time in which to extinguish so mighty a conflagration. It is
comparatively easy to suggest an explanation of the sudden outbreak; it
is not equally easy to understand how it can have been subdued in a few
weeks. A good-sized iron casting in one of our foundries takes nearly as
much time to cool as sufficed to abate the celestial fires in Nova
Cygni!

On this ground it seemed not unreasonable to suppose that perhaps Nova
Cygni was not really a very extensive conflagration. But, if such were
the case, the star must have been comparatively _near_ to the earth,
since it presented so brilliant a spectacle and attracted so much
attention. It therefore appeared a plausible object for a parallax
research; and consequently a series of observations were made some years
ago at Dunsink. I was at the time too much engaged with other work to
devote very much labour to a research which might, after all, only prove
illusory. I simply made a sufficient number of micrometric measurements
to test whether a _large_ parallax existed. It has been already pointed
out how each star appears to describe a minute parallactic ellipse, in
consequence of the annual motion of the earth, and by measurement of
this ellipse the parallax--and therefore the distance--of the star can
be determined. In ordinary circumstances, when the parallax of a star is
being investigated, it is necessary to measure the position of the star
in its ellipse on many different occasions, distributed over a period of
at least an entire year. The method we adopted was much less laborious.
It was sufficiently accurate to test whether or not Nova Cygni had a
_large_ parallax, though it might not have been delicate enough to
disclose a small parallax. At a certain date, which can be readily
computed, the star is at one end of the parallactic ellipse, and six
months later the star is at the other end. By choosing suitable times in
the year for our observations, we can measure the star in those two
positions when it is most deranged by parallax. It was by observations
of this kind that I sought to detect the parallax of Nova Cygni. Its
distance from a neighbouring star was carefully measured by the
micrometer at the two seasons when, if parallax existed, those distances
should show their greatest discrepancy; but no certain difference
between these distances could be detected. The observations, therefore,
failed to reveal the existence of a parallactic ellipse--or, in other
words,