iable.
The variations of some [Page 221] stars are so marked as to
challenge investigation. b Lyrae (Fig. 81) has two maxima and minima
of light. In three days it rises from magnitude 4-1/2 to 3-1/2; in a
week falls to 4, and rises to 3-1/2; and in three days more drops to
4-1/2: it makes all these changes in thirteen days; but this period
is constantly increasing. The variations of one hundred and
forty-three stars have been well ascertained.

[Illustration: Fig. 81.--Constellation Lyra, showing place of the
Ring Nebula.]

Mira, or the Wonderful, in the Whale (Fig. 68), is easily found when
visible. Align from Capella to the Pleiades, and as much farther,
and four stars will be seen, situated thus:

*
* * *

The right-hand one is Mira. For half a month it shines as a star
of the second magnitude. Then for three months it fades away, and
lost to sight; going down even to the eleventh magnitude. But after
five months its resurrection morning mes; and in three months
more--eleven months in all--our Wonderful is in its full glory
in the heavens. It its period and brilliancy are also variable.
The star Megrez, d in the Great Bear, has been growing dim [Page
222] for a century. In 1836 Betelguese was exceedingly variable, and
continued so till 1840, when the changes became much less
conspicuous. Algol (Fig. 68) has been already referred to. This
slowly winking eye is of the second magnitude during 2d. 14h. Then
it dozes off toward sleep for 4h. 24m., when it is nearly invisible.
It wakes up during the same time; so that its period from maximum
brilliancy to the same state again is 2d. 20h. 48m. Its recognizable
changes are within five or six hours. As I write, March 25th, 1879,
Algol gives its minimum light at 9h. 36m. P.M. It passes fifteen
minima in 43d. 13m. There will therefore be another minimum May 7th,
at 9h. 49m. Its future periods are easy to estimate. Perhaps it has
some dark body revolving about it at frightful speed, in a period of
less than three days. The period of its variability is growing
shorter at an increasing rate. If its variability is caused by a
dark body revolving about it, the orbit of that body is contracting,
and the huge satellite will soon, as celestial periods are reckoned,
commence to graze the surface of the sun itself, rebound again and
again, and at length plunge itself into the central fire. Such an
event would evolve heat enough to make Algol flame up into a star of
the fir