f gas
jets burning at each hour up to two or three o'clock in the morning. The
next step was to divide the region into a number of sub-districts and
institute a house-to-house canvass to ascertain precisely the data and
conditions pertinent to the project. When the canvass was over, Edison
knew exactly how many gas jets there were in every building in the
entire district, the average hours of burning, and the cost of light;
also every consumer of power, and the quantity used; every hoistway to
which an electric motor could be applied; and other details too numerous
to mention, such as related to the gas itself, the satisfaction of
the customers, and the limitations of day and night demand. All this
information was embodied graphically in large maps of the district, by
annotations in colored inks; and Edison thus could study the question
with every detail before him. Such a reconnaissance, like that of a
coming field of battle, was invaluable, and may help give a further idea
of the man's inveterate care for the minutiae of things.

The laboratory note-books of this period--1878-80, more
particularly--show an immense amount of calculation by Edison and his
chief mathematician, Mr. Upton, on conductors for the distribution of
current over large areas, and then later in the district described.
With the results of this canvass before them, the sizes of the main
conductors to be laid throughout the streets of this entire territory
were figured, block by block; and the results were then placed on the
map. These data revealed the fact that the quantity of copper required
for the main conductors would be exceedingly large and costly; and,
if ever, Edison was somewhat dismayed. But as usual this apparently
insurmountable difficulty only spurred him on to further effort. It
was but a short time thereafter that he solved the knotty problem by an
invention mentioned in a previous chapter. This is known as the "feeder
and main" system, for which he signed the application for a patent on
August 4, 1880. As this invention effected a saving of seven-eighths of
the cost of the chief conductors in a straight multiple arc system, the
mains for the first district were refigured, and enormous new maps were
made, which became the final basis of actual installation, as they were
subsequently enlarged by the addition of every proposed junction-box,
bridge safety-catch box, and street-intersection box in the whole area.

When this patent, afte