roper motion. In virtue of that proper
motion, the two components are carried together over the sky at the rate
of five seconds annually. A proper motion of this magnitude is extremely
rare, yet we do not say it is unparalleled, for there are some few stars
which have a proper motion even more rapid; but the remarkable duplex
character of 61 Cygni, combined with the large proper motion, render it
an unique object, at all events, in the northern hemisphere.

When Bessel proposed to undertake the great research with which his name
will be for ever connected, he determined to devote one, or two, or
three years to the continuous observations of one star, with the view of
measuring carefully its parallactic ellipse. How was he to select the
object on which so much labour was to be expended? It was all-important
to choose a star which should prove sufficiently near to reward his
efforts by exhibiting a measurable parallax. Yet he could have but
little more than surmise and analogy as a guide. It occurred to him that
the exceptional features of 61 Cygni afforded the necessary presumption,
and he determined to apply the process of observation to this star. He
devoted the greater part of three years to the work, and succeeded in
discovering its distance from the earth.

Since the date of Sir John Herschel's address, 61 Cygni has received the
devoted and scarcely remitted attention of astronomers. In fact, we
might say that each succeeding generation undertakes a new discussion of
the distance of this star, with the view of confirming or of criticising
the original discovery of Bessel. The diagram here given (Fig. 94) is
intended to illustrate the recent history of 61 Cygni.

When Bessel engaged in his labours, the pair of stars forming the double
were at the point indicated on the diagram by the date 1838. The next
epoch occurred fifteen years later, when Otto Struve undertook his
researches, and the pair of stars had by that time moved to the
position marked 1853. Finally, when the same object was more recently
observed at Dunsink Observatory, the pair had made still another
advance, to the position indicated by the date 1878. Thus, in forty
years this double star had moved over an arc of the heavens upwards of
three minutes in length. The actual path is, indeed, more complicated
than a simple rectilinear movement. The two stars which form the double
have a certain relative velocity, in consequence of their mutual
attraction.