counted for, and even the weight and the
size of Algol and its dark companion be determined.

There are, however, other classes of variable stars, the fluctuation of
whose light can hardly be due to occasional obscuration by dark bodies.
This is particularly the case with those variables which are generally
faint, but now and then flare up for a short time, after which temporary
exaltation they again sink down to their original condition. The periods
of such changes are usually from six months to two years. The best known
example of a star of this class was discovered more than three hundred
years ago. It is situated in the constellation Cetus, a little south of
the equator. This object was the earliest known case of a variable star,
except the so-called temporary stars, to which we shall presently refer.
The variable in Cetus received the name of Mira, or the wonderful. The
period of the fluctuations of Mira Ceti is about eleven months, during
the greater part of which time the star is of the ninth magnitude, and
consequently invisible to the naked eye. When the proper time has
arrived, its brightness begins to increase rather suddenly. It soon
becomes a conspicuous object of the second or third magnitude. In this
condition it remains for eight or ten days, and then declines more
slowly than it rose until it is reduced to its original faintness, about
three hundred days after the rise commenced.

More striking to the general observer than the ordinary variable stars
are the _temporary stars_ which on rare occasions suddenly make their
appearance in the heavens. The most famous object of this kind was that
which blazed out in the beginning of November, 1572, and which when
first seen was as bright as Venus at its maximum brightness. It could,
indeed, be seen in full daylight by sharp-sighted people. As far as
history can tell us, no other temporary star has ever been as bright as
this one. It is specially associated with the name of Tycho Brahe, for
although he was not the discoverer, he made the best observations of the
object, and he proved that it was at a distance comparable with that of
the ordinary fixed stars. Tycho described carefully the gradual decline
of the wonderful star until it disappeared from his view about the end
of March, 1574, for the telescope, by which it could doubtless have been
followed further, had not yet been invented. During the decline the
colour of the object gradually changed; at first i