ed the Hoosac Tunnel, in Western Massachusetts,
February 9, 1875, completing another artery between East and West. The
tunnel passed through the Hoosac Mountain, a distance of four miles and
three-quarters, and had been in process of boring, though not
continuously, about fifteen years.

[Illustration: Rail line looping through a steep, narrow valley.]
The Big Loop on the Georgetown Branch of the Union Pacific, Colorado.

[Illustration: Portrait.]
Charles F. Brush.

[Illustration: Portrait.]
Moses G. Farmer.

[1875-1878]

The lighting of large spaces by electricity in a profitable manner may
be dated from 1875. The possibility of producing a brilliant light with
this fluid had been well known to physicists ever since Sir Humphry
Davy's experiments in 1813, but no method of generating the electricity
cheaply had hitherto been invented. Utilizing among others the
inventions of Dr. C. W. Siemens, Mr. Charles G. Brush, of Cleveland, 0.,
gave to the world in 1875 his remarkably efficient dynamo or generator,
and from that time the illumination of streets and squares by
electricity began to be somewhat common. There was, to be sure another
difficulty to be overcome, even for lighting on a grand scale, that of
maintaining a steady and continuous light. In this the Jablochkoff
candle, used in the Paris streets by 1878, was measurably successful. It
was a voltaic arc arrangement, in which, by making each of the two
carbon pencils alternately positive and negative, their ends were
consumed with equal rapidity and so kept perpetually the same distance
apart.

[Illustration: Portrait.]
Thomas A. Edison.

[Illustration: Three men watching a train passing through a long tunnel.]
The Hoosac Tunnel Lit by Glow Lamps, after the Plan of the Marr
Construction Company.

But the voltaic light was too brilliant for a small area. How to divide
and subdue it so as to render it suitable for house lighting, was still
a difficult problem. Farmer, Sawyer, Mann, and Edison, all attacked it
at nearly the same time, going back with singular accord from the
voltaic arc principle to that of incandescence in a vacuum. Edison, the
prodigy of the century in inventive genius, was the most successful.
Besides improving the dynamo, he perfected with little difficulty a
cheap vacuum-globe. After long experimenting he succeeded in the more
arduous task of securing an automatic checking of the current before it
became hot enough to c