Page:Proceedings of the Royal Society of London Vol 1.djvu/249

Rh be either a single body of great magnitude; or it may be produced by the joint attraction of a great number of stars united into one condensed group ; or, lastly, it may be formed by the union of several groups, which, he says, will create a still more powerful centre of gravitation.

Dr. Herschel now proceeds to the determination of the quantity and velocity of the solar motion: and, calculating upon the principles laid down in the course of the present paper, assuming, as we have already stated, that the solar motion holds a middle rank among the sidereal velocities, it appears that we have sufﬁcient reason to ﬁx upon the quantities of the solar motion to be such as, by an eye placed at right angles to its direction, and at the distance of Sirius from us, would be seen to describe annually an arch of 1116992 of a degree ; and its velocity, till we are acquainted with the real distance of the fore-mentioned star, can therefore only be expressed by the proportional number of 11 16992.

Before he concludes, our author remarks, that the middle rank among the sidereal velocities, which he has assigned to the sun, agrees sufﬁciently with the phenomena that were to be explained. Thus the apparent velocities of Arcturus and Aldebaran, without a solar motion, are to each other as 208 to 12; but, according to the assumed solar motion, it appears, that when the deception arising from the parallactic eﬂ'ect is removed, these velocities are to each other only as 179 to 85, or 2 to 1. And although Arcturus still remains a star which moves with great velocity, yet it has been shown, in the eleventh table, that we have three or four stars with nearly as much motion, and ﬁve with more. The solar motion also removes the deception by'which the motion of a star, of the conse- quence of ex Orionis, is so concealed as hardly to show any velocity; whereas, by computation, we ﬁnd that it really moves at a rate which is fully equal to the motion of the sun.

It will now be found, Dr. Herschel says. that we are within the reach of a link of the chain which connects the principles of the solar and sidereal motions with those that are the cause of orbitual ones: the probable motions of the sun and of the stars in orbits consequently becomes a subject that may receive the assistance of arguments supported by observations. And he further observes, that what he has said in a former paper, where the sun is placed among the insulated stars, does not contradict the present idea of its forming a part of a very extensive system. The insulation there ascribed to the sun relates merely to a supposed binary combination with some neighbouring star; and it has been already proved, by the example of Arcturus, that the solar motion cannot be occasioned, or accounted for, by a periodical revolution of the sun and the above, or any other star, about their common centre of gravity.