ist
sidereal systems composed of two stars revolving about each other in
regular orbits, and constituting which may be called, to distinguish
them from double stars, which are only optically double, binary stars.
That which since then has been most assiduously watched, and has offered
phenomena of the greatest interest, is _gamma Virginis_. It is a star of
the vulgar third magnitude, and its component individuals are very
nearly equal, and, as it would seem, in some slight degree variable. It
has been known to consist of two stars since the beginning of the
eighteenth century, the distance being then between six and seven
seconds, so that any tolerably good telescope would resolve it. When
observed by Herschel in 1780 it was 5.66 seconds, and continued to
decrease gradually and regularly, till at length, in 1836, the two stars
had approached so closely as to appear perfectly round and single under
the highest magnifying power which could be applied to most excellent
instruments--the great refractor of Pulkowa alone, with a magnifying
power of a thousand, continuing to indicate, by the wedge-shaped form of
the disc of the star, its composite nature.

By estimating the ratio of its length to its breadth, and measuring the
former, M. Struve concludes that at this epoch the distance of the two
stars, centre from centre, might be stated at .22 seconds. From that
time the star again opened, and is now again a perfectly easily
separable star. This very remarkable diminution, and subsequent
increase, of distance has been accompanied by a corresponding and
equally remarkable increase and subsequent diminution of relative
angular motion. Thus in 1783 the apparent angular motion hardly amounted
to half a degree per annum; while in 1830 it had decreased to 5 degrees,
in 1834 to 20 degrees, in 1835 to 40 degrees, and about the middle of
1836 to upwards of 70 degrees per annum, or at the rate of a degree in
five days.

This is in entire conformity with the principles of dynamics, which
establish a necessary connection between the angular velocity and the
distance, as well in the apparent as in the real orbit of one body
revolving about another under the influence of mutual attraction; the
former varying inversely as the square of the latter, in both orbits,
whatever be the curve described and whatever the law of the attractive
force.

It is not with the revolutions of bodies of a planetary or cometary
nature round a solar centre