VOL. LXXX.] PHILOSOPHICAL TRANSACTIONS. 6l7 



what a degree of accuracy it must require to distinguish objects that are so mi- 

 nute, and at the same time so faint, on account of their nearness to the disc of 

 the planet. On the whole therefore, he had not any one instance that could in- 

 duce him to believe the ring was not of an uniform thickness ; that is, equally 

 thick at equal distances from the centre, and of an equal diameter throughout 

 the whole of its construction. The idea of protuberant points on the ring of 

 Saturn indeed is of itself sufficient to render the opinion of their existence inad- 

 missible, when we consider the enormous size such points ought to be of, for us 

 to see them at the distance we are from the planet. 



From these supposed luminous points however. Dr. H. was, by imperceptible 

 steps, brought to the discovery of 2 satellites of Saturn, which had escaped un- 

 noticed, on account of their little distance from the planet, and faintness; 

 which latter is partly to be ascribed to their smallness, and partly to being so 

 near the light of the ring and disc of Saturn. Strong suspicions of the exist- 

 ence of a 6th satellite he had long entertained ; and if he had been more at lei- 

 sure 1 years before, when the discovery of the 1 Georgian satellites took him off 

 the pursuit, he would certainly have been able to announce its existence as early 

 as the igth of August, 1787, when at 22^ 18"" 56% he saw, and marked it 

 down, as being probably a 6th satellite, which was then about 12° past its 

 greatest preceding elongation. 



In 1788 very little could be done towards a discovery, as his 20 feet speculum 

 was so much tarnished by zenith sweeps, in which it had been more than usually 

 exposed to falling dews, that he could hardly see the Georgian satellites. In 

 hopes of great success with the 40 feet speculum. Dr. H. deferred the attack on 

 Saturn till that should be finished ; and having taken an early opportunity of di- 

 recting it to Saturn, the very first moment he saw the planet, which v/as the 

 28th of last August, he was presented with a view of 6 of its satellites, in such 

 a situation, and so bright, as rendered it impossible to mistake them, or not to 

 see them. The retrograde motion of Saturn amounted to nearly 44- minutes 

 per day, which made it very easy to ascertain whether the stars he took to be 

 satellites really were so ; and, in about 2 hours and a half, he had the pleasure of 

 finding that the planet had visibly carried them all away from their places. He 

 continued his observations constantly, whenever the weather would permit ; and 

 the great light of the 40 feet speculum was now of so much use, that he also, 

 on the 17th of September, detected the 7th satellite, when it was at its greatest 

 preceding elongation. 



As soon as Dr. H. had observations enough to make tables of the motion of 

 these new satellites, he calculated their place backwards, and soon found that 

 many suspicions of these satellites, in the shape of protuberant points on the 



VOL. XVI. 4 K 



