h system. Let one example of his work be cited. In 1896
a cable was laid between Lavernock, near Cardiff, on the Bristol
Channel, and Flat Holme, an island three and a third miles off. As the
channel at this point is a much-frequented route and anchor ground, the
cable was broken again and again. As a substitute for it Mr. Preece, in
1898, strung wires along the opposite shores, and found that an electric
pulse sent through one wire instantly made itself heard in a telephone
connected with the other. It would seem that in this etheric form of
telegraphy the two opposite lines of wire must be each as long as the
distance which separates them; therefore, to communicate across the
English Channel from Dover to Calais would require a line along each
coast at least twenty miles in length. Where such lines exist for
ordinary telegraphy, they might easily lend themselves to the Preece
system of signalling in case a submarine cable were to part.

Marconi, adopting electrostatic instead of electro-magnetic waves, has
won striking results. Let us note the chief of his forerunners, as they
prepared the way for him. In 1864 Maxwell observed that electricity and
light have the same velocity, 186,400 miles a second, and he formulated
the theory that electricity propagates itself in waves which differ from
those of light only in being longer. This was proved to be true by
Hertz, who in 1888 showed that where alternating currents of very high
frequency were set up in an open circuit, the energy might be conveyed
entirely away from the circuit into the surrounding space as electric
waves. His detector was a nearly closed circle of wire, the ends being
soldered to metal balls almost in contact. With this simple apparatus he
demonstrated that electric waves move with the speed of light, and that
they can be reflected and refracted precisely as if they formed a
visible beam. At a certain intensity of strain the air insulation broke
down, and the air became a conductor. This phenomenon of passing quite
suddenly from a non-conductive to a conductive state is, as we shall
duly see, also to be noted when air or other gases are exposed to the X
ray.

Now for the effect of electric waves such as Hertz produced, when they
impinge upon substances reduced to powder or filings. Conductors, such
as the metals, are of inestimable service to the electrician; of equal
value are non-conductors, such as glass and gutta-percha, as they
strictly fence in a