One of the most remarkable facts in the economy of the telegraph is,
that the line, when connected with a battery in action, propagates
the hydro-galvanic waves in either direction without interference. As
several successive syllables of sound may set out in succession from
the same place, and be on their way at the same time, to a listener at
a distance, so also, where the telegraph-line is long enough, several
waves may be on their way from the signal station before the first one
reaches the receiving station; two persons at a distance may pronounce
several syllables at the same time, and each hear those emitted by the
other. So, on a telegraph-line of two or three thousand miles in length
in the air, and the same in the ground, two operators may at the same
instant commence a series of several dots and lines, and each receive
the other’s writings, though the waves have crossed each other on the
way.