The main object of the railroad engineer is to
reduce his road as nearly as possible to a level. High
grounds are to be cut down, and embankments raised
across the lower lands. When a mountain intervenes,
through which an open cut is impracticable, the ex-
pedient of a tunnel has to be adopted. When a deep
valley lays in the way, and an embankment is not
feasible, then there must be a viaduct. And when an
arm of the sea, such as the MenaiSlTa\\.9»,'\\^^ \a\i^



20 RAILROADS OF THE UNITED STATES.
crossed, it must be overleaped on iron tubes swung in
mid air. Of the eight thousand and fifty-five miles
of railroad in operation in England six years ago,
seventy miles passed through tunnels, and more than
fifty miles over viaducts; while of railroad bridges
there had been built some three thousand large and
costly structures.
Incredible difficulties have been encountered by
engineers in carrying embankments of earth across low
grounds. These grounds, in many places, under a
fair, green surface, have been found to conceal the re-
mains of ancient bogs and swamps, sometimes of great
depth. Thus, on one English railroad, about six
hundred tons of stone and earth were daily used to
form an embankment across a valley, and morning
after morning, for many weeks, the material deposited
on the preceding day was found to have disappeared.
