THE MINOR PLANETS.

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distance, lose much of their aggregating power,—there may hemyriads upon myriads of Minor Planets having a similar origin,and of a considerably larger hulk than those which we havediscussed in this lecture, but all invisible owing to their greatdistance from us. If so, great as are the wonders which wesee in the system of which we form a part, and numerous as arethe little planets which we have discovered, there may belongto it wonders still greater, and planets still more numerous, yetunseen.

Let the origin of these minor members of the Sun’s familybe what it may, there is good evidence that the attraction ofJupiter has, in certain cases, remarkably perturbed their orbitsin the course of long, long ages, and in an especially interestingmanner. It can be shown, by mathematical investigationshardly suitable for explanation here, that a Minor Planetmoving at such a distance from the Sun, that a certain wholenumber of its periods would be equal, or almost exactly equal,to an integral number of those of Jupiter , would periodicallycome into such a position, that Jupiter ’s perturbing effect wouldbe repeated upon it, again and again, in a very similar manner.Its orbit would consequently be so disturbed as to be quitechanged in its period, eccentricity, and distance from the Sun.It would no longer be found where it originally was; or,possibly, the planet might be brought into collision and unionwith some other which it would not otherwise have approached.In either case, gaps would be formed in the series at suchdistances as would correspond to such periodic times. Sucha process would doubtless have been most effective, if it besupposed to have occurred while the Minor Planets were beinggradually formed out of nebulous or meteoric matter; in fact,while they were much more numerous and smaller than theyprobably are at present.

Professor Daniel Kirkwood was, we believe, the first to notice(when not much more than one-third of the present numberwere known) that such gaps in the series actually existed ; andM. Niesten, in his recent article in the Annuaire of the Obser-vatory of Brussels for 1881, has shown that the same factstill holds good, especially at distances equal to and 3£

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