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281
Heroes of the Telegraph
·
John Munro
113
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.
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