provided with a door, erected on the pier or other site fixed
upon for the observations. A hole must be formed in the floor
and a galvanized iron or timber tube about 10 in square
reaching to below low water level fixed underneath, so that
when the float is suspended from the recording instrument it
shall hang vertically down the centre of the tube. The shed
and tube must of course be fixed securely to withstand wind and
waves. The inside of the tube must be free from all projections
or floating matter which would interfere with the movements of
the float, the bottom should be closed, and about four lin
diameter holes should be cleanly formed in the sides near to
the bottom for the ingress and egress of the water. With a
larger number of holes the wave action will cause the diagram
to be very indistinct, and probably lead to incorrectness in
determining the actual levels of the tides; and if the tube is
considerably larger than the float, the latter will swing
laterally and give incorrect readings.


A bench mark at some known height above ordnance datum should
be set up in the hut, preferably on the top of the tube. At
each visit the observer should pull the float wire down a short
distance, and allow it to return slowly, thus making a vertical
mark on the diagram, and should then measure the actual level
of the surface of the water below the bench mark in the hut, so
that the water line on the chart can be referred to ordnance
datum. He should also note the correct time from his watch, so
as to subsequently rectify any inaccuracy in the rate of
revolution of the drum.