54 



Astronomical Scie?ice ; 



[July 



would be called proper motion, but, for distinction sake, I propose to 

 introduce the terms true and apparent. 



Now the true proper motion of several of the (improperly called) 

 fixed stars, has been long since recognized, and the apparent has been 

 talked of as possible, but has not yet been established. With a view to 

 enquire into this question, I selected a large catalogue of stars for ob- 

 servation, about two years ago, a great many of which have already 

 been observed ; whence it appears, that, out of 3,003 stars, there are 

 135 which exhibit true proper motion in right ascension, and 128 in de- 

 clination. The remaining stars having (if any) but a very small pro* 

 per motion. 



To understand this the better, it will be as well here to explain, that 

 the places of the stars, from the Madras observations for the year 1835, 

 have been compared with the places determined by Piazzi for the year 

 1800 ; the difference between these places being divided by 35 ((he 

 numbers of years elapsed), gives the annual variation of each star ; the 

 difference between which and the computed annual precession, leaves 

 the proper motion. It happens, however, that (each observer having 

 committed an error, more or less, in each observation) the amount of 

 proper motion, thus determined from any single star, will be mixed up 

 with the accidental error of observation. Hence it will follow that all 

 the proper motions, which are less than the possible errors of observa- 

 tion, may be in reality the error of observation, and not proper motion. 

 This is true, as I have already said, for a single observation, but, from 

 the mean of a great many, we may hope much to reduce, if not to get 

 rid of, the error of observation. 



It will hence be understood that the 135 cases of proper motion in 

 right ascension, andl28 in declination, above mentioned, are those cases 

 in which the difference between the variation and the precession ex- 

 ceeds the largest amount to which the error of observation reaches. 

 On looking over these proper motions, they occur so indiscriminately, 

 among bright or faint stars, as often + as — , or at one part of the hea- 

 vens as frequently as at another ; so that we at once must set them 

 down as true proper motions. 



^ With regard to the remaining 2,868 stars, which exhibit proper mo- 

 tion, below the value at which we have rated the error of observation 

 m right ascension, and the 2,875 cases (under ths eame conditions) in 

 declination, should these too occur indifferently, + or — &c, it would 

 follow that, whilst taking the mean to get rid of the error of observa- 

 tion, we shall at the same time get rid of the proper motions. If, how- 

 ever, the proper motions arise from motion of the solar system, the 

 quantities we determine will be apparent proper motion, and, taking 

 the stars at various part of the heavens in groups, the effect would 

 (ail having the same tendency) accumulate. With this in view, the 

 means have been taken in every hour of right ascension as follows ; 



