620 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1790. 



exactly ranging with the shadow of it on the body ; generally one on each side, 

 with a white belt close to it. The step from equatorial belts to a rotation on an 

 axis is so easy, and, in the case of Jupiter, so well ascertained, that he hesitates 

 not to take the same consequence for granted here. But, if there could remain 

 a doubt, the observations of June 19, 20, and 21, 1780, where the same spot 

 was seen in 3 different situations, would remove it completely. 



There is another argument, of equal validity with the former, which Dr. H. 

 now mentions. It is founded on the following observations, and will show that 

 Saturn, like Jupiter, Mars, and the Earth, is flattened at the poles ; and there- 

 fore ought to be supposed to turn on its axis. July 22, 1776, he thought Sa- 

 turn was not exactly round. May 31, 1781, it appeared as if the body of 

 Saturn was at least as much flattened as that of Jupiter. August 18, 1787, the 

 body of Saturn is of unequal diameters, the equatorial one being the longest. 

 Sept. 14, 1789, 23^^ 36™ 32", having reserved the examination of the 2 diame- 

 ters of Saturn to the present as the most favourable time, he measured them 

 with the 20 feet reflector, and a good parallel-wire micrometer. 



Equatorial diameter. 



1st measure, 2l".94 



2d 23 .11 



3d 21 .73 



4th 22 .85 



Mean 22 .81 



Polar diameter. 



1st measure, 20",57 



2d 20.10 



3d 21 .16 



Mean 20 .61 



By this it appears that Saturn is considerably flattened at the poles. And as 

 the greatest measures were taken in the line of the ring and of the belts, we are 

 assured that the axis of the planet is perpendicular to the plane of the ring ; and 

 that the equatorial diameter is to the polar, nearly as 11 to 10. 



We may also infer the real diameter of Saturn from these measures, which 

 are perhaps more to be depended on than any that have hitherto been given. But 

 as in his journal Dr. H. had measures that were repeatedly taken 10 years past, 

 not only of the diameter of Saturn, but of the ring, and its opening, by which 

 its inclination may be known ; as well as of the distance of the 4th and 5th, 

 and other satellites, which will be of great use in ascertaining the quantity of 

 matter contained in the planet, he reserves a full investigation of these things for 

 another opportunity. 



One beautiful observation of the transit of the shadow of the 4th satellite 

 over the disc of Saturn, he adds, to conclude this paper. Last night, Novem- 

 ber 2, 1789, at 23*^ 13™ sidereal time, being always in quest of any appearance 

 that may afford the means of ascertaining the rotation of Saturn on an axis, he 

 discovered a black spot on the following margin of the disc of that planet. At 

 23h 21™^ he perceived a protuberance on the south preceding edge of the disc. 



