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Fkoji June, 1855, till June, 185G.
The daily rising and falling of the waters of the ocean are, in general, denominated
Tides. The rising is called the Flood, or Flux ; and when it ceases to rise, High
Water. The falling is called the Ebb, or Eeflux ; and when it ceases to fall, it is
said to be Low Water. The interval between the times of High and Low Water is
called a Tide.
The Tides are produced by the attraction of the sun and moon, principally that of
the latter object. The cohesion of fluids being much less than that of solid bodies,
they more easily yield to the power of gravity ; in consequence of which the waters
immediately below the moon are drawn up in a protuberance, producing a full tide,
or what is commonly called High Water, at the spot where it happens. According to
this theory, you would imagine that we should have full tide only once in twenty-four
hours, that is, every time that we were below the moon ; while we find that we have
two tides in the course of twenty-four hours, and that it is High Water with us and
with our antipodes at the same time.
This opposite tide is rather more difficult to explain than that which is drawn up
beneath the moon. In order to render the question more simple, let us suppose the
earth to be everywhere covered by the ocean, as in the following figure: —
M is the moon, A B C D the
earth. Now, the waters on the
surface of the earth about A being
more attracted than in any other
part, will be elevated, the at-
traction of the moon at B and C
being less ; but still it will be
greater there than at D, which
is the part most distant from the
The body of the earth will therefore be drawn away from the waters at D, leaving
a protuberance similar to that at A : so that the tide, A, is produced by the waters
receding from the earth, and the tide, D, by the earth receding from the waters.
The influence of the sun on the tides is less than that of the moon ; for, observe,
that the tides rise in consequence of the moon attracting one part of the waters more
forcibly than another part. It is this inequality of attraction which produces full and
ebb tides. Now, the distance of the sun is so great, that the whole globe of the jarth
is comparatively but as a point ; and the difference of its attraction for that part of
the waters most under its influence, and that part least subject to it, is but trifling.
No part of the waters will be much elevated above, or much depressed below, their
general surface by its action. The sun has, however, a considerable effect on the
tides ; and increases or diminishes them as it acts in conjunction with, or in opposi-
tion to, the moon.
According to the calculations of the celebrated La Place, in his Mecanique Celeste,
the mean force of the sun in raising the tides is to the mean force of the moon as 1
to 1^-; and Emerson, in page 423 second edition of his Treatise on Fhixio'ns, calcu-
lates the height of the solar tide to be 1-63 feet, and the height by the moon's influ-
ence to be 7-28 feet ; consequently, the height of their joint force to be 8-91 feet.
The height here calculated is supposed to be at such places where the sun and moon
are vertical, and also in the equinoctial. In places at a distance from the equator,
the height will be less as the latitude is greater. Further, their height will also be
less according to the sun and moon's declination from the equator.
From what has already been remarked regarding the tides,' it is obvious that,
when the moon is in Perigee, or at her nearest distance from the earth, she attracts
strongest, and therefore raises the tides most. The contrary happens when she is in
Apogee, or at her greatest distance from the earth, because of her weaker attraction.
At new moon, when she is in conjunction with the sun, and at full moon, when they
are in opposition, the tides are raised by the joint attraction of both luminaries, and
therefore will produce the highest, or what are called Spring Tides. And Neap
Tides are produced by the difference of their force when in quadrature. The Spring

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