144 The Planet Saturn. 



the eye nearer S. than ~N., Bond II : by micrometer, 8""5 and 

 9"-4.— 1852, Sept. 29, Bond II. found the ball projecting 

 somewhat more over the edge of the ring in front (S.) than 

 behind it. Dee. 1, distance between intersections of outer 

 edge of A with ball greater to the eye S. than 1ST.: by 

 micrometer, 10" # 68 and 9"'ll (exceedingly fine definition). 

 It is conceivable that the two hemispheres may be unequal in 

 density ; and the singular irregularity in terrestrial attraction 

 which has recently been discovered round Moscow furnishes 

 an analogy which might possibly find its counterpart in Saturn 

 on a much larger scale, so as to dissociate the centre of gravity 

 from the centre of figure of the ball ; but unfortunately such 

 an explanation would only be applicable if the deviation were 

 always in the same direction. 



Whether the whole ring-system may be encompassed by 

 any perceptible amount of atmosphere is an enquiry worthy 

 of attention. Ijl considered it probable, not only because 

 even the smallest satellites appeared distinctly protuberant on 

 either side of the thin ring-line, but because in June, 1807, 

 when the ring crossed the S. hemisphere so as to leave only a 

 segment of 1"*3 or 1"*4 in breadth visible, that portion seemed 

 curved or bulged outwards, while the opposite N. pole retained 

 it usual flattened appearance. This observation, which was 

 frequently repeated, was confirmed by his son, as well as by 

 Professor Wilson with another telescope. A refraction of 

 ig' would, he thought, be adequate to the effect. The same 

 appearance was remarked by Gruithuisen, 1817, and in the 

 same direction. It may be questioned, however, whether this 

 is anything more than an optical deception, the counterpart of 

 the quadrilateral figure of the ball. Were it a reality, its 

 effect would increase with the decrease of the outstanding 

 segment, till the last visible portion of the latter would exhibit 

 an unmistakable amount of distorted elevation. Nothing of 

 the kind, however, is on record. On the contrary, Bond II. 

 noted, 1853, Dec. 15, that the N. pole was invisible, when the 

 S. limb just touched the outer edge of A : circumstances under 

 which refraction round either ball or ring would surely have 

 made itself known. 



The idea, however, of an atmosphere is not without support 

 from evidence of another nature. 1828, Jan. 22, Kater was 

 led to the same conclusion, by noticing that duriug an obser- 

 vation of several hours, in a perfectly favourable evening, he 

 could find no trace of the subdivisions upon A which had struck 

 him so much on a former occasion. 



Other phenomena also may bo referred to the presence of 

 such an envelope. But before describing them we must obtain 

 some idea of the cause of the multiform aspect of the ring 



