02 PHILOSOPHICAL TRANSACTIONS. [ANNO 170(5. 



Herculis as a periodical star. The precision of the characters introduced in that 

 catalogue is such, that the smallest alteration in the lustre of the stars may be dis- 

 covered, by a proper attention to their expressions : the variation in the light of «. 

 Herculis is however pretty considerable, and cannot easily be mistaken, when 

 strictly compared to a proper standard. The star most conveniently situated for this 

 purpose is x Ophiuchi ; and as I have had no reason, during the time of my obser- 

 vations, to doubt the uniformity of its lustre, I have made use of it in the com- 

 parisons; which seem to be sufficiently decisive, with regard to the periodical 

 variations of the light of x Herculis. Other stars besides x Ophiuchi have also been 

 consulted; but the unsteadiness of their light would draw me into difficulties, which 

 at present it will be proper to avoid. The observations are found to contain at 

 least 4 regular changes of the alternate increase and decay of the apparent lustre of 

 our new periodical star, deduced from a comparison of its brightness with that of 

 x Ophiuchi. 



In order, from the table of observations, to obtain the time of the period, if we 

 first take all the successive observations from Sept. 16, till Nov. 28, they show 

 very clearly that the star has completely gone through all its changes. For, ad- 

 mitting a maximum of the light of a Herculis to have been Sept. 16, we find a 

 minimum on Oct. 25 ; and a 2d maximum about Nov. 28. The period therefore 

 is of somewhat more than 2 months duration. But as changeable stars are subject 

 to temporary inequalities, which will render a determination of the length of a 

 period, from a single series of changes, liable to considerable errors, we shall now 

 take the assistance of the most distant observations. By an inspection of the 

 table, we find again the first maximum to have been about Sept. 16, 1795; and 

 the 4th the 14th of May, 1796. This being an interval of 241 days, in which 4 

 successive changes have been gone through, we obtain about 60^ days for the 

 duration of the period. In confirmation of this computation, the table shows that 

 our periodical star was very faint in August 1795; bright about the middle of Sep- 

 tember; faint towards the end of October; bright the latter part of November; 

 faint in December; bright in January, 1796; not observed in February; bright in 

 March ; faint in April ; and lastly, bright again in May. This is just what should 

 have happened according to the above determination, which, as we have seen, 

 gives a period of 8 weeks, 4^ days. Greater accuracy can only be obtained by 

 future observations. 



On the rotary motion of the stars on their axes. — The rotation of the fixed stars 

 on their axes has been lately mentioned in a paper, where I could not have an 

 opportunity to enter into the reasons why it ought to be admitted*. The rotatory 

 motion of stars on their axes is a capital feature in their resemblance to the sun. It 

 appears now, that we cannot refuse to admit such a motion, and that indeed it may 

 be as evidently proved as the diurnal motion of the earth. Dark spots, or large 

 portions of the surface, less luminous than the rest, turned alternately in certain 

 directions, either towards or from us, will account for all the phenomena of 



• Phil. Trans, for the year 1795,—Orig. 



