Mr. Birt's Observations of ct Cassiopeiae. 311 



observations give the maximum from November 1 2, 1838, to January 

 22, 1839; and calculating from July 7, 1831, twelve periods, the 

 maximum would be obtained on December 4, 1838. On the whole, 

 Mr. Birt concludes that the period of 225 days may be regarded as 

 a first approximation, which may receive correction from a compari- 

 son with the earlier observations of Sir William Herschel. 



On the Variability and Periodic Nature of the Star a Orionis. By 

 Sir John F. W. Herschel, Bart President. 



" In a communication which was read to this Society on the 10th 

 of May last, I pointed out the star a Cassiopeia as variable and 

 periodical. That the fluctuations in splendour of this star should 

 have escaped general notice is not extraordinary, since the difference 

 between its greatest and least brightness can hardly be estimated 

 at more than half a magnitude. But that a periodical variation to 

 a very much greater extent, in so important and remarkable a star as 

 a Orionis, should, up to this time, have been completely Unnoticed 

 by astronomers, does appear to me, I confess, not a little extraor- 

 dinary, and might be taken as an argument to show, more than any 

 thing, the comparatively neglected state of this highly interesting 

 branch of Phyical Astronomy. Perhaps, however, in this, as in many 

 other cases, the very prominence of the object has been the cause 

 of its being neglected ; as it might easily be supposed by any one 

 entering on this research, that had a star so familiar to every practi- 

 cal astronomer presented any striking peculiarity of this kind, it 

 could not but have been observed. Hence, while the attention of 

 observers has been directed, and with success, to much inferior stars, 

 it seems to have been taken for granted, that among stars of the 

 first magnitude nothing, in fact, remained to be discovered. 



" Having bestowed much attention, during my residence at the 

 Cape, on the estimation of the magnitudes of the southern stars, 

 both by direct photometrical measurements, assigning numerical 

 values to about sixty or seventy of them, selected as offering con- 

 venient gradations of brightness, and also by very assiduous and 

 often-repeated comparisons by the naked eye, with the view to com- 

 pleting a graduated scale down to the fifth magnitude, at least, it 

 became important to connect these magnitudes by similar compari- 

 sons with those of the northern hemisphere, by means of stars in 

 the vicinity of the equator admitting of observation at both sta- 

 tions. My method in these observations has been invariably on 

 each night to establish, in the first instance, a sort of skeleton-scale, 

 beginning with the stars of the first magnitude actually visible, and 

 extending as far as was judged convenient for the occasion, then 

 filling in this scale by the insertion of fresh stars between the mem- 

 bers. The stars of the first magnitude actually above the horizon 

 at the time of commencing observation were first arranged, and 

 others of that magnitude inserted among them as they rose and 

 gained altitude. 



" On the very clear and brilliant night of the 26th November last, 

 being engaged in a process of this kind, I was surprised, and I may 

 almost say startled, by the extraordinary splendour of a Orionis, 



