ons daily, the expense of manual labor to move such an immense
quantity of rock, sand, and ore would be absolutely prohibitive. Hence,
it became necessary to invent a system of conveyors that would be
capable of transferring this mass of material from one place to another.
And not only must these conveyors be capable of carrying the material,
but they must also be devised so that they would automatically receive
and discharge their respective loads at appointed places. Edison's
ingenuity, engineering ability, and inventive skill were equal to the
task, however, and were displayed in a system and variety of conveyors
that in practice seemed to act with almost human discrimination. When
fully installed throughout the plant, they automatically transferred
daily a mass of material equal to about one hundred thousand cubic feet,
from mill to mill, covering about a mile in the transit. Up and down,
winding in and out, turning corners, delivering material from one to
another, making a number of loops in the drying-oven, filling up bins
and passing on to the next when they were full, these conveyors in
automatic action seemingly played their part with human intelligence,
which was in reality the reflection of the intelligence and ingenuity
that had originally devised them and set them in motion.

Six of Edison's patents on conveyors include a variety of devices that
have since came into broad general use for similar work, and have been
the means of effecting great economies in numerous industries of widely
varying kinds. Interesting as they are, however, we shall not attempt to
describe them in detail, as the space required would be too great. They
are specified in the list of patents following this Appendix, and may be
examined in detail by any interested student.

In the same list will also be found a large number of Edison's patents
on apparatus and methods of screening, drying, mixing, and briquetting,
as well as for dust-proof bearings, and various types and groupings
of separators, all of which were called forth by the exigencies and
magnitude of his great undertaking, and without which he could not
possibly have attained the successful physical results that crowned his
labors. Edison's persistence in reducing the cost of his operations is
noteworthy in connection with his screening and drying inventions, in
which the utmost advantage is taken of the law of gravitation. With
its assistance, which cost nothing, these opera