2IO CELESTIAL MEASURINGS AND WEIGHINGS. 



as a "proper motion," or slow progressive movement 

 pr.pcr to itself as an individual: smaller indeed than 

 those already specified in its apparent amount, but by 

 no means inconsiderable, being sufficient to displace it 

 by about two minutes in angle or one-fifteenth part of 

 the apparent diameter of the moon per century : corre- 

 sponding at the distance of the star to no less an amount 

 of actual linear travel than 1,900,000 miles per diem. 

 This movement, in the absence of all apparent reason to 

 the contrary, was of course presumed to be uniform and 

 rectilineal ; but as instruments improved, as observations 

 became more exact, and their calculation more scrupu- 

 lous and refined, this became at first doubtful, and at 

 length tlemonstrably incorrect Not to dwell on the 

 steps of the proof, it became apparent that the visible 

 path of the star, mapped down from year to year and 

 from century to century, is not a straight line, but is 

 affected by a small and regularly recurring undulation, 

 alternately carrying it to a small distance above and 

 below the medial line, similar to those represented in 

 Fig. i : the performance of one complete undulation 

 occupying about 49^ years, and the excursions to and 

 fro on either side of the medial line being about one- 

 sixtieth part of the linear distance passed over in the 

 same interval. 



(36.) It was impossible to ascribe this phenomenon 

 (as in the case of our star in the Swan) to parallax. 

 Were this its origin, the undulations (as above explained) 

 would be annual, instead of extending over a period of 

 nearly fifty years ; and moreover that cause of apparent 



