734 PHILOSOPHICAL TRANSACTIONS. [aNNO I79O, 



As observations, carefully made, should always take the lead of theories. Dr. 

 H. will not be concerned if such lucid spots as he is now going to admit, should 

 seem to contradict what has been said in his last paper, concerning the idea of 

 inequalities, or protuberant points. We may however remark, that a lucid^ 

 and apparently protuberant point, may exist without any great inequality in the 

 ring. A vivid light, for instance, will seem to project greatly beyond the limits 

 of the body on which it is placed. If therefore the luminous places on the ring 

 should be such as proceed from very bright reflecting regions, or, which is more 

 probable, owe their existence to the more fluctuating causes of inherent fires 

 acting with great violence, we need not imagine the ring of Saturn to be very 

 uneven or distorted, in order to present us with such appearances as will be re- 

 lated. In this sense of the word, then, we may still oppose the idea of 

 protuberant points, such as would denote immense mountains of elevated 

 surface. 



On comparing together several observations, a few trials show that the brightest 

 nd best observed spot agrees to a revolution of 10^ 32"^ ]5**4; and, calculating 

 its distance from the centre of Saturn, on a supposition of its being a satellite, 

 we find it 17'''.!227, which brings it on the ring. It is therefore certain, that un- 

 less we should imagine the ring to be sufficiently fluid to permit a satellite to 

 revolve in it, or suppose a notch, groove, or division in the ring, to suffer the sa- 

 tellite to pass along, we ought to admit a revolution of the ring itself. The density 

 of the ring indeed may be supposed to be very inconsiderable by those who ima- 

 gine its light to be rather the effect of some shining fluid, like an aurora borealis, 

 than a reflection from some permanent substance; but its disapparition in general, 

 and in the telescopes its faintness when turned edgeways, are in no manner favour- 

 able to this idea. When we add also, that this ring casts a deep shadow on the 

 planet, is very sharply defined both in its outer and inner edge, and in brightness 

 exceeds the planet itself, it seems to be almost proved, that its consistence cannot 

 be less than that of the body of Saturn ; and that consequently, no degree of 

 fluidity can be admitted sufficient to permit a revolving body to keep in motion 

 for any considerable time. 



A groove might affbrd a passage, especially as on a former occasion we have al- 

 ready considered the idea of a divided ring. A circumstance also which seems 

 rather to favour this idea is, that in some observations a bright spot has been 

 seen to project equally on both sides, as the satellites have been observed to do 

 when they passed behind the ring. But, on the other hand, we ought to con- 

 sider that the spot has often been observed very near the end of the arms of 

 Saturn's ring, and that the calculated distance is consequently a little too small 

 for such appearances, and ought to be IQ or 20" at least. We should also at- 

 tend to the size of the spot, which seems to be variable; for it is hardly to be 

 imagined that a satellite, brighter than the 6th, and which could be seen with 



