Page:Proceedings of the Royal Society of London Vol 1.djvu/218

Rh Dr. Herschel has delineated the meeting of the arches, arising from a calculation of the proper motions of the 36 stars in Dr. Mas- kelyne’s Catalogue, on a celesﬁal globe ; and ﬁnds that, in the northern hemisphere, no less than ten of those intersections are made by stars of the ﬁrst magnitude, in a very limited part of the heavens, about the constellation of Heﬁcules. Upon all the remaining surface there is not the least appearance of any other than a promiscuous situation of intersections, and only one of these'made by arches of principal stars.

A table is then given of the calculated situations of the above- mentioned ten intersections in right ascension and north polar di- stance; and it is observed, that if the intersections made by the proper motions of some large stars of the next order, and the arches in which the stars of the ﬁrst magnitude move, are examined, no less than ﬁfteen unite with the former ten in pointing out the same part of the heavens as a parallactic centre. This. Dr. Herschel thinks, can hardly fail to be considered as a convincing proof of the motion here treated of.

The changes in the position of double stars are next considered; and these, Dr. Herschel thinks, it will be more eligible to ascribe to the effect of parallax than to admit so many separate motions in the different stars, especially as the parallactic motions of at least half of the 56 double stars described by him, point out the same apex of a solar motion by their direction to its opposite parallactic centre.

Dr. Herschel then remarks, that if the proper motions of the stars were such as they appear to be, they would exhibit an incongruous mixture of great velocity and extreme slowness. Of this incongruity, several instances are enumerated; but it will, he says, be shown, when the direction and velocity of the solar motion are explained, that these incongruities are mere parallactic appearances.

With respect to the occultation of a small star by a large one, Dr. Herschel will, he says, prove, when the solar motion is esta- blished, that the vanishing of the small star near 5 Cygni is, as far as We can judge at present, only a parallactic appearance.

Dr. Herschel now proceeds to consider the direction of the solar motion: the expedience of admitting such motion being, he thinks, after what has been said, no longer questionable.

He begins by proving, that when the proper motions of two stars are given, an apex may be found, to which, if the sun be supposed to move with a certain velocity, the two given motions may be resolved into apparent changes, arising from sidereal parallax; the stars remaining perfectly at rest. The mode of proving this, in which Arcturus and Sirius are used as examples, will not admit of abridgement. But, from the nature of proper motions, it follows, that when a third star does not lead us to the same apex as the other two, its apparent motion cannot be resolved by the effect of parallax alone: and, although we may account for the proper motion of the third star, Capella for instance, by retaining the same apex of the solar motion which explained the apparent motions of the other