126 PHILOSOPHICAL TRANSACTIONS, [aNNO 1792. 



unnecessary here to repeat them. The mean of all the observations give 

 8' 3\".Q7, for the mean apparent distance of the 5th satellite from Saturn. 



Dr. H. forbears making deductions from this result, with respect to the quan- 

 tity of matter contained in the planet, as possibly the orbit of the satellite may 

 be considerably elliptical; in which case measures taken in opposite parts of that 

 orbit will be required, before we can make a strict application of the laws of 

 centripetal forces. 



//. Miscellaneous Observations. By JVilUam Herschel, LL. D., FRS. p. 23. 



Account of a comet. — Last Thursday evening, Dec. 15, 1791, about half 

 after 8 o'clock, while observing Saturn, his sister. Miss Herschel, looked over 

 the heavens, and discovered a pretty large, telescopic comet, in the breast of 

 Lacerta. Dr. H. viewed it in his 7-feet reflector, and with that instrument 

 settled its place and rate of moving. At y" 42"^ 4^8 true mean time, it pre- 

 ceded a small telescopic star 1 1^3 in time, and was 1' 41" south of the same. 

 It follows the 2d of Flamsteed's stars in the constellation of Lacerta, 1"" 41'.5 in 

 time; and is 45' 40". 8 more south than the same. The apparent motion of the 

 comet was direct, and at the rate of about 3'" of time in right ascension and a 

 little more than 1° in polar distance per day. He examined it with a 20-feet 

 reflector, and found it to consist of a great light, pretty regularly scattered about 

 a condensed small part of 5 or 6" in diameter; which resembled a kind of nu- 

 cleus, but had not the least appearance of a solid body. Besides the scattered, 

 and gradually diminishing light, which reached nearly to a distance of 3' every 

 way beyond the bright centre, there was also a faintly extended, ill defined, 

 pretty broad ray, of about 15' in length, directed towards the north following 

 part of the heaven, which might be called the tail of the comet. 



On the periodical appearance of o Ceti. — The changeable star in the neck of 

 the whale, o Ceti, continues its variations as usual, but with some considerable 

 irregularities of brightness. In the year 1779, we have seen that it excelled a. 

 Arietis so far as almost to rival Aldebaran, and continued in that state a full 

 month. In 1780, its greatest brightness was only like that of S Ceti. In the 

 year 1781, it did not come up to the brightness of <?. In 1/82, this star increased 

 to the size of (3 Ceti, and continued bright for more than 20 days. In 1783, it 

 did not only vanish to the naked eye, as usual, but disappeared so completely, 

 that he could not find it with a telescope which permitted not a star of the 10th 

 magnitude to escape. When it increased again, it did not amount to the bright- 

 ness of S. In 1784, it was only of the 8th magnitude in a 20-feet reflector. In 

 1789, it arrived to the brightness of a Piscium, or rather excelled it. In 1790, 

 the greatest brightness was almost equal to that of a Ceti. In the present year. 



