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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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