The spring tides around the
coast of England are four inches higher on the average at the
time of new moon than at full moon, the average rise being
about 15 ft, while the average rise at neaps is 11 ft 6 in.
The height from high to low water of spring tides is
approximately double that of neap tides, while the maximum
height to which spring tides rise is about 33 per cent. more
than neaps, taking mean low water of spring tides as the datum.
Extraordinarily high tides may be expected when the moon is new
or full, and in her position nearest to the earth at the same
time as her declination is near the equator, and they will be
still further augmented if a strong gale has been blowing for
some time in the same direction as the flood tide in the open
sea, and then changes when the tide starts to rise, so as to
blow straight on to the shore. The pressure of the air also
affects the height of tides in so far as an increase will tend
to depress the water in one place, and a reduction of pressure
will facilitate its rising elsewhere, so that if there is a
steep gradient in the barometrical pressure falling in the same
direction as the flood tide the tides will be higher. As
exemplifying the effect of violent gales in the Atlantic on the
tides of the Bristol Channel, the following extract from "The
Surveyor, Engineer, and Architect" of 1840, dealing with
observations taken on Mr.
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