544 BIBLIOGRAPHY OF HERSCHEL's WRITINGS. 



Herschel, W.: Synopsis of the Writings of— Continued. 

 A. I). Vol. p. 



1789 79 251 Class VII No, 18 to No. 55 



253 VIII No. 41 to No. 78 



255 Notes. 



255 P. S. — The planet Saturn has a sixth satellite revolving round it ia 

 about 32 hours 48 minutes. Its orbit lies exactly in the plane of 

 the ring. An account of its discovery with the 40-foot reflector, 

 etc. , will be presented to the Royal Society at their nest meeting. 



William Herschel. 



1790 80 1 Account of the discovery of a sixth and seventh satellite of the planet 



Saturn; with remarks on the construction of its ring, its atmosphere, its 

 rotation on an axis, and its spheroidical figure. By William Her- 

 schel, LL. D., F. R. S. Read November 12, 1789. 



1 In a postscript to my last paper I announced to the Royal Society the 



discovery of a satellite of Saturn. I have now the honor to present 

 them, with an account of two satellites instead of one, and I have 

 called them the sixth and seventh, although their situation very 

 probably entitles them to be called the first and second. 



2 These have not been before discovered on account of the difficulty of 



seeing them with a less telescope than the 40-foot. 



2 I began to observe Saturn in 1774, and on March 17 I saw it with a 



.5^-foot reflector, as in Plate I, fig. 1. Fig. 2 shows it on April 3, 

 1774, without its ring. In 1775 I saw the ring gradually open 

 (using a 7-foot reflector). Fig. 3, Plate II, shows the aj>pearauce 

 178S [should be 1778], June 20 (with a very good 10-foot reflector). 



3 The black belt on the ring of Saturn is not in the middle of ita 



breadth, nor is the ring subdivided by many such lines, but there 

 is one single, considerably broad belt upon the ring which is per- 

 manently in its jflace. 

 3 My observations show that this belt (at least on the north half of the 

 ring, where I have alone observed it) is not like the belts oi Jupiter 

 or those of Saturn, subject to variations of color or figure, but is 

 most probably owing to some permanent construction of the surface 

 of the ring itself. 



3 It is not the shadow of a chain of mountains, since it is visible all 



round the ring. The same argument will hold good against sup- 

 posed caverns or concavities. It is pretty evident that it is con- 

 tained between two concentric circles. See fig. 4, Plate II, drawing 

 of 1780, May 11. 



4 As to the surmise of two rings, it does not appear eligible to venture 



on so artificial a construction, by way of explaining a phenomenon, 

 which does not absolutely demand it. 



4 As yet we do not know of any rotation of the ring which may be of 



such a proper velocity as [might lend some support to the idea of 

 two detached rings]. 



5 If the southern side of the ring should be differently marked, it would 



negative the idea of two rings. Even if it should be marked the 

 same in every respect, it would be best to wait for the occultation 

 of some considerable star by Saturn, when, if the ring be divided, 

 it will be seen between the openings of the ring, as well as between 

 the ring and Saturn. 

 5 We may certainly affirm that the ring is no less solid than the planet 

 itself. The reasons which prove the solidity of one serve to prove 

 that of the other. 



