VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 457 



tion in the star Algol or (3 Persei, of a nature hitherto, I believe, unnoticed. 

 The first time I saw it vary was Nov. 12, 1782, between 8 and 9 o'clock at 

 night, when it appeared of about the 4th magnitude ; but the next day it was 

 of the 2d magnitude, which is its usual appearance. On Dec. 28, I perceived 

 it vary again thus ; at 5-L h in the evening, it was about the 4th magnitude, as on 

 the 12th of Nov. but at 8± h I was much surprized to find it so quickly increased 

 as to appear of the 2d magnitude. The usual and greatest magnitude of Algol 

 is this ; of the 2d magnitude, much less bright than a. Persei, and not so much as 

 y Andromedae ; brighter than a. Cassiopeae and (3 Arietis, and nearly the same, if 

 not rather brighter, than «. Pegasi and (3 Cassiopeae ; rather less bright than 

 y Cassiopeae, and much brighter than £ Persei and |3 Trianguli. The relative 

 brightness of the stars to which I compared it during the progress of its varia- 

 tion, is as follows ; a. Cassiopeae is the brightest, and of near the 2d magnitude; 

 (3 Arietis is the next, and of between the 2d and 3d magnitude ; then 1 Persei 

 and (3 Trianguli, both of the 3d magnitude ; £ Persei is somewhat less bright 

 than i Persei, and also of the 3d magnitude ; $ Persei is less than £ Persei, and 

 rather of between the 3d and 4th magnitude ; ^ Persei, which Algol is equal to 

 at its least brightness, is not so bright as S Persei, and of about the 4th 

 magnitude. 



Observations on Algol, as to its Brightness and Magnitude. 



Jan. 14, 1783. — At 6 h. it was varied from its usual brightness, but rather brighter than /3 Arietis. 



At 6§ h. equal to /3 Arietis, but rather a little less bright, and of between the 2d and 3d mag- 

 nitude. 



At 7\ h. 3d magnitude ; not so bright as p Arietis, and equal to /3 Trianguli. 



At 7| h. nearly die same as at 7 5 ; but rather less bright than /3 Trianguli. 



At 8| h. between the 2d and 3d magnitude ; not quite so bright as /3 Trianguli, and rather less than 

 i and £ Persei, but a little brighter than ^ and j Persei. 



At 9 \ h. about the 4th magnitude, and equal to j Persei. 



The weather was cloudy till llf h. when it appeared to be of the 3d magnitude ; much brighter 

 than j Persei, and rather brighter than y Persei. 



At 12j h. between the 2d and 3d magnitude ; brighter than Z, and i Persei and /3 Trianguli. 



Jan. 17. — At 7§ h. of the 3d magnitude, equal to t Persei, and rather less than /3 Trianguli. 



At 8 h. a very little brighter than t Persei, and equal to & Trianguli. 



At 8| h. rather brighter than /3 Trianguli, but the sky was not favourable. 



Jan. 31. — At 10j h. varied from its usual brightness, but with some doubt. 



At lljh. certainly less bright ; much less than y Andromedae, but brighter than £ and t Persei, 

 and of between the 2d and 3d magnitude. 



At 12| h. 3d magnitude, and rather brighter than £ and e Persei. 



At 13 h. about the brightness of £ Persei, and much brighter than » Persei. 



At 14j h. the 4th magnitude, and equal to f Persei, but afterwards increased. 



Feb. 6. — At 5 1 h. rather brighter than /3 Arietis, and between the 3d and 4th magnitude. 



At 61 h. 3d mag. ; not so bright as fi Arietis, but brighter than /3 Trianguli and t Persei. 



At 6\ h. about the same brightness as /3 Trianguli and t Persei. 



VOL. XV. 3 N 



