ction and at the
same rate, across the sky. An example of this kind of unanimity was
alleged by him in the five intermediate stars of the Plough; and that
the agreement in thwartwise motion is no casual one is practically
demonstrated by the concordant radial velocities determined at Potsdam
for four out of the five objects in question. All of these approach the
earth at the rate of about eighteen miles a second; and the fifth and
faintest, Delta Ursae, though not yet measured, may be held to share
their advance. One of them, moreover, Zeta Ursae, alias Mizar,
carries with it three other stars--Alcor, the Arab "Rider" of the horse,
visible to the naked eye, besides a telescopic and a spectroscopic
attendant. So that the group may be regarded as octuple. It is of vast
compass. Dr. Hoeffler assigned to it in 1897[1628]--although on grounds
more or less hypothetical--a mean parallax corresponding to a
light-journey of 192 years, which would give to the marching squadron a
total extent of at least fourteen times the distance from the sun to
Alpha Centauri, while implying for its brightest member--Eta
Ursae Majoris--the lustre of six hundred suns. The organising principle
of this grand scheme must long remain mysterious.

It is no solitary example. Particular association, indeed--as was
surmised by Michell far back in the eighteenth century--appears to be
the rule rather than an exception in the sidereal system. Stars are
bound together by twos, by threes, by dozens, by hundreds. Our own sun
is, perhaps, not exempt from this gregarious tendency. Yet the search
for its companions has, up to the present, been unavailing. Gould's
cluster[1629] seems remote and intangible; Kapteyn's collection of solar
stars proved to have been a creation of erroneous data, and was
abolished by his unrelenting industry. Rather, we appear to have secured
a compartment to ourselves for our long journey through space. A
practical certainty has, at any rate, been gained that whatever
aggregation holds the sun as a constituent is of a far looser build than
the Pleiades or Praesepe. Of all such majestic communities the laws and
revolutions remain, as yet, inaccessible to inquiry; centuries may
elapse before even a rudimentary acquaintance with them begins to
develop; while the economy of the higher order of association, which we
must reasonably believe that they unite to compose, will possibly
continue to stimulate and baffle human curiosity to the en