bers) six billions of miles a year. Four and a
third such measures are needed to span the abyss that separates us from
the nearest fixed star. In other words, light takes four years and four
months to reach the earth from Alpha Centauri; yet Alpha Centauri lies
some ten billions of miles nearer to us (so far as is yet known) than
any other member of the sidereal system!

The determination of parallax leads, in the case of stars revolving in
known orbits, to the determination of mass; for the distance from the
earth of the two bodies forming a binary system being ascertained, the
seconds of arc apparently separating them from each other can be
translated into millions of miles; and we only need to add a knowledge
of their period to enable us, by an easy sum in proportion, to find
their combined mass in terms of that of the sun. Thus, since--according
to Dr. Doberck's elements--the components of Alpha Centauri revolve
round their common centre of gravity at a mean distance nearly 25 times
the radius of the earth's orbit, in a period of 88 years, the attractive
force of the two together must be just twice the solar. We may gather
some idea of their relations by placing in imagination a second luminary
like our sun in circulation between the orbits of Neptune and Uranus.
But systems of still more majestic proportions are reduced by extreme
remoteness to apparent insignificance. A double star of the fourth
magnitude in Cassiopeia (Eta), to which a small parallax is ascribed on
the authority of O. Struve, appears to be above eight times as massive
as the central orb of our world; while a much less conspicuous pair--85
Pegasi--exerts, if the available data can be depended upon, no less than
thirteen times the solar gravitating power.

Further, the actual rate of proper motions, so far as regards that part
of them which is projected upon the sphere, can be ascertained for stars
at known distance. The annual journey, for instance, of 61 Cygni _across
the line of sight_ amounts to 1,000, and that of Alpha Centauri to
446 millions of miles. A small star, numbered 1,830 in Groombridge's
Circumpolar Catalogue, "devours the way" at the rate of at least 150
miles a second--a speed, in Newcomb's opinion, beyond the gravitating
power of the entire sidereal system to control; and Mu Cassiopeiae
possesses above two-thirds of that surprising velocity; while for both
objects, radial movements of just sixty miles a second were disclosed
by