Malta to Alexandria
cable gave rise to researches on the resistance and electrification of
insulating materials under pressure, which formed the subject of a paper
read before the British Association in 1863. The effect of pressure
up to 300 atmospheres was observed, and the fact elicited that the
inductive capacity of gutta-percha is not affected by increased
pressure, whereas that of india-rubber is diminished. The electrical
tests employed during the construction of the Malta and Alexandria
cable, and the insulation and protection of submarine cables, also
formed the subject of a paper which was read before the Institution of
Civil Engineers in 1862.

It is always interesting to trace the necessity which directly or
indirectly was the parent of a particular invention; and in the great
importance of an accurate record of the sea-depth in which a cable
is being laid, together with the tedious and troublesome character of
ordinary sounding by the lead-line, especially when a ship is actually
paying out cable, we may find the requirements which led to the
invention of the 'bathometer,' an instrument designed to indicate the
depth of water over which a vessel is passing without submerging a line.
The instrument was based on the ingenious idea that the attractive power
of the earth on a body in the ship must depend on the depth of water
interposed between it and the sea bottom; being less as the layer of
water was thicker, owing to the lighter character of water as compared
with the denser land. Siemens endeavoured to render this difference
visible by means of mercury contained in a chamber having a bottom
extremely sensitive to the pressure of the mercury upon it, and
resembling in some respects the vacuous chamber of an aneroid barometer.
Just as the latter instrument indicates the pressure of the atmosphere
above it, so the bathometer was intended to show the pull of the earth
below it; and experiment proved, we believe, that for every 1,000
fathoms of sea-water below the ship, the total gravity of the mercury
was reduced by 1/3200 part. The bathometer, or attraction-meter, was
brought out in 1876, and exhibited at the Loan Exhibition in South
Kensington. The elastic bottom of the mercury chamber was supported by
volute springs which, always having the same tension, caused a portion
of the mercury to rise or fall in a spiral tube of glass, according to
the variations of the earth's attraction. The whole was kept at an e