184 



Proceedings of the Royal Irish Academy. 



its discovery, haying been about 500 days in 1708, 487 days in 1785, 

 461 days in 1825, and 437 days in 1870, and it seems to be still de- 

 creasing. He gives the following formula, from Argelander, for the 

 calculation of the time of Maximum : — 



1815. 9. 2-48 + 469-3363 ^- 0'^-44351 E- + O'^-OOigSl E-. 



According to Schmidt, the minimum occurs about 200 days before 

 the maximum. 



Dr. Gould, at Cordoba, has studied the changes of this variable, 

 and finds that the period is rapidly diminishing, the diminution 

 amounting to more than 9 hours in each period, and that there is a 

 symmetric perturbation which completes its cycle in 72 years. He 

 gives the following formula of sines for the calculation of the time of 

 maximum, counting from the beginning of the year 1875 : — 



T= 35''-6 + 4:S4-4:45n - 0•37974?^2 + B2^-0 sin (5°/^ + 10°) 



+ 2'^-6 sin {10°n + 324°) + 6^-8 sin (15n + 205"). 



This formula fixes a maximum for 1884, August 9. 



Heis gives the minimum magnitude as 11 m. Smyth (Cel. Cycle) 

 calls the star /jl Hydrae ; but this is a mistake, the real /x (42 Fl) being 

 a 4 m. star a little south of a line joining X Hydra with a Crateris. 



The following are the observed Maxima of E, Hydras since the year 

 1853:— 



Obseeved M a xt wa of R. Hxdks. 



Year. 



Date 



of 



Maximum. 



Brightness. 



Authority. 



Remarks. 



1853. 



Mar. 27 



"sehi'heU" 



Winnecke. 



Au-svers. A. jST., 1238. 



,, 



Mar. 30 



.. 



ScTimidt. 



A. N., 1547. 



1856. 



Dec. 15 



.. 



„ 



). )) 



1858. 

 1859. 

 1859. 



Feb. 27-5 

 May 20-5 

 Last S"-* May 



5-4 



Au\7ers. 



A. N., 1410. Unsiclier, zu 



Olmutz bestimmt. 

 A. N., 1236. Genau, zu 



Athens bestimmt. 

 A. N., 1238. 



1860. 

 1863. 



Aug. 15 

 Jan. 19 





Schmidt. 



4 or 5 steps fainter^than y 



Hydi-se. A. iST., 1300. 

 A. N., 1410. 



1864. 



April 14 





j> 





1865. 



July 4 





j> 



A. N., 1547. Period about 

 447-8 days. 



