The top 12
in of rods and the cord and neck of the bottle, as the case may
be, should be painted red, as this colour renders floats more
conspicuous when in the water and gives considerable assistance
in locating their position, especially when they are at some
distance from the observer.
A deep-sea float designed by Mr. G. P. Bidden for ascertaining
the set of the currents along the base of the ocean has
recently been used by the North Sea Fisheries Investigation
Committee. It consists of a bottle shaped like a soda-water bottle,
made of strong glass to resist the pressure of the water, and
partly filled with water, so that just sufficient air is left
in it to cause it to float. A length of copper wire heavy enough
to cause it to sink is then attached to the bottle, which is then
dropped into the sea at a defined place. When the end of the wire
touches the bottom the bottle is relieved of some of its weight
and travels along with the currents a short distance above the bed
of the sea. About 20 per cent. of the bottles were recovered, either
by being thrown up on the beach or by being fished up in trawl nets.
[Illustration: FIG. 8.--DETAIL OF WOOD TIDAL FLOAT 10 FEET
DEEP.]
A double float, weighing about 10 lb complete, was used for the
tidal observations for the Girdleness outfall sewer, Aberdeen.
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