with the rude signal fires
of the primitive savage. As the first land telegraphs joined village to
village, and city to city, the crossing of water came in as a minor
incident; the wires were readily committed to the bridges which spanned
streams of moderate width. Where a river or inlet was unbridged, or a
channel was too wide for the roadway of the engineer, the question
arose, May we lay an electric wire under water? With an ordinary land
line, air serves as so good a non-conductor and insulator that as a rule
cheap iron may be employed for the wire instead of expensive copper. In
the quest for non-conductors suitable for immersion in rivers, channels,
and the sea, obstacles of a stubborn kind were confronted. To overcome
them demanded new materials, more refined instruments, and a complete
revision of electrical philosophy.
As far back as 1795, Francisco Salva had recommended to the Academy of
Sciences, Barcelona, the covering of subaqueous wires by resin, which
is both impenetrable by water and a non-conductor of electricity.
Insulators, indeed, of one kind and another, were common enough, but
each of them was defective in some quality indispensable for success.
Neither glass nor porcelain is flexible, and therefore to lay a
continuous line of one or the other was out of the question. Resin and
pitch were even more faulty, because extremely brittle and friable. What
of such fibres as hemp or silk, if saturated with tar or some other good
non-conductor? For very short distances under still water they served
fairly well, but any exposure to a rocky beach with its chafing action,
any rub by a passing anchor, was fatal to them. What the copper wire
needed was a covering impervious to water, unchangeable in composition
by time, tough of texture, and non-conducting in the highest degree.
Fortunately all these properties are united in gutta-percha: they exist
in nothing else known to art. Gutta-percha is the hardened juice of a
large tree (_Isonandra gutta_) common in the Malay Archipelago; it is
tough and strong, easily moulded when moderately heated. In comparison
with copper it is but one 60,000,000,000,000,000,000th as conductive. As
without gutta-percha there could be no ocean telegraphy, it is worth
while recalling how it came within the purview of the electrical
engineer.
In 1843 Jose d'Almeida, a Portuguese engineer, presented to the Royal
Asiatic Society, London, the first specimens of gutta-percha brought to
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