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Page 5
Leave it there alone, and withdraw millions of miles into space:
it looks smaller and smaller. We lose sight of those distinctive
spires of flame, those terrible movements. It only gives an even
effulgence, a steady unflickering light. Turn one quarter round.
Still we see our world, but it is at one side.
Now in front, in the utter darkness, suddenly create another world
of the same size, and at the same distance from you. There they
stand--two huge, lone bodies, in empty space. But we created them
dowered with attraction. Each instantly feels the drawing influence
of the other. They are mutually attractive, and begin to [Page 8]
move toward each other. They hasten along an undeviating straight
line. Their speed quickens at every mile. The attraction increases
every moment. They fly swift as thought. They dash their flaming,
seething foreheads together.
And now we have one world again. It is twice as large as before,
that is all the difference. There is no variety, neither any motion;
just simple flame, and nothing to be warmed thereby. Are our creative
powers exhausted by this effort?
[Illustration: Fig. 1.--Orbit A D, resulting from attraction, A
C, and projectile force, A B.]
No, we will create another world, and add another power to it that
shall keep them apart. That power shall be what is called the force
of inertia, which is literally no power at all; it is an inability
to originate or change motion. If a body is at rest, inertia is
that quality by which it will forever remain so, unless acted upon
by some force from without; and if a body is in motion, it will
continue on at the same speed, in a straight line, forever, unless
it is quickened, retarded, or turned from its path by some other
force. Suppose our newly created sun is 860,000 miles in diameter.
Go away 92,500,000 miles and create an earth eight thousand miles
in diameter. It instantly feels the attractive power of the sun
drawing it to itself sixty-eight [Page 9] miles a second. Now, just
as it starts, give this earth a push in a line at right angles with
line of fall to the sun, that shall send it one hundred and
eighty-nine miles a second. It obeys both forces. The result is that
the world moves constantly forward at the same speed by its inertia
from that first push, and attraction momentarily draws it from its
straight line, so that the new world circles round the other to the
starting-point. Continuing under the operation of both forces, the
worlds can never come together or fly apart.
They circle about each other as long as these forces endure; for
the first world does not stand still and the second do all the
going; both revolve around the centre of gravity common to both.
In case the worlds are equal in mass, they will both take the same
orbit around a central stationary point, midway between the two.
In case their mass be as one to eighty-one, as in the case of the
earth and the moon, the centre of gravity around which both turn
will be 1/81 of the distance from the earth's centre to the moon's
centre. This brings the central point around which both worlds
swing just inside the surface of the earth. It is like an apple
attached by a string, and swung around the hand; the hand moves
a little, the apple very much.
Thus the problem of two revolving bodies is readily comprehended.
The two bodies lie in easy beds, and swing obedient to constant
forces. When another body, however, is introduced, with its varying
attraction, first on one and then on the other, complications are
introduced that only the most masterly minds can follow. Introduce
a dozen or a million bodies, and complications arise that only
Omniscience can unravel.
[Page 10]
[Illustration: Fig. 2.]
Let the hand swing an apple by an elastic cord. When the apple
falls toward the earth it feels another force besides that derived
from the hand, which greatly lengthens the elastic cord. To tear
it away from the earth's attraction, and make it rise, requires
additional force, and hence the string is lengthened; but when
it passes over the hand the earth attracts it downward, and the
string is very much shortened: so the moon, held by an elastic cord,
swings around the earth. From its extreme distance from the earth,
at A, Fig. 2, it rushes with increasing speed nearly a quarter of a
million of miles toward the sun, feeling its attraction increase
with every mile until it reaches B; then it is retarded in its
speed, by the same attraction, as it climbs back its quarter of
a million of miles away from the sun, in defiance of its power,
to C. All the while the invisible elastic force of the earth is
unweariedly maintained; and though the moon's distances vary over a
range of 31,355 miles, the moon is always in a determinable place.
A simple revolution of one world about another in a circular orbit
would be a problem of easy solution. It would always be at the
same distance from its centre, and going with the same velocity.
But there are over sixty causes that interfere with such a simple
orbit in the case of the moon, all of which causes and their
disturbances must be considered in calculating such a simple matter
as an eclipse, or predicting the moon's place as the sailors guide.
One of the most puzzling of the irregularities [Page 11] of our
night-wandering orb has just been explained by Professor Hansen, of
Gotha, as a curious result of the attraction of Venus.
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