MAKING AND LAVING SUBMARINE CABLES . 1 73 
took place in 1873 on the line of the Direct Spanish cable that 
connects Spain and England, the result being a suspension of 
communication for several weeks. It was found, on making 
the repairs, that several miles of the cable were buried, and these 
of course were lost. In such cases it is necessary to select 
another route, and generally the greater the depth the less 
the chance of disturbance. 
Elevated temperature on tropical coasts .—The temperature of 
a cable is usually greater near the shores and at low depths, 
and hence the shore cables of very warm seas should never be 
made with gutta-percha cores. This substance becomes plastic 
at 86° F. (a temperature that is sometimes found at the 
Antilles and in Indian seas), and the core is then liable to dis¬ 
placements, which may result in a loss of insulation. India- 
rubber stands heat better than gutta-percha at elevated 
temperatures, and has better insulating powers. 
Destructive Animals. — Teredo navalis; Xylophaga; 
Limnoria lignorum. —Among submarine animals, to the ravages 
of which cables are liable, none are more formidable than the 
various kinds of worms and small crustaceans that abound in 
every sea. 
The teredo navalis and its congener the xylophaga , discovered 
by Professor Huxley in 1860 in one of the Levant cables, bore 
into the cable, and even into the gutta-percha, wherever the 
outside wires allow them sufficient passage. 
The teredo is a worm that makes itself a tubular shelter by 
secreting calcareous matter, whilst the xylophaga is 
characterised by its bivalve form. The xylophaga does not 
penetrate far into the gutta-percha, but buries one of its valves 
in i*t. In wires of small size, the gutta-percha may thus be 
sufficiently entered upon to cause a considerable loss of current 
(fig. 113). 
The teredo and the xylophaga have been met with on the 
Mediterranean cables, and in the Atlantic Ocean and northern 
seas. There are, undoubtedly, several species of it which have 
hitherto not been fully examined as to their destructive powers 
on cables. 
The teredo norvegica (fig 113) is a worm of considerable size, 
provided at the head with two shell-like valves, by which it is 
enabled to gnaw through the hardest wood. This worm 
