ained in the text.]

[Illustration: _Photo: G. Brocklehurst._

THE AEGIR ON THE TRENT

An exceptionally smooth formation due to perfect weather conditions. The
wall-like formation of these tidal waves (see next page also) will be
noticed. The reason for this is that the downward current in the river
heads the sea-water back, and thus helps to exaggerate the advancing
slope of the wave. The exceptional spring tides are caused by the
combined operation of the moon and the sun, as is explained in the
text.]

[Illustration: _Photo: G. Brocklehurst._

A BIG SPRING TIDE, THE AEGIR ON THE TRENT]

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.

The Action of Sun and Moon

It would be beyond our scope to attempt to explain the complete theory,
but we may mention one obvious factor which must be taken into account.
Since the moon, by its gravitational attraction, produces tides, we
should expect that the sun, whose gravitational attraction is so much
stronger, should also produce tides and, we would suppose at first
sight, more powerful tides than the moon. But while it is true that the
sun produces tides, it is not true that they are more powerful than
those produced by the moon. The sun's tide-producing power is, as a
matter of fact, less than half that of the moon. The reason of this is
that _distance_ plays an enormous role in the production of tides. The
mass of the sun is 26,000,000 times that of the moon; on the other hand
it is 386 times as far off as the moon. This greater distance more than
counterbalances its greater mass, and the result, as we have said, is
that the moon is more than twice as powerful. Sometimes the sun and moon
act together, and we have what are called spri