Europe. A few months later, Dr. W. Montgomerie, a surgeon, gave other
specimens to the Society of Arts, of London, which exhibited them; but
it was four years before the chief characteristic of the gum was
recognized. In 1847 Mr. S. T. Armstrong of New York, during a visit to
London, inspected a pound or two of gutta-percha, and found it to be
twice as good a non-conductor as glass. The next year, through his
instrumentality, a cable covered with this new insulator was laid
between New York and Jersey City; its success prompted Mr Armstrong to
suggest that a similarly protected cable be submerged between America
and Europe. Eighteen years of untiring effort, impeded by the errors
inevitable to the pioneer, stood between the proposal and its
fulfilment. In 1848 the Messrs. Siemens laid under water in the port of
Kiel a wire covered with seamless gutta-percha, such as, beginning with
1847, they had employed for subterranean conductors. This particular
wire was not used for telegraphy, but formed part of a submarine-mine
system. In 1849 Mr. C. V. Walker laid an experimental line in the
English Channel; he proved the possibility of signalling for two miles
through a wire covered with gutta-percha, and so prepared the way for a
venture which joined the shores of France and England.

[Illustration: Fig. 58.--Calais-Dover cable, 1851]

In 1850 a cable twenty-five miles in length was laid from Dover to
Calais, only to prove worthless from faulty insulation and the lack of
armour against dragging anchors and fretting rocks. In 1851 the
experiment was repeated with success. The conductor now was not a single
wire of copper, but four wires, wound spirally, so as to combine
strength with flexibility; these were covered with gutta-percha and
surrounded with tarred hemp. As a means of imparting additional
strength, ten iron wires were wound round the hemp--a feature which has
been copied in every subsequent cable (Fig. 58). The engineers were fast
learning the rigorous conditions of submarine telegraphy; in its
essentials the Dover-Calais line continues to be the type of deep-sea
cables to-day. The success of the wire laid across the British Channel
incited other ventures of the kind. Many of them, through careless
construction or unskilful laying, were utter failures. At last, in 1855,
a submarine line 171 miles in length gave excellent service, as it
united Varna with Constantinople; this was the greatest length of
satisfactory