dently driven by a direct-coupled engine. Mr. Sprague
compared the relations thus established between electricity and the
high-speed engine not to those of debtor and creditor, but rather to
those of partners--an industrial marriage--one of the most important
in the engineering world. Here were two machines destined to be joined
together, economizing space, enhancing economy, augmenting capacity,
reducing investment, and increasing dividends.

While rapid progress was being made in this and other directions, the
wheels of industry were humming merrily at the Edison Tube Works, for
over fifteen miles of tube conductors were required for the district,
besides the boxes to connect the network at the street intersections,
and the hundreds of junction boxes for taking the service conductors
into each of the hundreds of buildings. In addition to the immense
amount of money involved, this specialized industry required an enormous
amount of experiment, as it called for the development of an entirely
new art. But with Edison's inventive fertility--if ever there was a
cross-fertilizer of mechanical ideas it is he--and with Mr. Kruesi's
never-failing patience and perseverance applied to experiment and
evolution, rapid progress was made. A franchise having been obtained
from the city, the work of laying the underground conductors began in
the late fall of 1881, and was pushed with almost frantic energy. It
is not to be supposed, however, that the Edison tube system had then
reached a finality of perfection in the eyes of its inventor. In his
correspondence with Kruesi, as late as 1887, we find Edison bewailing
the inadequacy of the insulation of the conductors under twelve hundred
volts pressure, as for example: "Dear Kruesi,--There is nothing wrong
with your present compound. It is splendid. The whole trouble is
air-bubbles. The hotter it is poured the greater the amount of
air-bubbles. At 212 it can be put on rods and there is no bubble. I have
a man experimenting and testing all the time. Until I get at the proper
method of pouring and getting rid of the air-bubbles, it will be waste
of time to experiment with other asphalts. Resin oil distils off easily.
It may answer, but paraffine or other similar substances must be put in
to prevent brittleness, One thing is certain, and that is, everything
must be poured in layers, not only the boxes, but the tubes. The tube
itself should have a thin coating. The rope should also have a coat