total (12 m) or: 1 m In the second hour the water level would rise by another 2 twelfths of the total (12 m) or: 2 m In the third hour the water level would rise by another 3 twelfths of the total (12 m) or: 3 m This gives the increase in the water level by 3:00 p.m. as 6 metres. This represents only the increase - the total depth of the water (relative to chart datum) will include the 2 m depth at low tide: 6 m + 2 m = 8 metres. The calculation can be simplified by adding twelfths together and reducing the fraction beforehand: Rise of tide in three hours Daylength If midwinter sunrise and set are at 09:00 and 15:00, and midsummer at 03:00 and 21:00, the daylight duration will shift by 0:30, 1:00, 1:30, 1:30, 1:00 and 0:30 over the six months from one solstice to the other. Likewise the day length changes by 0:30, 1:00, 1:30, 1:30, 1:00 and 0:30 each month. More equatorial latitudes change by less, but still in the same proportions; more polar by more. Caveats The rule is a rough approximation only and should be applied with great caution when used for navigational purposes. Officially produced tide tables should be used in preference whenever possible. The rule assumes that all tides behave in a regular manner, this is not true of some geographical locations, such as Poole Harbour or the Solent where there are "double" high waters or Weymouth Bay where there is a double low water. The rule assumes that the period between high and low tides is six hours but this is an underestimate and can vary anyway. References Rules of thumb Tide tables de:Gezeitenrechnung#Zwölftel-Regel