erschel
observed a star, No. 55 in Flamstead's Catalogue, in the constellation
Hercules. In 1790 the same star was observed by the same astronomer, but
since that time no search has been able to detect it. The stars 80 and
81 of the same catalogue, both of the fourth magnitude, have likewise
disappeared. In May, 1828, Sir John Herschel missed the star No. 42, in
the constellation Virgo, which has never since been seen. Examples might
be multiplied, but it is unnecessary."[273]

The demonstration of the variableness of the light-giving power of suns
is completed by the phenomena of the class called _Variable Stars_;
though the best astronomers are now agreed that _variability, and not
uniformity_, in the emission of light, is the general character of the
stars.[274] But the variations which occur before our eyes impress us
more deeply than those which require centuries for their completion. Sir
John Herschel has observed, and graphically described, one such instance
of variation of light.

"The star Eta Argus has always hitherto been regarded as a star of the
second magnitude; and I never had reason to regard it as variable. In
November, 1837, _I saw it, as usual_. Judge of my surprise to find, on
the sixteenth of December, that _it had suddenly become a star of the
first magnitude_, and almost equal to Rigel. It continued to increase.
Rigel is now not to be compared with it. It exceeds Arcturus, and is
very near equal to Alpha Centauri, being, at the moment I write, the
fourth star in the heavens, in the order of brightness."[275] It has
since passed through several variations of luster. Humboldt gives a
catalogue of twenty-four of such stars whose variations have been
recorded.

"A strange field of speculation is opened by this phenomenon. Here we
have a star fitfully variable to an astonishing extent, and whose
fluctuations are spread over centuries, apparently in no settled period,
and with no regularity of progression. What origin can we ascribe to
these sudden flashes and relapses? What conclusions are we to draw as to
the comfort or habitability of a system depending for its supply of
light and heat on such an uncertain source? Speculations of this kind
can hardly be termed visionary, when we consider that, from what has
been before said, we are compelled to admit a community of nature
between the fixed stars and our own sun; and when we reflect, that
geology testifies to the fact of extensive changes having ta