If, now, in that excellent field for the experiment we are essaying, the mind's eye, we add a second planet outside of the single sphere which we have so far supposed to journey about the sun, or rather about the common centre of gravity, we perceive at once that we have introduced an element which leads to a complication of much importance. The new sphere would, of course, pull upon the others in the measure of its gravitative value--i.e., its weight. The centre of gravity of the system would now be determined not by two distinct bodies, but by three. If we conceive the second planet to journey around the sun at such a rate that a straight line always connected the centres of the three orbs, then the only effect on their gravitative centre would be to draw the first-mentioned planet a little farther away from the centre of the sun; but in our own solar system, and probably in all others, this supposition is inadmissible, because the planets have longer journeys to go and also move slower, the farther they are from the sun. Thus Mercury completes the circle of its year in eighty-eight of our days, while the outermost planet requires sixty thousand days (more than one hundred and sixty-four years) for the same task. The result is not only that the centre of gravity of the system is somewhat displaced--itself a matter of no great account--but also that the orbit of the original planet ceases to be circled and becomes elliptical, and this for the evident reason that the sphere will be drawn somewhat away from the sun when the second planet happens to lie in the part of its orbit immediately outside of its position, in which case the pull is away from the solar centre; while, on the other hand, when the new planet was on the other side of the sun, its pull would serve to intensify the attraction which drew the first sphere toward the centre of gravity. As the pulling action of the three bodies upon each other, as well as upon their equatorial protuberances, would vary with every change in their relative position, however slight, the variations in the form of their orbits, even if the spheres were but three in number, would be very important. The consequences of these perturbations will appear in the sequel.