SOUNDING MACHINES

131

Another one, made of brass or copper, contains a spring, against
which a piston is forced up by the pressure of the water. The depth
is indicated by a pointer, which shows how far up the tube the piston
has been, and the corresponding number of fathoms is read off the tube
itself.

Most of the foregoing sounders can be used while the shi]? is going
at full speed, but the makers of some of them advise sli wing down to 8
or 10 knots.

Fig. 4.—Side Boom to Enable Soundings to be made from the Bridge.
Some winches are fitted with a dial which registers the amount of
wire run out. A Table is provided which, when entered with the two
known factors, viz., speed of ship as the base line of a right-angled
plane triangle, and the wire run out as the hypotenuse, gives the depth
of water in fathoms (approximately) as the perpendicular side. It is
assumed that the sinker drops vertically to the bottom at the rate of
about 15 feet per second.
HOW TO TAKE A CAST WITH A PATENT SOUNDING
MACHINE.
Pass the sounding wire through the pulley on the rail at the stern
of the ship, or on the side-boom shackle the wire on to the short piece ot
rope spliced into the lead and to which is attached the brass container.
Put the glass tube into the container open end downwards. When ready,
hang the lead over the stern, release the brake on the winch and the wire
will run out free wheel Keep feeling the -wire with the brass Łeeieu