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_THE SEAS
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‘
;
read o a dial.
When a very large amount of wire is oñ
—
the drum, its own weight comes dangerously near its
breaking strain and there is a risk that, when it and the
lead weight attached are hauled in, a break may occur.
This
difculty is overcome by emplôying a specially
designed weight which consists of a number of separate
sinkers, and on the lead striking bottom these are released
and the Weight to be hauled up is thus considerably reduced.
In very recent years a much improved method of sounding
'
_
has come into practice.
This method is an outstanding
example of the application of knowledge gained by research
in other branChes of science, being a combination of the
_
use of sound and electricity.
It is known as “ echo sound—
.
ing
”
and the principle of it is that a sound is sent vertically
downwards to the
a very
delicat'e instrument
picks up the sound once m0re as it is reected back from
the bottom, in other words it receives the echo. Now the
rate of travel of the
of sound through” water is
known (about 4,900 feet per second) and by noting the
interval of time between
rst transmission of the
,
sound and the reception of its echo it is possible to calculate
_
the distance through which it has passed and hence the
depth*of the
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In _shallow water thisinterval of
time becomes extremelysmall, and it is only by the use of
_
elaborate electrical apparatus that the time can be measured.
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With such an instrument a ship can travel swiftly through
the water s0unding almost continuously as she goes, and the
time occupied in taking a reading is; a striking contrast to
that required With weight and line.
_
The original word to
_
“Sound ” (which is, considered to be a form of the Old
English ';Word “
sund,” meaning
“
swimming ’.’) would
nowadays appear to have been Very happily chosen and
could not express better what is actually—being done when
a depth measurement is being
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