TIDES.
Anyone who has spent a summer at the seashore has observed that the
water level of the ocean changes twice in about twenty-four hours, or
perhaps it would be a better statement to say that it is continually
changing and that twice in twenty-four hours there is a point when it
reaches its highest level and another when it reaches its lowest. It
swings back and forth like a pendulum, making a complete oscillation
once in twelve hours. When we come to study this phenomenon closely we
find that it varies each day, and that for a certain period of time the
water will reach a higher level each succeeding day until it culminates
in a maximum height, when it begins to gradually diminish from day to
day until it has reached a minimum. Here it turns and goes over the same
round again. It will be further observed that the time occupied between
one high tide and the next one is a trifle over twelve hours. That is to
say, the two ebbs and flows that occur each day require a little more
than twenty-four hours, so that the tidal day is a little longer than
the solar day. It corresponds to what we call the lunar day.
As all know, the moon goes through all its phases once in twenty-eight
days. The tide considered in its simplest aspect is a struggle on the
part of the water to follow the moon. There is a mutual attraction of
gravitation between the earth and the moon. Because the water of the
earth is mobile it tends to pile up at a point nearest the moon. But the
earth as a whole also moves toward the moon, and more than the water
does, keeping its round shape, while its movable water (practically
enveloping it) is piled up before it toward the moon and left
accumulated behind it away from the moon. So that in a rough way it is a
solid sound earth, surrounded by an oval body of water: the long axis of
the oval representing the high tides, which, as they follow the moon,
slide completely around the earth once in every twenty-four hours. Thus,
there are really two high tides and two low tides moving around the
earth at the same time; and this accounts for the two daily tides.
We have accounted for the time when they occur in the fact that the
water attempts to follow the moon, but this does not account for the
gradual changes in the amount of fluctuation from day to day. The
problem is complicated by the fact that the sun also has an attraction
for the earth as well as the moon. But from the fact that the sun is
something like 400 times further from the earth than the moon is, and
also the fact that the attraction of one body for another varies
inversely as the square of the distance, the moon has an immense
advantage over the sun, although so much smaller. If the power of the
moon were entirely suspended, or if the moon were blotted out of
existence, there would still be a tide. The fluctuation between high and
low tide would not be nearly so great as it is at present, but it would
occur at the same time each day, because it would be wholly a product of
the sun.
It will be easily seen that these two forces acting upon the water at
the same time will cause a complicated condition in the movement of the
waters of the ocean. There will come a time once in twenty-eight days
when the sun and the moon will act conjointly, and both will pull in the
same direction at the same time upon the water. This joint action of the
sun and moon produces the highest tide, which is called the "spring"
tide. From this point, however, the tides will grow less each day,
because the relation of the sun and moon is constantly changing, owing
to the fact that it requires 365 days for the sun to complete his
apparent revolution around the earth, while the moon does her actual
course in twenty-eight days. When the sun and moon have changed their
relative positions so that they are at right angles to each other with
reference to the earth--at a quarter-circle apart--the sun and moon will
be pulling against each other; at least this is the point where the moon
is at the greatest disadvantage with reference to its ability to attract
the water.
Because one-quarter around the earth the sun is creating his own tide,
which to that extent counteracts the effect produced by the moon, the
tide under the moon at this point is at its lowest point and is called
the "neap" tide. When the moon has passed on around the earth to a point
where it is opposite to that of the sun--at a half-circle apart--there
will be another spring tide, and then another neap tide when it is on
the last quarter, and from that point the tide will increase daily until
it reaches the point where the sun and moon are in exact line with
reference to the earth's center, when another spring tide occurs. From
this it will be seen that there are two spring tides and two neap tides
in each twenty-eight days. This is the fundamental law governing tides.
There are many other conditions that modify tidal effects. Neither the
sun nor the moon is always at the same distance from the earth. So that
there will be a variation at times in high and low tides. For instance,
it will happen sometimes that when both the sun and moon are acting
conjointly they will both be at their nearest point to the earth, and
when this is the case the spring tide will be much higher than usual.
For many years the writer has observed that artesian wells, made by deep
borings of small diameter into the earth to a water supply, have a daily
period of ebb and flow, as well as a neap and spring tide, the same as
the tides of the ocean, except that the process is reversed. The time of
greatest flow of an artesian well will occur at low tide in the ocean.
This might be accounted for from the fact that when the tide is at its
height the moon is also pulling upon the crust of the earth, which would
tend to take the pressure off the sand rock which lies one or two
thousand feet below the surface and through which the flow of water
comes, and thus slacken the flow. When the moon is in position for low
tide, the crust of the earth would settle back and thus produce a
greater pressure upon the water-bearing rock. This is the only theory
that has suggested itself to the writer that would seem to account for
these phenomena.
Looked at from one standpoint, it is easy to account for tidal action.
But when we attempt to make up a table giving the hour and minute as
well as the height of the tide at that particular time we find that we
have a very complicated mathematical problem. However, tables are made
out so that we know at just what time in the day a tide will occur every
day in the year.