austral heavens as does the great ``Coal-sack'' described in Chapter
1. But it is their isolation that makes them so remarkable, for their
composition is essentially galactic, and if they were included within
its boundaries they would not appear more wonderful than many other
parts of the Milky Way. Placed where they are, they look like masses
fallen from the great stellar arch. They are full of nebulæ and
star-clusters, and show striking evidences of spiral movement.

Star-swarms, which are also characteristic features of the Galaxy,
differ from star-clouds very much in the way that their name would
imply -- i.e., their component stars are so arranged, even when they
are countless in number, that the idea of an exceedingly numerous
assemblage rather than that of a cloud is impressed on the observer's
mind. In a star-swarm the separate members are distinguishable because
they are either larger or nearer than the stars composing a ``cloud.''
A splendid example of a true star-swarm is furnished by Chi Persei, in
that part of the Milky Way which runs between the constellations
Perseus and Cassiopeia. This swarm is much coarser than many others,
and can be seen by the naked eye. In a small telescope it appears
double, as if the suns composing it had divided into two parties which
keep on their way side by side, with some commingling of their members
where the skirts of the two companies come in contact.

Smaller than either star-clouds or star-swarms, and differing from
both in their organization, are star-clusters. These, unlike the
others, are found outside as well as inside the Milky Way, although
they are more numerous inside its boundaries than elsewhere. The term
star-cluster is sometimes applied, though improperly, to assemblages
which are rather groups, such, for instance, as the Pleiades. In their
most characteristic aspect star-clusters are of a globular shape --