GENERAL MOVEMENT OF THE STARS. 221 



positions sufficiently exact, for what relates to these proper move- 

 ments. 



Sir William Herschel, in studying this subject with the sagacity 

 which distinguished him, arrived successively at two discoveries of 

 very high importance. 



In the first place, about the year 1783, he showed that when these 

 proper movements of the stars are considered collectively, a reason 

 may be assigned for a great part of them by admitting that our sun 

 has itself a movement in space in a certain direction. This movement, 

 by a simple effect of perspective, would produce a gradual apparent, 

 separation among the stars on the side to which the sun. was directing 

 its course, and an apparent diminution of relative distance among those 

 from which it Avas receding. Though long contested, even by astrono- 

 mers of great merit, the movement in question seems now placed 

 beyond dispute, since the labors of Argelander, Lundahl, Otto Struve, 

 Bravais, Galloway, and Msedler, directed to this subject, have given 

 it full confirmation, and indicated its direction towards a point in the 

 constellation Hercules, very near to that assigned by Herschel 

 himself.* 



The second great discovery of Ilerschel in reference to this subject 

 dates from the first years of the present century, and relates to those 

 stars which appear to the naked sight so near one another as to be 

 blended into one, which, from this circumstance, we denominate double, 

 triple, and multiple, stars. It was shown that in examining these 

 closely there might be detected, at the end of a certain number of 

 years, evident changes in the relative positions of the stars composing 

 some of these groups, from which we may conclude that such stars 

 form systems of suns revolving around their common centre of gravity. 

 It is well known what an extension has been given to this interesting 

 part of science by the subsequent researches of astronomers, in which 

 Sir John Herschel, Sir James South, Messrs. Dawes, Hind, Jacob, &c., 

 of the British empire; the elder and younger Struve, Savary, Encke, 

 Bessel, Kaiser, Msedler, &c, on the continent of Europe, have par- 

 ticularly distinguished themselves by their labors of observation and 

 calculation. 



From divers considerations set forth in the memoir above cited, and 

 from comparing together the proper movements of some hundred 

 stars, M. Masdler had, in 184*!, already attained a firm persuasion that, 

 besides the causes of apparent displacement just spoken of, the 

 collective body of stars visible to us has a real and common move- 

 ment of revolution around a centre, situated in the group of the 

 Pleiades, and corresponding to the star Alcyone, or yj of Taurus, of 

 the third magnitude, the most brilliant of the numerous stars of 

 that group. 



* We feel satisfaction in recording that among the first to confirm the results obtained 

 by Herschel respecting the proper movement of the sun were two French philosophers, 

 Pierre Prevost and Frederic Maurice, in a memoir inserted in the collection of the Academy 

 of r>eiiin for 1801. M. Arago has shown that the idea of the possibility of such a move- 

 ment had been already announced by Fontenelle, Bradley, Mayer, and Lambert, though he 

 fully recognizes Herschel as the first who proved its existence. 



