The actually observed phenomena are vastly more complicated, and the
complete theory bears very little resemblance to the simple form we have
just outlined. Everyone who lives in the neighbourhood of a port knows,
for instance, that high water seldom coincides with the time when the
moon crosses the meridian. It may be several hours early or late. High
water at London Bridge, for instance, occurs about one and a half hours
after the moon has passed the meridian, while at Dublin high water
occurs about one and a half hours before the moon crosses the meridian.
The actually observed phenomena, then, are far from simple; they have,
nevertheless, been very completely worked out, and the times of high
water for every port in the world can now be prophesied for a
considerable time ahead.