weight; that is, every body has a tendency to fall
somewhere, that somewhere being determined by
the attraction exercised by other surrounding
bodies. In other words, universal gravitation
exists—a wonderful fact discovered by Newton.
Now, a cannon-ball, shot out with slight velocity,
would soon fall to the ground in consequence of
the earth's attraction; and the greater the
velocity with which it was projected, the greater
would be the distance at which it would fall to
the ground. Until we can conceive it shot with
such a velocity that it would travel round and
round the earth without ever falling upon it,
the attractive centre being strong enough to
prevent it from flying away altogether, and yet
riot strong enough to pull it down to itself.

Exactly as that cannon-ball would move round
the earth, so does the earth travel round the sun,
her speed of travelling being at the rate of about
eighteen miles and a half per second; and the
reciprocal attractions of the sun and the earth,
and of the earth and the cannon-ball respectively,
are not altogether too dissimilar for rough
comparison, Represent the sun by a circle three
inches in diameter, the earth will be represented
by the full stop at the close of this sentence.
The volume of the one is one million three
hundred thousand times that of the other.

You have also, ladies and gentlemen, learned
that the earth travels round the sun in an ellipse
or oval; but the ellipticity, or length of the oval
—like the flattening of the earth at the poles—
has been much exaggerated in popular astronomies.
You have seen your gardener trace an
oval flower-bed. He fixes in the ground a couple
of pegs connected by a slack bit of string; and,
with a stick which keeps the string always
stretched, he marks you out the oval required.
The further apart the pegs are fixed, the longer
will be the resulting oval; on the other hand,
by bringing the pegs closer and closer together,
you will at last get an ellipse which is hardly to
be distinguished from a circle.

Such an ellipse is the earth's orbit round the
sun. The places of the pegs are called the foci
or focuses. The sun is not in the centre of the
ellipse, but in one of the foci. The earth, therefore,
is sometimes a little nearer to the sun
(when it approaches the focus where the sun is
placed) than at other times (when it goes away
from it); and the nearer it is, the greater is the
force of the sun's attractive power. The earth
reaches the point of her ellipse, which is nearest
to the sun, about the 1st of January, and that
most distant from it about the 1st of July; but
the difference of those distances is only trifling.
Your eye would be unable to distinguish an exact
tracing of the earth's orbit round the sun from
the circumference of a perfect circle. While
performing the whole tour of this circular orbit,
the earth spins completely round nearly three
hundred and sixty-six times, each complete spin
making one of our days.

We are now approaching, ladies and gentlemen,
the principal cause of our lengthening day.
The earth is not alone in her celestial travels.
She is accompanied by an attendant much
smaller certainly, but also very much nearer to
her than the sun—so much so, in short, as to
exert a considerably stronger attractive force.
For the earth, take a globe six inches in
diameter; the relative size of the moon will be
shown by a ball a trifle more than an inch and a
half through. The earth's volume is only about
forty-nine times that of the moon. The moon,
everybody knows, revolves round the earth, as
the earth revolves round the sun; but, at the
same time, the earth goes a little out of her way
in consequence of the presence of the moon. In
reality, they revolve round one another like two
persons performing a waltz; and the couple
annually whirl together round the sun in the
circular orbit just described. In the moon's motion
there exists, however, one remarkable peculiarity.
While revolving round the earth, she also
revolves on her own axis in such a way as always
to keep the same face turned towards the earth.

The earth also revolves on her axis, but much
more rapidly than the moon, making several
turns (some eight-and-twenty) while she makes
only one. The consequence is that, to the
inhabitants of the earth, the moon appears
successively to rise in the east (or thereabouts) and to
set in the west. If, on the contrary, the earth
had always the same hemisphere turned towards
the moon, people living on the side next the
moon would always behold it above the horizon.
The moon would never rise nor set for them.
There exists, therefore, this essential difference
between the motions of the earth and the moon:
the moon travels round the earth, always
presenting the same face to her; whilst the earth
turns on her axis, continually and successively
presenting the different portions of her surface
to the moon.

As the moon thus waltzes round the earth
while both are waltzing together round the sun,
the moon successively occupies different
positions with respect to the sun. Sometimes (at
new moon) she is on the same side as the sun,
and sometimes (at full) on the opposite.

Sometimes, therefore, her attraction pulls together
with the sun's, and sometimes in a contrary
direction. These pullings can effect a visible
action only on that portion of the earth's surface
which is capable of yielding to it, namely, the
waters. And so, ladies and gentlemen, we have
tides. On the shores of the ocean you must
have observed that, for six hours or thereabouts,
the waters flow, and then ebb for the six hours
following.

Everything in nature attracts everything else.
The earth attracts the moon, and the moon
attracts the earth. If the earth were entirely
solid, the moon's attraction would have the same
effect as if applied to a single rigid body. But
the earth is not entirely solid. It is covered
with a certain quantity of water (forming a thin
stratum relatively to the dimensions of our
globe), which constitutes our seas and oceans.

The moon's attraction does not act under
exactly the same conditions on the solid and the