156 SPECIAL RESULTS IN THE URANOLOGJCAL PORTION 



greater number, variable stars of the 6th to the 9th mag- 

 nitudes, as the Variabiles, Coronse, Yirgiuis, Cancri, and 

 Aquarii. The maximum of the star x Cygni undergoes 

 great fluctuations. 



That variable stars are very irregular in their periods had 

 long been known ; but that in the midst of tin's apparent ir- 

 regularity their variations are yet subject to definite laws, 

 has for the first time been made out by Argelander. He 

 hopes to demonstrate the truth of his views in this respect 

 in detail in an extensive treatise devoted expressly to the 

 subject. He now considers that two perturbations in the 

 period of ^ Cygni, one of 100 and the other of 8' 5 single 

 periods, are more probable than one of 108. Whether such 

 disturbances originate in alterations in the luminous process 

 going on in the atmosphere of the star, or in the period of 

 revolution of a planet revolving round the fixed star or sun 

 X Cygni, and affecting the form of its photosphere by at- 

 traction, remains indeed still uncertain. The greatest ir- 

 regularities in the variation of lustre are certainly presented 

 by the star " Variabilis Scuti" in Sobieski's Shield, as this 

 star sometimes diminishes from 5*4m. down to 9m. and 

 once, according to Pigott, disappeared entirely at the end of 

 the last century. At other times its fluctuations have only 

 been between 6 -5m. and 6m. The maximum brightness 

 observed in % Cygni has varied between 6*7m. and 4m., 

 and that of Mira, between 4m. and 2*lm. On the 

 other hand, 3 Cephei has shewn in the length of its periods 

 an extraordinary degree of regularity, greater than in any 

 other variable star, as has appeared by 87 minima observed 

 between the 10th of October, 1840, and the 8th of January, 

 1848, and others still more recent. In e Aurigae the alte- 

 ration of brightness, ( 276 ) as found by an indefatigable ob- 



