Page:Sir William Herschel, his life and works (1881).djvu/172

150 After all allowances had been made for the effects of precession and other displacements of a star's position which were produced by motions of the earth, it was found that there were still small outstanding differences which must be due to the motion of the star itself—its proper motion. The quantity of this motion was not well known for any star when researches began. Before they were concluded, however, had deduced the proper motions of thirty-six stars—the fundamental stars, so called—which included in their number Sirius, Procyon, Arcturus, and generally the brightest stars.

It is à priori evident that stars, in general, must have proper motions, when once we admit the universality of gravitation. That any fixed star should be entirely at rest would require that the attractions on all sides of it should be exactly balanced. Any change in the position of this star would break up this balance, and thus, in general, it follows that stars must be in motion, since all of them cannot occupy such a critical position as has to be assumed. If but one fixed star is in