122 I'HILOSOPHICAL TRANSACTIONS. [aNNO i7Q'2. 



may be so minute, that no otiier time than when the plane of the ring is ex- 

 posed as much as it can possibly be, will do to ascertain such phenomena. 



It remains now to consider the observations that have been made by M. 

 Cassini, Mr. Short, and Mr. Hadley. Without being in possession of the 

 original observations of M. Cassini, it cannot be decided whether the black list 

 which he saw was the same which Dr. H. observed. M. de la Lande says (Ast. 

 vol. 3, page 441) that Cassini saw it divided by a small black line into 2 equal 

 parts. M. de la Place (Memoire sur la Theorie de I'Anneau de Saturne) men- 

 tions that Cassini saw that the breadth of the ring divided into 2 parts almost 

 equal. It should seem from this, that M. Cassini was not particularly attentive 

 to the proportions of the division; in which case his observations and Dr. H.'s 

 will agree perfectly well; but if he has any where expressly mentioned, tiiat the 

 ring was divided into equal parts, so that we may be certain he was particularly 

 attentive to that circumstance, it will follow evidently that the ring, since his 

 time, has undergone a very capital change in its construction. 



Mr. Short assured M. de la Lande, that he had seen many divisions on the 

 ring, with his telescope of 12-feet. A thing of such consequence, and so new, 

 ought certainly to have been given in a more satisfactory and circumstantial way 

 than only by communicating it, from memory, in conversation, to another 

 person. Besides, it is well known that many telescopes will give double and 

 treble images, and that especially those which have large apertures are subject 

 to tremors, which multiply small lines. For these reasons, we can hardly take 

 into account observations that seem not to be sufficiently established. What 

 has been said is however by no means intended to undervalue Mr. Short's ob- 

 servations; and this. Dr. H. hopes, will be evident, when it is remembered how 

 scrupulously he has just before set aside 4 of his own, because he thought them 

 not sufficiently confirmed. Mr. Hadley's observation of the division of the 

 ring, with a 5-l feet Newtonian reflector, which was certainly a very excellent 

 instrument, agrees perfectly well with Dr. H.'s. 



From what has been said, it does not appear that there is a sufficient ground 

 for admitting the ring of Saturn to be of a very changeable nature; and probably 

 its phenomena will hereafter be so fully explained, as to reconcile all observa- 

 tions. In the mean while, we must withhold a final judgment of its construc- 

 tion, till we can have more observations. Its division however into 2 very un- 

 equal parts, can admit of no doubt; and the following are measures taken of 

 the diameter of the larger or outer ring. 



Oct. 7, 1791- Correction of the 10-feet ist measure 54".115 



clock.— 'V'' 1 6'. 5. —Measures of the ring of Sa- 2d 52 .537 



.^, ^, r n w 3d 52 .875 



turn with the 20-feet reflector, at O'' 37"', as an- 4th 54 .079 



nexed. When this measure is reduced to what ^tl> 52 903 



it would be at the mean distance of Saturn from 7th ..... !! 53 am 



the earthj we have 46".832. Mean .... 53 .30"(> 



