VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 459 



appears also, that this variation probably recurs about every 2 days and 21 hours. 

 This last conclusion will be rendered more conspicuous by the following table ; 

 the first column of which shows the days, and exact time of the day, when Algol 

 was observed to be very near, or at its least brightness ; the 2d column marks the 

 different intervals of time elapsed between the several observations ; the 3d ex- 

 hibits the quotient arising from a division of these intervals by a certain number 

 of revolutions, each of 2 days and 21 hours, which number of revolutions are 

 expressed in the last column. 



The day and time when Algol The different intervals The quotients of the Number of 



was observed at or near its between the several divisions of the 2d revolu- 



least brightness. observations. column by the 4th. tions. 



1782 Nov. 12" 8 J" 



Dec. 28 5 \ 45" 21 h 2 d 20.8" \6 



1783 Jan. 14 91 17 3j 2 20.6 6 



31 14.1 17 5 2 20.8 6 



Feb. 6 8" 5 17| 2 21 2- 



23 12+ 17 4 2 20.6 6 



26 91 2 21 J 2 21.5 1 



Mar. 21 8.1 22 23 2 20.9 8 



April 10 10+ 20 li 2 20.8 ^ 



13 8 2 22 2 22* 1 



May 3 9\ 20 1 2 20.7 7 



The results in the 3d column agree so nearly, that there is the greatest pro- 

 bability, not to say certainty, that the singular and quick variation of this star, 

 during the space of 7 hours, as above-mentioned, recurs regularly and periodically 

 about every 2 days and nearly 20f hours. 



Whether this singular phenomenon is always the same ; or whether it occurs 

 only some years, and ceases entirely in others (as may be presumed from the ac- 

 count of Montanari and Maraldi ;) and whether in this case it recurs in regular 

 periods of time or otherwise ; are curious objects of investigation, which can only 

 be determined by a long and regular course of observations for many years. If it 

 were not perhaps too early to hazard even a conjecture on the cause of this varia- 

 tion, I should imagine it could hardly be accounted for otherwise than either by 

 the interposition of a large body revolving round Algol, or some kind of motion 

 of its own, by which part of its body, covered with spots or such like matter, is 

 periodically turned towards the earth. 



* The difference of upwards of an hour in this quotient will easily be reduced to the others by re- 

 marking, that Algol was observed on the 10th and 13th of April, not when it was at, but only near, 

 its least brightness : and indeed all the little differences of the rest will vanish by making a reasonable 

 allowance of the same kind. — Orig. 



END OF THE SEVENTY-THIRD VOLUME OF THE ORIGINAL. 



3 N 2 



