The most suitable period for taking these observations is from
about the middle of March to near the end of June, as this will
include records of the high spring equinoctial tides and the
low "bird" tides of June. A chart similar to Fig. 6 should be
prepared from the diagrams, showing the rise and fall of the
highest spring tides, the average spring tides, the average
neap tides, and the lowest neap tides, which will be found
extremely useful in considering the levels of, and the
discharge from, the sea outfall pipe.

The levels adopted for tide work vary in different ports.
Trinity high-water mark is the datum adopted for the Port of
London by the Thames Conservancy; it is the level of the lower
edge of a stone fixed in the face of the river wall upon the
east side of the Hermitage entrance of the London Docks, and is
12 48 ft above Ordnance datum. The Liverpool tide tables give
the heights above the Old Dock Sill, which is now non-existent,
but the level of it has been carefully preserved near the same
position, on a stone built into the western wall of the Canning
Half Tide Dock. This level is 40 ft below Ordnance datum. At
Bristol the levels are referred to the Old Cumberland Basin
(O.C.B.), which is an imaginary line 58 ft below Ordnance
datum. It is very desirable that for sewage work all tide
levels should be reduced to Ordnance datum.

A critical examination of the charts obtained from the tide-
recording instruments will show that the mean level of the sea
does not agree with the level of Ordnance datum. Ordnance datum
is officially described as the assumed mean water level at
Liverpool, which was ascertained from observations made by the