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The intrinsic brightness of some of the larger Minor Planetsappears to vary. This may be a result of their rotation upontheir axes, which may periodically turn portions of theirsurfaces of different reflective power towards us ; or it mayarise from temporary changes in the vapours of a cloud-ladenatmosphere. Decided variability has been suspected in thelight of Frigga, No. 77 ; as well as in some which are amongstthe largest in the group.*
It is, however, much more important to consider what maybe the total mass or weight of these bodies collectively, thanto estimate that of any individual one. And from this pointof view we may ask ;—Is it possible that they may, by theirunited attraction, perturb the movements of Mars and Jupiter,although individually their influence may be quite inappre-ciable ? If so, such a result would be of the highest interest.
But, if we assume for them, upon an average, a diameterof 80 miles (which is most probably very considerably inexcess of the truth), the volume of any one will clearly fallshort of that of the Earth by the cube of the number 100 ; theEarth ’s diameter being 100 times 80 miles, and the volumes ofspheres varying as the cubes of their diameters. It wouldconsequently take 1,000,000 of such Minor Planets to forma globe as large as the Earth ; and the 229 already dis-covered would only form a planet of about T1 ] ffff th of its size.Moreover, unless they are composed of meteoric iron, or ofsome other metallic substances, it is hardly likely that theirconstituent materials are so densely pressed together, as in theEarth ’s larger globe. In that case their average density wouldbe much less, and the united weight of the 229 would fallconsiderably short even of the ^^th part of the weight ofthe Earth . It is therefore in no wise surprising that noperturbations produced by their action have as yet beendetected in the movements of Jupiter and Mars .
We understand that Le Yerrier has shown that an amount ofmatter of a weight equal to one-fourth of that of the Earth maypossibly exist in the space between Mars and Jupiter. If so,
* See Astronomische Nachrichtm, No. 2314; quoted in the Observatory for August, 1880, p. 518.