own in Fig. 91. There are in Cygnus five principal stars,
which form a constellation of rather remarkable form.

The last constellation which we shall here describe is that of Aquila or
the Eagle, which contains a star of the first magnitude, known as
Altair; this group can be readily found by a line from Vega over b
Cygni, which passes near the line of three stars, forming the
characteristic part of the Eagle.

We have taken the opportunity to indicate in these sketches of the
constellations the positions of some other remarkable telescopic
objects, the description of which we must postpone to the following
chapters.

CHAPTER XIX.

THE DISTANT SUNS.

Sirius Contrasted with the Sun--Stars can be Weighed, but not in
general Measured--The Companion of Sirius--Determination of the
Weights of Sirius and his Companion--Dark Stars--Variable and
Temporary Stars--Enormous Number of Stars.

The splendid pre-eminence of Sirius has caused it to be observed with
minute care from the earliest times in the history of astronomy. Each
generation of astronomers devoted time and labour to determine the exact
places of the brightest stars in the heavens. A vast mass of
observations as to the place of Sirius among the stars had thus been
accumulated, and it was found that, like many other stars, Sirius had
what astronomers call _proper motion_. Comparing the place of Sirius
with regard to the other stars now with the place which it occupied one
hundred years ago, there is a difference of two minutes (127") in its
situation. This is a small quantity: it is so small that the unaided eye
could not see it. Could we now see the sky as it appeared one century
ago, we should still see this star in its well-known place to the left
of Orion. Careful alignment by the eye would hardly detect that Sirius
was moving in two, or even in three or in four centuries. But the
accuracy of the meridian circle renders these minute quantities evident,
and gives to them their true significance. To the eye of the astronomer,
Sirius, instead of creeping along with a movement which centuries will
not show, is pursuing its majestic course with a velocity appropriate to
its dimensions.

Though the velocity of Sirius is _about_ 1,000 miles a minute, yet it
is sometimes a little more and sometimes a little less than its mean
value. To the astronomer this fact is pregnant with information. Were
Sirius an isolated star, attended only b