diminished, at the greater distance, in the
proportion of 49 to 81; that is, if 81 represent the
attractive force at 7 miles' distance—at 9 miles it
will only he 49. This rapid diminution of
attractive force with increase of distance
explains hcnv the immense Sun, an enormous way
off', may have a weaker pull on an object at the
surface of the Earth than the small Moon, which
is comparatively close at hand.

As the Earth revolves round the Sun, not in a
circle but in an oval, her distance from the Sun is
constantly varying slightly; the sainething obtains
with respect to the Moon and the Earth. The
distances, therefore, from the Earth to the Sun
and from the Earth to the Moon, when
mentioned in leagues or miles, must be understood
to be the mean or average distances. The mean
distance of the Earth from the Sun is 95,576,240
miles. The mean distance of the Moon from
the Earth is 239,100 miles. Consequently, were
there a railroad from the Earth to the Moon,
with trains going at the rate of 30 miles an hour,
it would require 7970 hours, or 332 days and 2
hours, or nearly 11 months, travelling night and
day, to pay a visit to the hills and dales of our
bright attendant. At the same speed of
locomotion, to reach the surface of the Sun would
occupy a period of 363 years (of 365 days each),
249 days, 17 hours, and 40 minutes. Many
generations of men must be born and die on the
road, in a railway carriage incessantly dashing
along at a pace of 30 miles an hour, before their
posterity could arrive at the great central luminary.
The Sun is about four hundred times
further from the Earth than the Moon is. The
important point is the great diminution of the
attractive force of so large a mass, which is the
consequence of so wide an interval.

It happens, then, that two distinct forces, the
attraction of the Sun and the attraction of the
Moon, are continually pulling at the entire mass
of the Earth on which we dwell. The solid
portion of the globe—rocks and dry land,
mountains and continents—hold together, and obey
the combined attractions impressed upon them,
all in one piece, in a rigid state. But it is
otherwise with the liquid portion of our globe,
the outspread oceans, which do not hold together
rigidly as if they were frozen, but which flow in the
direction of the attractive force, by the same
law which causes a brook to stream down a
mountain-side, in obedience to the Earth's
attraction. And, as the Oceans cover so large
a proportion of the surface of the globe, the
entire globe may be roughly compared to an
india-rubber ball which is pulled out of shape by
a couple of strings. There will be a bulging
out at the places where the strings pull hardest.

It is thus that the attractions of the Sun and
the Moon pull the watery parts of the Earth
out of shape. The Ocean is raised in a tidal
wave or waves; for, there is a solar tidal wave,
caused by the Sun, and a lunar tidal wave,
caused by the Moon. The latter is about three
times as givat as the former, in consequence of
the nearness of the Moon, and the distance of
the Sun. If the Earth, Sun, and Moon, all
remained perfectly still in relation to each other,
the waters so raised would remain in a permanent
heap; there would be a permanent alteration
of the shape of the globe, and that is all.
But, as the Earth revolves on her axis, the solar
tidal wave carried on by that movement, like
everything else on the Earth's surface, rushes
back after the attractive force of the Sun; and,
as the Moon revolves round the Earth, there is
a similar rushing of the lunar tidal wave, caused
partly by the Earth's revolution on her axis, and
partly by the Moon's changed place in her orbit,
In these complicated influences lie some of the
causes which produce our changing and yet
periodical tides.

The strength of the tides, as well as the hour
of the day at which they occur, is governed by
the place of the Moon in her orbit, whether
she be in syzygy or in quadrature (in conjunction
or opposition). Let not those hard words
frighten us; for the explanation of a word its
etymology or derivation often clears away a
difficulty. As the Moon revolves round the
earth in a plane which is nearly the same as the
plane of the ecliptic or the plane of the Earth's
orbit round the Sun, it follows that, once in
every lunar revolution, the relative position
of the Sun, Moon, and Earth, must be this,
E——- M——- S, which is called "in conjunction"
(with the Sun), and once this, M——- E——S,
which is called "in opposition." The former
takes place at every New Moon, the latter at
every Full Moon. Both are called syzygy, from
a Greek word meaning " a yoking together." The
Sun and Moon draw in couples; they both pull
in one direction. The lunar and the solar
tidal waves are combined; the waters rise to
an unusual height; spring tides are the result.
Of course their contemporaneous absence during
the interval between two high waters, occasions
an unusually low ebb. The highest tides of all
will occur at conjunction, because there is then
a combination of attractive force as well as an
identity in its direction.

But, when the Moon has performed a quarter
of her orbit from either of those positions, that
is at her first quarter or her third, the three
heavenly bodies (for the Earth is a heavenly
body) are no longer in the same line. On
  the contrary, they are at squares, in quadrature,
forming a right angle of which the Earth is the

                       M
corner, thus,   E l_____S,    or      E l????????S.   The Sun and
                                                                   M

the Moon, then pulling different ways, will
reduce each other's tidal wave. Union makes
strength; division weakness. The tides are weak,
or neap; the oscillation of the sea is less; high
water is never so high, and low water is never so
low, at quadratures as at syzygies.

Wherever the movement of the waters of the
Ocean is not impeded by islands, capes, straits,
and other similar obstacles, the tides are
observed to have three distinct periods: the daily
period, the monthly period, and the annual
period. Of the two first we have already
spoken, as the flux and the reflux, and the spring