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17.10 Tides
The term tide is applied to the periodical rising and
falling of the water of the
ocean caused by the attraction of the sun and moon. Periodical alterations in the
direction of the wind, and periodical variations in atmospheric pressure,
may give rise
to alterations in the level of the sea, but true tides are attributed (are due to) to
astronomical causes. It is supposed that the attraction of the sun and moon may affect
not only the waters of the ocean but also the
solid crust of the earth, producing an
alternating change in its shape, but so small as to be difficult of detection.
Anyone living at the seaside must have observed the
gradual advance and retreat
of the sea about twice in the 24 hours, or to be more exact, twice in 24 hours 50
minutes, the average interval between two successive high waters being 12 hours 25
minutes. The time of high-water thus changes from day to day, and is evidently
related to the position of the moon, which passes the meridian on an average 50
minutes later on each succeeding day. The height to which
the water rises varies also
from day to day, the range from high-water to low-water being greatest about the
time of full moon and new moon, when the tides are called “spring-tides”, and least
about the time of the moon’s first and third quarters, when the tides are called “neap-
tides”. The tide generating effect of the moon is more
than double that of the sun,
because of the very much greater distance of the sun, in spite of its greater mass.
When the sun and the moon are both on the same side of the earth and when they are
diametrically opposed to each other their tide-generating effects are additive, but
when they are at right angles to each other the effects are subtractive, so that the
spring-tides have a range three times greater than the neap-tides.
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