HISTORY OF PHYSICAL ASTRONOMY.

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rotation, he found that the belts generally presented a very alteredappearance after the lapse of five hours. It followed, therefore, that thetime of rotation was somewhere about' ten hours. In order to arrive ata result of greater precision, he watched the motion of the spot whichappeared near the southern belt, and he found that it returned to thesame position on the disk exactly in 12 d 0 h 4 m . During this lapse oftime it had effected twenty-nine complete rotations, consequently theperiod of a single rotation was 9 h 50 m . Cassini attempted on subsequentoccasions to determine the time of rotation, but the results obtained byhim did not exhibit so exact an accordance with each other as wasdesirable. It appeared, from this circumstance, impossible to withholdthe conclusion that the spots, besides participating in the rotatory motionof the planet, had also a proper motion on liis surface. The accurateobservations of Sir William Herschel lead to a similar conclusion. Thatillustrious astronomer carefully watched the spots of the planet in theyears 1778-79, but the various results at which he arrived relative to thetime of rotation fluctuated between 9 h 55 m 40 s and 9 h 50 m 48 s ; thediscordance, therefore, amounted, in some instances, to 4 m 52 s , a quantitytoo great by far to be ascribable wholly to errors of observation *.Scliroeter experienced similar anomalies, in the course of his attempt todetermine the rotation of the planet. He concluded, from his observations,that the most probable value of the period of rotation is 9 L 55 m 33 s .6. Inthe years 1834-5, Mr. Airy made observations on the planet, at Cambridge,for the purpose of determining the time of rotation. He perceive^ a darkspot below the apparent lower belt, which seemed to him to be well adaptedfor establishing this point. His observations comprehended 225 rotations,commencing December 16, 1834, and terminating March 19, 1835. Theperiod of rotation was found by means of them to be 9 h 55 m 21 s .3 f. Mr.Airy considers that the probable error of this result does not exceedfour seconds.

Since Jupiter revolves round an axis, it might he expected that hisfigure would be flattened at the poles ; and further, it might be inferred,from his immense magnitude and the rapidity of his rotation, that theflattening would exceed that of the terrestrial spheroid. The results ofobservation have been found to accord with both these conclusions. In1691 Cassini remarked that Jupiter appeared to him to have an ovalshape. The greater diameter extended east and west, and it seemed to himto exceed the diameter at right angles to it by about a fifteenth part J.The remark of that illustrious astronomer was soon confirmed by theobservations of La Hire, Roemer, and Picard. The first determinationof the ellipticity of the planet by careful measurement is due to Pound,whose results assign to it a mean value equal to §. In the present dayM. Struve has found, by means of micrometrical measurements made withFraunhofer’s great refractor, that the apparent magnitude of the equa-torial diameter, as seen from the sun, at the mean distance 5.20279, isequal to 38 // .327, and that the apparent magnitude of the polar diameter,at the same distance, is equal to 35".538||. These results give xy.yy forthe ellipticity of the planet. We have seen that Laplace found fromtheory, that if the equatorial axis of the planet be represented by unity,

* Phil. Trans., 1781. f Mem. Ast. Soc., vol. ix., p. 5.

+ Anc. Mem. Acad, des Sciences, tome ii., p. 130.

§ Principia, lib. iii., prop. 19. || Mem. Ast. Soc., vol. hi., p. 301.