THE MINOR PLANETS.

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mission, copied (and at the same time reduced) from one pub-lished in his article. The scale is intended to represent meandistances varying from two to four times that of the Earth .The number of Minor Planets known in August 1880, whoseorbits correspond to the various distances lying between theselimits, is indicated by the small dots which are seen on theleft of the scale. The gaps at 2*50 (or 2|), and 3-25 (or 3£),as well as another at about 2| times the Earth ’s distance, areespecially evident; while some others not quite so decided maybe noticed, a little short of, and a little beyond, three times thesame distance.

It is a very interesting confirmation of the above hypothesisas to the influence of Jupiter in the production of these gapsin the series of Minor Planets, that the principal divisions inthe rings of Saturn, as we shall show in Lecture XIV., cor-respond to those distances from the planet at which the minutesatellites (by myriads of which the rings are probably consti-tuted) would be.in like manner affected by the commensurabilityof their periods with those of some of the nearer of Saturn’smoons ; while other smaller gaps caused by these, or by someof the other moons, seem on rare occasions to be faintlyvisible in the indications which have been noticed of additionalconcentric divisions in the rings.

After all we may ask:—Are the Minor Planets to be lookedupon as useless, or nearly useless, members of our system ?Certainly not! If our space permitted, we might discussseveral important benefits already derived from them.

For instance, we showed in Lecture I., page 18, with referenceto the determination of the Sun’s distance, that the accuracywith which the minuteness of their star-like discs enables us tomeasure their apparent distances from neighbouring stars, maybe of great advantage if we use such of their number asapproach the nearest to the Earth , for observations of the samekind as those which Mr. Gill made of the planet Mars in 1877,in the Isle of Ascension. The minor planet ^Ethra might havebeen so used in February of last year (see Nature, Feb. 24th,1881), when it was only about 78,000,000 miles from the Earth .In the present year, 1882, on August 24th, Victoria, No. 12,