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