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Page 41
[Illustration: Fig. 55.--The Aurora as Waving Curtains.]
Investigation has discovered far more mysteries than it has explained.
It is possible that the same cause that produces sun-spots produces
aurora in all space, visible in all worlds. If so, we shall see
more abundant auroras at the next maximum of sun-spot, between
1880-84.
_The Delicate Balance of Forces._
A soap-bubble in the wind could hardly be more flexible in form
and sensitive to influence than is the earth. On the morning of
May 9th, 1876, the earth's crust at Peru gave a few great throbs
upward, by the action of expansive gases within. The sea fled,
and returned in great waves as the land rose and fell. Then these
waves fled away over the great mobile surface, and in less than
five hours they had covered a space equal to half of Europe. The
waves ran out to the Sandwich Islands, six [Page 145] thousand
miles, at the rate of five hundred miles an hour, and arrived there
thirty feet high. They not only sped on in straight radial lines,
but, having run up the coast to California, were deflected away into
the former series of waves, making the most complex undulations.
Similar beats of the great heart of the earth have sent its pulses
as widely and rapidly on previous occasions.
The figure of the earth, even on the ocean, is irregular, in consequence
of the greater preponderance of land--and hence greater density--in the
northern hemisphere. These irregularities are often very perplexing
in making exact geodetic measurements. The tendency of matter to
fly from the centre by reason of revolution causes the equatorial
diameter to be twenty-six, miles longer than the polar one. By this
force the Mississippi River is enabled to run up a hill nearly
three miles high at a very rapid rate. Its mouth is that distance
farther from the centre of the earth than its source, when but
for this rotation both points would be equally distant.
If the water became more dense, or if the world were to revolve
faster, the oceans would rush to the equator, burying the tallest
mountains and leaving polar regions bare. If the water should become
lighter in an infinitesimal degree, or the world rotate more slowly,
the poles would be submerged and the equator become an arid waste.
No balance, turning to 1/1000 of a grain, is more delicate than
the poise of forces on the world. Laplace has given us proof that
the period of the earth's axial rotation has not changed 1/100
of a second of time in two thousand years.
[Page 146]
_Tides._
But there is an outside influence that is constantly acting upon
the earth, and to which it constantly responds. Two hundred and
forty thousand miles from the earth is the moon, having 1/81 the
mass of the world. Its attractive influence on the earth causes the
movable and nearer portions to hurry away from the more stable and
distant, and heap themselves up on that part of the earth nearest
the moon. Gravitation is inversely as the square of the distance;
hence the water on the surface of the earth is attracted more than
the body of the earth, some parts of which are eight thousand miles
farther off; hence the water rises on the side next the moon. But
the earth, as a whole, is nearer the moon than the water on the
opposite side, and being drawn more strongly, is taken away from
the water, leaving it heaped up also on the side opposite to the
moon.
A subsidiary cause of tides is found in the revolution of the earth
and moon about their common centre of gravity. Revolution about
an axis through the centre of a sphere enlarges the equator by
centrifugal force. Revolution about an axis touching the surface
of a flexible globe converts it into an egg-shaped body, with the
longer axis perpendicular to the axis of revolution. In Fig. 56 the
point of revolution is seen at the centre of gravity at G; hence,
in the revolution of earth and moon as one, a strong centrifugal
force is caused at D, and a less one at C. This gives greater height
to the tides than the attraction of the moon alone could produce.
[Page 147]
[Illustration: Fig. 56.]
If the earth had no axial revolution, the attractive point where
the tide rises would be carried around the earth once in twenty-seven
days by the moon's revolution about the earth. But since the earth
revolves on its axis, it presents a new section to the moon's attraction
every hour. If the moon were stationary, that would bring two high
tides in exactly twenty-four hours; but as the moon goes forward,
we need nearly twenty-five hours for two tides.
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