full. In fact, these conveyors in automatic action seemed to play their part with human intelligence.