Page:Proceedings of the Royal Society of London Vol 1.djvu/266

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The observations made by Dr. Herschel last year, on the ﬁgure of Saturn, having drawn the attention of astronomers to the subject, a further investigation of it appeared to him to be necessary. Those, he says, who compare his former ﬁgures of the planet (in which the particular shape of the body was not meant to be represented,) with that annexed to his last paper, and wonder at the difference between them, have not attended to the measures he had given of the equa- torial and polar diameters of it; these, as established in 17 89, are the same as in his last ﬁgure, which diﬁ'ers only in having the ﬂat- tening at the pole a little more extended on both sides; and as his attention, in 17 89, was entirely taken up with an examination of the two principal diameters of the planet, it is not, he thinks, extraordi- nary that the singularity of its shape should then have been over- looked by him.

After some observations on the magnifying powers necessary to be used in observing the ﬁgure of Saturn, Dr. Herschel proceeds to relate the observations made by him. for that purpose, in the months of April, May, and the beginning of June, of the present year. He ﬁrst, however, gives an observation made in the year 1788, from which it appears, that he had then observed the shape of Saturn not to be spheroidical (like that of Mars or Jupiter), but much ﬂattened at the poles, and a little ﬂattened at the equator.

The observations made this year by our author, agree with those made last year, in establishing, that the ﬂattening at the poles of Saturn is more extensive than it is in Jupiter: also that the curva- ture in high latitudes is greater than in the last-mentioned planet; but, on the contrary, the curvature at the equator is rather less in Saturn than it is in Jupiter.

In the observation of May 16, of the present year, the greatest curvature in the disc of Saturn appeared to be at the latitude of about 40°.

Upon the whole, the ﬁgure of Saturn may, Dr. Herschel says, be called a spheroid, while that of Jupiter may be called an ellipsoid.

Our author now proceeds to notice some observations he has made on the periodical changes in the colour of the polar regions of Saturn. From those made in the years 1793, 1794, and 1796, when the south pole of the planet had been long exposed to the inﬂuence of the sun, it appeared that the regions about that pole had lost their former whiteness; and that the whiteness of the northern hemisphere was increased. Those made in the present year, when the north pole of Saturn is exposed to the sun, show that its regions have lost much of their brightness; while those about the south pole have regained their former colour, and are brighter, and whiter, than the equatorial parts.

Respecting the atmosphere of Saturn, Dr. Herschel observes, that