15 



is subject to periodical changes, alternately increasing and 

 decreasing within certain limits." Since the reading of this 

 paper additional observations and a re-examination and 

 discussion of the former observations have brought to light 

 a phenomenon whicl], so far as I am aware, has not been 

 previously observed. Each list of minima in the paper 

 referred to consists of two series, the first of minima observed 

 from 1859 to 1868 at the Crumpsall Observatory, and the 

 second those observed in 1878 and 1879 at the observatory 

 in Birkdale, and in the re-examination I have recently made 

 of all my reductions and results I found that I had been in 

 error in assuming that the minimum of August 19, 1878, 

 was a principal minimum, and that the interval from 

 September 12, 1868, comprised 52 periods, whereas the 

 minimum of July 16, 1878, was a principal minimum, 

 and the interval from September 12, 1868, to that date 

 comprised 51 complete periods, giving a mean period of 

 70"47 days. It appears, therefore, that six minima which 

 I had entered in the table of principal minima ought to 

 have been entered in the table of secondary minima, and 

 four minima entered as secondary were really primary; 

 and that in fact a reversion of the two minima had taken 

 place in the interval from 1868 to 1878. Since I arrived 

 at this conclusion I have examined the results of Professor 

 Schonfeld's observations to 1874, published in Nos. 1907, 

 1992, and 2066 of the " Astronomische Nachrichten," and 

 find that they fully bear out this view, and as he states that 

 the period seems to have decreased up to 1870-71, and that 

 in 1874 it had again increased, there can be no doubt that 

 the number of periods between 1868, September 12, and 

 1878, July 16, was only 51, and that a reversion of the 

 minima took place in or about 1874 after an increase in the 

 length of the period had commenced. Schonfeld, indeed, 

 in his last communication alluding to the near approach to 

 equality of the two minima which had then taken place. 



