CLUSTERS OF STARS. 141 



tail of Scorjno, and the Altar (U. A. IGh. 45m.-19h.). All 

 clusters in and near the Milky Way are not, however, round 

 and globular ; there are many of irregular outline, with but 

 few stars and not a very dense center. In many globular 

 clusters the stars arc uniform in magnitude, in others they 

 vary. In some few cases they exhibit a fine reddish cen- 

 tral star* (R. A. 2h. 10m. ; N. Decl. 56° 21'). It is a dif- 

 ficult problem in dynamics to understand how such island- 

 worlds, with their multitude of suns, can rotate free and un 

 disturbed. Nebulous spots and clusters of stars appear sub- 

 ject to different laws in their local distribution, although the 

 former are now very generally assumed to consist of very 

 small and still more remote stars. The recojrnition of these 

 laws must specially modify the conjectures entertained of 

 what has been boldly termed the " structure of the heav- 

 ens." It is, moreover, worthy of notice, that, with an in- 

 strument of equal aperture and magnifying power, round 

 nebulous spots are more easily resolved into clusters of stars 

 than oval ones.f 



I will content myself with naming the following among 

 the isolated systems of clusters and swarms of stars. 



The Pleiades : doubtless known to the rudest nations from 

 the earliest times ; the inarinef's stars — Pleias, aixb tov 

 ttXeIv (from nXelv, to sail), according to the etymology of 

 the old scholiast of Aratus, who is probably more correct than 

 those modern writers who would derive the name from 7T?ieo^, 

 plenty. The navigation of the Mediterranean lasted from 

 May to the beginning of November, from the early rising to 

 the early setting of the Pbiades. 



Pra3sepe in Cancer : according to Pliny, nubecula quani 

 p7'cBsepia vacant inter Asellos, a vecpeXiov of the Pseudo- 

 Eratosthenes. 



The cluster of stars on the sword-hilt of Perseus, frequent- 

 ly mentioned by Greek astronomers. 



Coma Berenices, like the three former, visible to the naked 

 eye. 



A cluster of stars near Arcturus (No. 1663), telescopic: 

 R. A. 13h. 34m. 12s., N. Decl. 29^ 14' ; more than a thousand 

 stars from the tenth to the twelfth magnitude. 



Cluster of stars between rj and <^ Herculis, visible to the 

 naked eye in clear nights. A magnificent object in the tel- 

 escope (No. 1968), with a singular radiating margin ; R. A. 



* Outlines, $ 864-869, p. .591-596; Madler's Astr., s. 764, 

 t Observations at the Cape, $ 29, p. 19. 



