The datum of each chart varies as regards Ordnance datum, and in the
case of charts embracing a large area the datum varies along the coast.
The following table gives the fall during each half-hour of the
typical tides shown in Fig, 6 (see page 15), from which it will
be seen that the maximum rate occurs at about half-tide, while
very little movement takes place during the half-hour before
and the half-hour after the turn of the tide:--
Table I.
Rate of fall of tides.
State of Eqionoctial Ordinary Ordinary Lowest
Tide. Tides. Spring Tides. Neap Tides. Neap Tides.
High water -- -- -- --
1/2 hour after 0.44 0.40 0.22 0.19
1 " " 0.96 0.80 0.40 0.31
1-1/2 " " 1.39 1.14 0.68 0.53
2 " " 1.85 1.56 0.72 0.59
2-1/2 " " 1.91 1.64 0.84 0.68
3 " " 1.94 1.66 0.86 0.70
3-1/2 " " 1.94 1.66 0.86 0.70
4 " " 1.91 1.64 0.84 0.68
4-1/2 " " 1.35 1.16 0.59 0.48
5 " " 1.27 1.09 0.
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