the Ring of Saturn. 13 
■when seen at the mean distance of the sun, it will subtend an 
angle of 7' 25", 332. 
From the above proportions we also compute that this ring 
must be upwards of 204883 miles in diameter.* 
On the Rotation of the fifth Satellite of Saturn , on its Axis. 
In my frequent observations of the Saturnian system, I re- 
marked, that the 5th satellite is subject to a change of bright- 
ness. This having been noticed before by other observers, I 
did not at first pay so much attention to it as I soon afterwards 
found this circumstance deserved. When I saw' this satellite 
always assume the same brightness in the same part of its orbit, 
and perceived that its change was regular and periodical, it 
occurred to me very naturally, that the cause of this pheno- 
menon could be no other than a rotation upon its axis. It be- 
came necessary therefore to find out a method to determine the 
time of this rotation. 
In order to investigate this, I pursued the satellite with great 
attention, and marked all its changes of apparent bright- 
ness. The result of many observations is as follows. The 
light of the satellite is in full splendour during the time it runs 
through that part of its orbit which is between 68 and 129 de- 
grees past the inferior conjunction. In passing through this 
arch it does not fall above one magnitude short of the bright- 
ness of the 4th satellite. On the contrary, from about 7 de- 
* In this calculation I have used for the earth’s diameter the sum of the longer and 
shorter semi-axes which are given in Mr. Daley’s Paper, published in the last vo- 
lume of the Philosophical Transactions. If we compute the vacant space between the 
two rings immediately from the above dimensions of the outward ring, we shall have 
2839 miles; and this will certainly be more accurate than the result which has been 
drawn from the proportion of the J^readth of the ring to the diameter of Saturn. 
