“1. The planets grew out of scattered material. For had they arisen from already more or less complete nuclei these could not have borne to one another the general comensurate relation of mean motions existent to-day.
“2. Each brought the next one into being by the perturbation it induced in the scattered material at a definite distance from it.
“3. Jupiter was the starting point, certainly as regards the major planets; and is the only one among them that could have had a nucleus at the start, though that, too, may equally have been lacking.
“4. After this was formed Saturn, then Uranus, and then Neptune.” (This he shows from the densities of these planets.)
“5. The asteroids point unmistakably to such a genesis, missed in the making.
“6. The inner planets betray inter se the action of the same law, and dovetail into the major ones through the 2:5 relation between Mars and the asteroids.
“We thus close with the law we enunciated: Each planet has formed the next in the series at one of the adjacent commensurable-period points, corresponding to 1:2, 2:5, 1:3, and in one instance 3:5, of its mean motion, each then displacing the other slightly sunward, thus making of the solar system an articulated whole, an inorganic organism, which not only evolved but evolved in a definite order, the steps of which celestial mechanics enables us to retrace.
“The above planetary law may perhaps be likened to Mendelief’s law for the elements. It, too, admits of prediction. Thus in conclusion I venture to forecast that when the nearest trans-Neptunian planet is detected it will be found to have a major axis of very approximately 47.5 astronomical units, and from its position a mass comparable with that of Neptune, though probably less; while, if it follows a feature of the satellite systems which I have pointed out elsewhere, its excentricity should be considerable, with an inclination to match.”
The last paragraph we shall have reason to recall again.
This paper on the “Origin of the Planets” has been called the most speculative of Percival’s astronomical studies, and so it is; but it fascinated him, and is interesting not more in itself, than as an illustration of the inquiring and imaginative trend of his mind and of the ease with which intricate mathematical work came to the aid of an idea.