BOUNDING MACHINES 129
and so on up to 100 or 120 fathoms. The 100 fathoms is marked by
leather with 2 holes, or with a piece of rag, after which the knots are
repeated
The deep-sea lead weighs from 28 to 30 Ibs.
Both leads have cavities in their heels to admit of their being armed.
The deep-sea lead should always be armed before heaving, in order that
the nature of the bottom may be ascertained as well as the depth for
comparison with the chart.
TO TAKE A CAST OF THE DEEP SEA LEAD
The Ship's Way through the water is stopped, the lead is carried
forward, the line is passed forward on the weather side outside of every-
thing and made fast to the lead.
The Hands are stationed along the weather rail with a few bights of
the line in their hands When ready, the lead is dropped into the sea
and as the line is carried down by it each man in turn calls out "watch
there, watch," as the line leaves his hand.
The Officer in charge has the line in his hand at about the expected
depth, and when the strain comes of! the hues he gives the lead a dump
or two on the bottom to get a good up and down cast. He then
estimates the depth of water by the mark on the line, making
due allowance for the height of the ship's rail above the sea level.
The line is then wound up on the reel as it is hauled in to be ready for
another cast should one be required. The tallow arming on the heel
of the lead is cut off and the nature of the bottom, be it sand (s), shell
(sh), mud (m), gravel (g), stone (st), coral (crl) is compared with that
given on the chart against the depth obtained. The depth of water and
the nature of the sea bottom when taken into conjunction with the
estimated position of the ship give the navigator some further assurance
as to his actual position. Fishermen in the North Sea grope their way
about and recognise particular fishing banks in this way.
The cast being satisfactory the ship gets under way again.
PATENT SEA SOUNDING MACHINES.
Most patent sounding machines are based on the fact that the
pressure of the water on an immersed body increases with the depth to
which it is immersed.
Boyle and Marriotte's Law is made use of in some of them. Applied
to the sounding machine it may be stated briefly as follows:—Th*