Page:Proceedings of the Royal Society of London Vol 1.djvu/119

Rh rius, &c.—These indeed, as well as all other heavenly bodies, cannot be said to be entirely free from the inﬂuence of the stars surrounding them; but the character assigned to them is, that the attraction in one direction is so counteracted by a contrary inﬂuence of the same nature, as to be retained for many ages in a state almost equal to undisturbed rest. Dr. Herschel suspects that we are to look for solar systems only among those insulated stars.

2. Binary sidereal systems, or double stars—It is suffciently ob- vious that these are not stars seen nearly in the some visual ray, for these rays may be an immense distance from each other; but by these are meant two stars that are connected together by the in- ﬂuence of attraction. It is easy to prove, by the doctrine of gravi- tation, that two stars thus connected, and sufﬁciently distant from the inﬂuence of other celestial bodies, will perform revolutions round a common centre of motion; that hence they will always move in di- rections opposite and parallel to each other; and that their system, if not destroyed by some foreign cause, will remain permanent. This kind of rotation is exempliﬁed by the instance of our earth and the moon. Dr. Herschel proposes, on a future occasion, to communi- cate a series of observations made on double stars, whereby it will he seen that many of them have actually changed their situation with regard to each other, in a progressive course, denoting a periodical revolntion round each other, and that the motion of some of them is direct, While that of others is retrograde.

3. More complicated sidereal systems, or treble, quadruple, and multiple stars.—From the combination of two stars, it is easy to ad- vance a step further, and allow that three or more stars maybe con‘ nected in one mutual system of reciprocal attraction; and the com- putation for determining the common centre of their respective orbits is here exempliﬁed by a variety of hypothetical cases. The author at the same time asserts, that there is not a single night when in passing over the zones of the heavens by sweeping, he does not meet with numerous collections of such multiple stars, apparently insu- lated from other groups, and probably joined in some small sidereal system of their own.

4. Clustering stars—These are described as great collections of small stars that are profusely scattered over the milky way, by no means uniformly, but unequally dispersed in many separate allot- ments. An instance of one of these aggregates is given, which in a space of about 5° between [3 and y Cygni, contains above 331,000 stars. A more particular account of the milky way, we are pro- mised, will be the subject of a future communication.

5. Groups of stars.—-These differ from the preceding class by being collections of closely, and almost equally compressed stars, of any ﬁgure or outline; and from the next following, by showing no particular condensation that seems to point out any ideal centre of attraction.

6. Clusters of stars—These are generally round, and the compression of their stars indicates a gradual accumulation towards their