•ti"
i i
A P P E N D I X .
imperceptible, except by its effects, there can be little' doubt after
reflecting how small a lateral movement of an ocean would cause
immense commotion at its boundaiy, in consequence of the shght
elasticity of water, when free to move.
Now let the moon be supposed to move from M in fig. 2 to M in,
fig. 3. The highest point of the water would then be transferred
from F to K, during which transfer the water must fall at F and
rise at G : and so of other points. In this manner when the moon
causes a tide by her direct attraction, a wave or swelling, whose
crest is above the natural level of the sea, moves westward, until
it is stopped by a barrier of land. But when it recedes from that
barrier, how is the excess of the wave above the height of the sea
(when uninfluenced by the moon) transferred to the other side of
the same ocean ? There is no return wave : if there were, islands
intermediate would have an ebb, and a flood tide, every six hours ;
four floods in twenty-four hours ; but they have, on the contrary,
six hour tides, alternate ebb and flow, twice in twenty-four hours,
like those of the shores of continents, though generally smaller in
amount. Water cannot rise in one place unless it falls in another
—it does fall on one side of an ocean, while it rises on the other
—how then is the fluid transferred ? There is only one way—
which is by the mass oscillating. In the former case when the
moon passed over, it was a Hbratory movement, in this latter it is an
oscillation.
If it is shown, as I believe, that the ocean oscillates, we see that
there are two principal causes of tides—one the direct 'raising of
water by the moon : and the other, oscillation excited by that temporary
derangement of the natural level of the sea.
From the preceding facts and deductions combined with the commonly
received laws of fluids and gravitation, the following conclusions
may be drawn :—
1. Every large body of water is affected by the attraction of the
moon, and sun, and has tides caused by their action.
2. Bodies of water are not only raised, or accumulated, vertically,
by the attraction of the moon and sun; but they are also drawn
laterally by them.
3. When a large body of water is prevented from continuing a
horizontal movement, it rises until whatever momentum it had
acquired ceases ; and then it sinks gradually.
A P P E N D I X . 289
4 The momentum acquired by a body of water in thus sinkmg
back to the position it should occupy, with reference to the earth's
attraction only, carries it beyond that position to one from which it
has a tendency to recoil again-and so to keep up an oscdlation
untU brought to rest by the friction of its bed. (Attraction of the
moon and sun not considered.)
5. The recurring influences of the moon and sun are checks on
these oscillations, and prevent their taking place more than once
between each separate raising of the water in consequence of their
attractions.
6. Difierent zones (or widths measured by latitude) of an ocean,
may move differently, each having waves and osciUations at times
differing from those of an adjoining zone, in consequence of one
having more or less longitude, depth, or freedom from obstacles than
another.
7. Original waves and oscillations combine with, and modify one
another, according to their relative magnitude, momentum, and
direction.
8. The natural tendency of tide-waves, and oceanic librations, is
from east to west ; and of oscillations, from west to east, and east
to west also : but derivative waves or oscfflations move in various
directions according to primary impulse, and local configuration of
the bed of an ocean.
Conformably to these conclusions, I will now try to explain a few
of the more remarkable, anomalies of tides, m various parts of the
world: taking it for granted that the reader is acquainted with
existing works on the subject, especially those of Mr. WheweU,*
and the brief but comprehensive and explanatory view taken by Sir
J . Herschel in his treatise on astronomy.f
I mentioned that between CaUao and the western shores of the
Pacific, in the paraUel of about 12° south, no comparison of times
can be trusted ? Why not ? may be asked. Four or five hours west
of Callao, there is a multitude of islands which checks the libration of
the ocean. Another tide wave forms westward of them, on a small
scale, and it is by this second tide, altered by derivative tides
• Published in the Philosophical Transactions.
+ Cabinet Cyclopedia. A Treatise on Astronomy, chap, xi., pp. 334,
5, 6, 7, 8,9.