SECT. XXXVI. VARIABLE STARS. 391 



instance of a variable star. It has the size of a star of the second 

 magnitude for two days and thirteen and a half hours, and then 

 suddenly begins to diminish in splendour, till, in about three hours 

 and a half, it is reduced to the size of a star of the fourth magni- 

 tude ; it then begins again to increase, and in three hours and 

 a half more regains its brightness, going through all these vicissi- 

 tudes in 2 d 20 h 48 m 54 s - 7. Sir John'Herschel and Mr. Goodricke, 

 by whom the variable nature of this star was discovered in 1782, 

 considered this to be a case strongly indicative of the revolution 

 of an opaque body, which, coming between us and Algol, cuts 

 off a large portion of the light. This star has been constantly 

 observed, and the more recent observations, compared with the 

 ancient ones, indicated a diminution in the periodic time. It is 

 even proved that this decrease is not uniformly progressive, but 

 is actually proceeding with accelerated rapidity, which, however, 

 will probably not continue, but will by degrees relax, and then be 

 changed into increase, according to the laws of periodicity, which, 

 as well as their causes, remain to be discovered. The first mini- 

 mum of this star, in 1844, happened on January 3rd, at 4 h 14 

 Greenwich time, y Hydras also vanishes and reappears every 494 

 days. /3 Lyrae was discovered to be variable, in 1784, by Mr. 

 Goodricke, and its period was ascertained by Argelander to be 

 12 d 21 h 53 m 10", in which time a double maximum and mini- 

 mum takes place, the two maxima being nearly equal, but the 

 two minima unequal ; besides these semi-periods, there is a slow 

 aberration of period, which appears to be periodical itself : from 

 its discovery to 1840 the time was continually lengthening, but 

 more and more slowly, till, in 1840, it ceased to increase, and has 

 since been slowly on the decrease. 



The -stars d Cephei and rj Aquilee, or Antinoi, were discovered 

 to be variable in 1784 ; their.respective periods, being 5 d 8 h 47 m 39" 

 and 7 d 4 h 13 m 53", have since been accurately determined. Besides 

 these, the variations of between 30 and 40 have been approximately 

 ascertained, and a great many more among the smaller stars have 

 been discovered to be variable by Mr. Hind, who has remarked 

 that many of those stars which continue visible at their minimum 

 appear hazy and indistinct, as though some cloudy or nebulous 

 medium intervened. Some of the variable stars are red, and 

 others present successive changes through blue, yellow, and red. 

 When the brightness is increasing the star has a blueish tinge, 



