Submarine cable laying and repairing

18 SUBMARINE CABLE LAYING AND REPAIEING.
bottoms except soft ooze, in which it sinks without impact.
Whenever the jaws come up unclosed after touching bottom, it
is a sure sign that the bottom is soft mud. In this particular
design the sinker is heaved up with the sounding wire.
On the occasion of the " Challenger " Expedition Mr. J. Y.
Buchanan devised instruments (for use in conjunction with
the sounding line) which indicated the depth by the barometric
method and gave very concordant results. These instruments
Fig. 13.—Lucas's Snapper, for bringing up Specimens of Sea Bottom.
were described by him in a lecture before the Chemical
Society in 1878, entitled "Laboratory Experiences on board the
' Challenger'" {Journal of the Chemical Society, October, 1878),
and are known as the water and mercury piezometers (Fig. 14).
This method was adopted for the reason that the sounding line
is sometimes affected by deep-water currents, which cause
errors in the measurement of depth. Generally the effect of
these currents is noticed by the streaming out of the line,
and the ship has to be manoeuvred to follow it and endeavour