w of retardation, and cleared up the
whole matter. He showed that the velocity of a signal through a given
core was inversely proportional to the square of the length of the
core. That is to say, in any particular cable the speed of a signal is
diminished to one-fourth if the length is doubled, to one-ninth if it
is trebled, to one-sixteenth if it is quadrupled, and so on. It was
now possible to calculate the time taken by a signal in traversing the
proposed Atlantic line to a minute fraction of a second, and to design
the proper core for a cable of any given length.

The accuracy of Thomson's law was disputed in 1856 by Dr. Edward O.
Wildman Whitehouse, the electrician of the Atlantic Telegraph Company,
who had misinterpreted the results of his own experiments. Thomson
disposed of his contention in a letter to the ATHENAEUM, and the
directors of the company saw that he was a man to enlist in their
adventure. It is not enough to say the young Glasgow professor threw
himself heart and soul into their work. He descended in their midst
like the very genius of electricity, and helped them out of all their
difficulties. In 1857 he published in the ENGINEER the whole theory of
the mechanical forces involved in the laying of a submarine cable, and
showed that when the line is running out of the ship at a constant speed
in a uniform depth of water, it sinks in a slant or straight incline
from the point where it enters the water to that where it touches the
bottom.

To these gifts of theory, electrical and mechanical, Thomson added a
practical boon in the shape of the reflecting galvanometer, or mirror
instrument. This measurer of the current was infinitely more sensitive
than any which preceded it, and enables the electrician to detect
the slightest flaw in the core of a cable during its manufacture and
submersion. Moreover, it proved the best apparatus for receiving the
messages through a long cable. The Morse and other instruments, however
suitable for land lines and short cables, were all but useless on the
Atlantic line, owing to the retardation of the signals; but the mirror
instrument sprang out of Thomson's study of this phenomenon, and was
designed to match it. Hence this instrument, through being the fittest
for the purpose, drove the others from the field, and allowed the first
Atlantic cables to be worked on a profitable basis.

The cable consisted of a strand of seven copper wires, one weighing
107 pounds a nauti