VOL. LXXX.] PHILOSOPHICAL TRANSACTIONS. 6l5 



Dr. H. comes now to one of the most remarkable properties in the con- 

 struction of the ring, which is its extreme thinness. The situation of Saturn, 

 for some months past, has been particularly favourable for an investigation of 

 this circumstance; and his experiments have been so complete, that there can 

 remain no doubt on this head. When nearly in the plane of the ring, he re- 

 peatedly saw the 1st, the 2d, and the 3d satellites, nay even the 6th and 7 th, 

 pass before and behind the ring in such a manner that they served as excellent 

 micrometers to estimate its thickness by. It may be proper to mention a few in- 

 stances, especially as they will serve to solve some phenomena that have been re- 

 marked by other astronomers, without having been accounted for in any manner 

 that could be admitted, consistently with other known facts. July 18, 1789, at 

 19^ 41" 9", sidereal time, the 1st satellite seemed to hang on the following arm, 

 declining a little towards the north, and gradually advanced on it towards the 

 body of Saturn ; but the ring was not so thick as the lucid point. July 23, at 

 19^ 41™ 8% the 2d satellite was a very little preceding the ring; but the ring ap- 

 peared to be less than half the thickness of the satellite. July 27, at 20** 15™ 

 12% the 2d satellite was about the middle, on the following arm of the ring, 

 and towards the south ; and the 6th satellite on the farther end, towards the 

 north; but the arm was thinner than either of them. August 29, at 22^ 12"* 

 0.5% the 3d satellite was on the ring, near the end of the preceding arm; and 

 the arm seemed not to be the 4th, at least not the 3d, part of the diameter of 

 the satellite, which, in the situation it was, seemed to be less than 1 single 

 second in diameter. At the same time the 7th satellite, at a little distance fol- 

 lowing the 3d, was seen in the shape of a bead on a thread, projecting on both 

 sides of the same arm : henee we are sure, that the arm also appeared thinner 

 than the 7 th satellite, which is considerably smaller than the 6th, which again is 

 a little less than the 1st satellite. August 31, at 20^ 48"* 26% the preceding arm 

 was loaded about the middle by the 3d satellite. October 15, at O'' 43™ 44", he 

 saw the 6th satellite, without obstruction, about the middle of the preceding 

 arm, though the ring was but barely visible with the 40-feet reflector, even while 

 the planet was in the meridian ; however, we were then a little inclined to the 

 plane of the ring, and the 3d satellite, when it came near its conjunction with 

 the 1st, was so situated that it must have partly covered the first a few minutes 

 after the time it was lost behind the house. In all these observations the ring 

 did not in the least interfere with the view of the satellites. October 16, Dr. 

 H. followed the 6th and 7th satellites up to the very disk of the planet; and the 

 ring, which was extremely faint, opposed no manner of obstruction to seeing 

 them gradually approach the disk, where the 7 th vanished at 21^ 46"^ 44', and 

 the 6th at 22*^ 36™ 44 ^ 



Many other instances might be brought, if necessary. There is however some 



