THE PLANET SATURN.

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of a brownish hue, while the polar regions, as a rule, are bluish.Spots are rarely visible upon it. Nevertheless some, both darkand light, have at times been sufficiently distinct and permanentto afford a determination of the period of the planet’s rotationupon its axis. In this way Sir W. Herschel obtained for itsduration 10 fi 16“ But the best determination yet made is byProfessor Asaph Hall , from a white spot visible during nearlya month, beginning with December 7th, 1876, which gave aresult 1“ 36 s less than Herschel’s, viz., 10 h 14“ 24 s .

Next to the appearance of the planet’s disc we feel temptedto proceed at once to discuss its wondrous rings,—so marvellousin themselves, so utterly unlike what we see in any other mem-ber of the Solar System . But we must first pause for awhileto mention, as briefly as possible, some of the accurate measure-ments of the size and shape, the weight and orbit, of Saturn ,which, in spite of its vast distance, the perfection of moderninstruments has secured.

The ball, or sphere, of this great planet measures (apart fromthe rings) about 74,000 miles in its equatoreal diameter. Butits polar diameter is less by between ^th and ^th part ; theproportionate compression at its poles being nearly twice asgreat as in Jupiter , but not so striking, owing to the smaller sizeof its apparent disc, and possibly from the effect upon the eyeof the contour of the rings.

The surface of the globe of Saturn is fully 80 times, and itsvolume about 750 times, that of the Earth . It will be remem-bered that, in the case of Jupiter , the corresponding numbers arerespectively about 120 and 1,300. Consequently the apparentvolume of Saturn is not very much more than one-half of thatof Jupiter . We must not, however, forget that our remarks, inLecture XII., as to the difference which may exist between thesize indicated by the visible disc and the real size of Jupiter ,may also be applicable to Saturn . Our measurements are onlyof that which, after all, may be an outlying vaporous, or cloudy,envelope, within which a more solid and smaller globe may, ormay not, exist. And in one respect this is all the more likelyto be the case with Saturn , inasmuch as the average densityof a globe extending to the boundary which we see, and of a