In some cases we may protect the plungers from the pressure of ooze,
etc., by guards fitted to the stem of the grapnel, but in practice we
have not found these to be necessary.
The water is allowed free access around and about each separate part, in
order that its pressure shall be equal on all sides. This arrangement
renders the grapnel as effectual in the deepest as in the shallowest
water.
By making the plungers in two pieces, with a rubber washer or its
equivalent between them, we prevent mud or ooze from getting behind and
interfering with their working. As the hole in the rubber surrounding
the contact-plate, by caused the passage of the pin through it, closes
up as soon as the pressure is removed, leaving in the rubber a fault of
exceedingly high resistance, the rubber does not require renewing.
In the rubber in which we embedded the contact-plate, we place a layer
or more of tinfoil or other easily pierced conducting surface, through
which the pin passes on its way to the contact-plate proper.
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