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