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
Ordnance Survey Department in March, 1844, but subsequent
records taken in May and June, 1859, by a self-recording gauge
on St. George's Pier, showed that the true mean level of the
sea at Liverpool is 0.068 ft below the assumed level. The
general mean level of the sea around the coast of England, as
determined by elaborate records taken at 29 places during the
years 1859-60, was originally said to be, and is still,
officially recognised by the Ordnance Survey Department to be
0.65 ft, or 7.8 in, above Ordnance datum, but included in these
29 stations were 8 at which the records were admitted to be
imperfectly taken. If these 8 stations are omitted from the
calculations, the true general mean level of the sea would be
0.623 ft, or 7.476 in, above Ordnance datum, or 0.691 ft above
the true mean level of the sea at Liverpool.
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