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.
The surface portion consisted of two sheet-iron cups soldered
together, making a float 9 in in diameter and 6 in deep. The
lower or submerged portion was made of zinc, cylindrical in
shape, 16 in diameter and 16 in long, perforated at intervals
with lin diameter holes and suspended by means of a brass chain
from a swivel formed on the underside of the surface float.

In gauging the currents the float is placed in the water at a
defined point and allowed to drift, its course being noted and
afterwards transferred to a plan. The time of starting should
be recorded and observations of its exact position taken
regularly at every quarter of an hour, so that the time taken
in covering any particular distance is known and the length of
travel during any quarter-hour period multiplied by four gives
the speed of the current at that time in miles per hour.