and capillary vessels which feed and nourish
the bodily organs.

CHAPTER XX.

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 att