stellations, and
its name is a Centauri. This star is, indeed, of memorable
interest in the history of the subject. Its parallax was first
determined at the Cape of Good Hope by Henderson; subsequent researches
have confirmed his observations, and the elaborate investigations of Dr.
Gill have proved that the parallax of this star is about three-quarters
of a second, so that it is only two-thirds of the distance of 61 Cygni.

61 Cygni arrested our attention, in the first instance, by the
circumstance that it had the large proper motion of five seconds
annually. We have also ascertained that the annual parallax is about
half a second. The combination of these two statements leads to a result
of considerable interest. It teaches us that 61 Cygni must each year
traverse a distance of not less than ten times the radius of the earth's
orbit. Translating this into ordinary figures, we learn that this star
must travel nine hundred and twenty million miles per annum. It must
move between two and three million miles each day, but this can only be
accomplished by maintaining the prodigious velocity of thirty miles per
second. There seems to be no escape from this conclusion. The facts
which we have described, and which are now sufficiently well
established, are inconsistent with the supposition that the velocity of
61 Cygni is less than thirty miles per second; the velocity may be
greater, but less it cannot be.

For the last hundred and fifty years we know that 61 Cygni has been
moving in the same direction and with the same velocity. Prior to the
existence of the telescope we have no observation to guide us; we
cannot, therefore, be absolutely certain as to the earlier history of
this star, yet it is only reasonable to suppose that 61 Cygni has been
moving from remote antiquity with a velocity comparable with that it has
at present. If disturbing influences were entirely absent, there could
be no trace of doubt about the matter. _Some_ disturbing influence,
however, there must be; the only question is whether that disturbing
influence is sufficient to modify seriously the assumption we have made.
A powerful disturbing influence might greatly alter the velocity of the
star; it might deflect the star from its rectilinear course; it might
even force the star to move around a closed orbit. We do not, however,
believe that any disturbing influence of this magnitude need be
contemplated, and there can be no reasonable doubt that 61 C