VOL. LXXVI.] PHILOSOPHICAL TRANSACTIONS. 85 



Now to give a short account of these stars, beginning with those of the first 

 clasar. 



The famous Nova of J 572 in Cassioperr — Several astronomers are of opinion, 

 that it has a periodical return, which Keill, and others have conjectured to 

 happen every 150 years. This is also my opinion. 



o Cell. — Since the end of ] 782 I have observed very exactly the decrease of 

 brightness of this star; but never have seen it of above the 6th magnitude. 

 Oct. 29, 1782, it was of the 7 th magnitude, and gradually decreased till Dec. 

 30, it being then of the 8 . Qth magnitude. J 783, Feb. 16, certainly less than 

 the 9th magnitude. 1783, Aug. 25, of the dth magnitude, and gradually de- 

 creased until Dec. 14, being then of the 10th magnitude, and equal to the little, 

 star close to it. I have deduced its period from the times when it was equal to a 

 certain star in the course of its decrease; the results were 320, 337, and 328 

 days; but M. Cassini determined its mean period with greater exactness to be 

 334 days. Mr. Goodricke saw it Aug. 9, 1782, of the 2d magnitude, rather 

 brighter than a and less than (3 Ceti. Sept. 5, it was of the 3d magnitude, 

 being equal to y Ceti. 



y^lgol. — The period of Algol, discovered by Mr. Goodricke, gave some new 

 light into the nature of the fixed stars. Its degree of brightness, when at its 

 minimum, is different in different periods; and I think, when at its full, it is 

 sometimes brighter than « Persei, and at other times less. 



Mayers N° 420, lateli/ discovered to he variable by M. Koch. — A few years 

 before 1782, M. Koch saw the N° 420 undoubtedly less than the N°419 of 

 Mayer's Catalogue. In February J 782, he found them both exactly of the same 

 brightness, therefore of the 7 th magnitude. From an extract of a letter I have 

 lately seen, the variable was of the Qth magnitude in April, 1 783, and of the 

 10th in April, 1784. 



Variable in Hydra. — Maraldi, in 1704, having found that this star had a 

 periodical variation, continued to examine it for several years, and concluded its 

 period to be about 2 years, though with considerable variations; in which he was 

 much mistaken, as appears from several observations, which show that its period 

 in all probability is tolerably regular, and only of 494 days. If Maraldi's obser- 

 vations of 1704 and 17O8 are exact to a month, and there is no reason to believe 

 otherwise, the period at that time seems to have been a few days longer than it 

 is at present, and therefore the one here deduced may be esteemed as the mean 

 period. 



Particulars of the changes it undergoes, are, 1 . When at its full brightness it is 

 of the 4th magnitude, and has no perceptible change for about a fortnight. 2. It 

 is about 6 months in increasing from the lOth magnitude, and returning to the 



