182 A CENTURY'S PROGRESS IN ASTRONOMY. 



particularly distinguished themselves in this 

 branch, and the number of known variables is 

 now very large, as every year brings fresh dis- 

 coveries, mostly by aid of photography. Many 

 of these newly-discovered variables are in star- 

 clusters and nebulae. 



Pickering proposed in 1880 the following 

 classification of variable stars, which has been 

 adopted all over the scientific world : Class 

 I., temporary star ; Class II., stars undergoing 

 in several months large variations, such as 

 Mira Ceti and U Orionis ; Class III., irregular 

 variables, such as Betelgeux and a Hercu- 

 lis; Class IV., short -period variables, such as 

 S Cephei, Geminorum, and ft Lyrse ; Class V., 

 11 Algol variables," which undergo variations last- 

 ing but a few hours. It is doubtful whether 

 new stars should be included in a classification 

 of variables, although in one case, at least, a 

 new star was found to be a long-period variable. 

 To these a sixth class may now be added. 

 This class, the detection of which is mainly due 

 to the profound investigations of Gore, is com- 

 posed of what have been termed "secular vari- 

 ables," which undergo slow fluctuations in 

 periods of many years, and sometimes of cen- 

 turies. This Class includes 8 Ursse Majoris, 

 Al - Fard, X Draconis, 6 Serpentis, e Pegasi, 



