Changes of Light in the Star Algol. 289 
that FLamsteap has alfo amongft other ftars obferved Algol, 
and in two places has marked it of lefs magnitude than at 
other times, vz. of the third magnitude, 1696, January 16. 
6h. 24’, and 1711, December 5. gh. 13’, both mean time 
and old ftile*. Sufpecting thefe might probably be days of 
Algol’s variation, I computed the interval between them, but 
could not find a period anfwerable to that which I have above’ 
determined. Upon examining more clofely the obfervations, 
I find, in that of 1696, he marked at the {fame time the mag- 
nitude of p Perfei; which, confidering efpecially the nearnefs 
of e Perfei to Algol, makes this obfervation to be relied on for 
its juftnefs, and lefs liable to any miftake of judgement ; 
whereas the other obfervation of December 5, 17 Ef. is more 
liable to error or doubtfulnefs, becaufe he did not then mark 
the magnitude of 9 Perfei, or of any ftar of the fame magni- 
tude near enough to Algol. Prefuming, therefore, on the juft- 
nefs of FLAMSTEAD’s obfervation of 1696, to think that it 
probably was made at a time when Algol varied, I compared 
it with one of mine, viz. October 25. 6h. 39’, 1783, and I 
find there is, in the interval between thofe obfervations, either 
11,176 periods, each of 2d. 20h. 49’ 18”; or 11,177, each 
of 2d. 20h. 48’ 56”. The laft, -as it approaches neareft to 
the refults of my beft obfervations, I think, is the exacteft de- 
termination of the period. This, however, all proceeds upon 
the fuppofition that Algol varied at the time of FLAMsTEAD’s 
obfervation, and alfo that the period is regular. 
* Hiftoria Coeleftis, vel. I, edit 1725, p. 284. and 534. © 
- Vou, LXXIV. Qq ache 
