VOL. LXXXII.] PHILOSOPHICAL TRANSACTIONS. 1'23 



Oct. 24, 1791. Correction of the AO-feet 

 clock + 25^4. — Measure of the ring of Saturn ist measure 53".9i+ 



with the 40-feet reflector. Power 370, at l'^ 3">. ^d^.^.^^. . . . 53 j6o 



Reduced to the mean distance of Saturn, the 

 measure is 4 7 ".241. 



Nov. 21, 1791. Correction of the AO-feet 



clock. 7'.8. — Another measure of the ring of 1st measure 50".627 



Saturn with the 40-feet reflector, power 370, gd'.V.V.V. 50 .808 



at 0*" 48"". Reduced to the mean distance of Mean 50 .492 



Saturn the measure is 45".803. 



Oct. 24 47".24i ^y "'^y ^^ forming more easily a comparative 



Nov. 21 — 45.803 idea of the stupendous size of this ring of Sa- 



Mean .... 46 .522 . t-« u 1 w j .u .• • , 



40-feet 46" 5''2 turn, Ur. ti. calculated the proportion it bears 



20-feet 46 .832 to the earth, and found that its diameter is to 



Meanofall..46 .677 that of the latter as 25.8914 to 1; and that 



consequently, 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 frequent 

 observations of the Saturnian system, Dr. H. remarked that the 5th satellite is 

 subject to a change of brightness. When he 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 very naturally, that the cause of this phe- 

 nomenon could be no other than a rotation on its axis. It became necessary 

 therefore to find out a method to determine the time of this rotation. To 

 investigate this, he pursued the satellite with great attention, and marked all its 

 changes of apparent brightness. The result of many observations was as fol- 

 lows. The light of the satellite was in full splendour during the time it ran 

 through that part of its orbit which is between 68 and 129° past the inferior 

 , conjunction. In passing through this arch it did not fall above 1 magnitude 

 short of the brightness of the 4th satellite. On the contrary, from about 7° 

 past the opposition till towards the inferior conjunction, it was not only less 

 bright than the 3d, but hardly, if at all, exceeded the 2d, or even the 1st satel- 

 lite; provided the latter were then about its greatest elongation, where its light 

 is least impeded by the brightness of the planet. On the whole, the alteration 

 seems to amount to what among the fixed stars, and with the naked eye, would 

 be called a change from the 5th to the 2d, and from the 2d to the 5th magnitude. 

 Having thus observed this satellite, for many of its revolutions round the 

 primary planet, to lose and regain its light regularly, it is evident that the time 

 of its rotation on its axis cannot differ much from that of its revolution round 



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