Royal Astronomical Society. 131' 



slants employed in the reduction of the observations themselves may 

 mislead us, and induce us to suspect a proper motion, when there is, 

 in fact, no foundation for such an opinion. Two catalogues, there- 

 fore, are not always enough to enable us to decide with confidence 

 that any particular star has a proper motion of its own ; a iluTd be- 

 comes necessary to determine the question with the requisite degree 

 of accuracy; and where these are discordant, ^jouTth\% absolutely 

 essential, in order to show where the error exists. 



Mr. Baily then proceeds to state, that he has selected the cata- 

 logues of Bradley and Piazzi for determining the proper motion ((u,') 

 of any proposed star for the^rsi period, viz. from 1755 to 1800, and 

 the catalogues of Piazzi and Pond for the proper motion (im') for the 

 second period, viz. from 1800 to 1830 ; at the same time slating, that 

 he has also adopted some recent determinations in right ascension, by 

 Professor Airy and Dr. Robinson, in addition to those of Mr. Pond. 

 The formulae by which the proper motions for these two periods are 

 deduced are as follows : viz. 



P - B p + V 



'^ "" 45 ~ 2 



'^ = ^0 — v'+-3-; 



where P denotes the place of the star in Piazzi's catalogue, B the 

 place of the star in Bradley's catalogue ; p the annual precession in 

 J 800, ir the annual precession in 1755, both taken from M. Bessel's 

 catalogue in the Fund. Astron.; and S the place of the star for the 

 year 1830, the right ascensions being reduced to the same equinoctial 

 point as those of B and P. 



It is in tliis manner that Mr. Baily has compared the places of all 

 those stars which are known (or which have been supposed) to be af- 

 fected with an annual proper motion of not less than about 0"'50 of 

 a great circle, and has given a list of upwards of three hundred of 

 such stars, with the amount of their annual proper motion (in right 

 ascension and declination) for the two periods above mentioned. The 

 author remarks, that when the annual proper motions of any given 

 star thus deduced, for the two periods, correspond with each other, 

 there is every reason to believe that such proper motion is correct ; 

 but that where they differ, (and this is by no means a rare occurrence, 

 at once showing the difficulty of the subject,) the probable conclusion 

 is, that such stars have very little proper motion, or perhaps none at 

 all ; and that, at all events, further observations are necessary to de- 

 cide so delicate yet so imjjortant a point. Mr. Baily concludes by re- 

 marking, that "our information is at present but slight, and not al- 

 " ways satisfactory. When future observations shall have increased 

 " our slock of knowledge, the subject may probably be taken up by 

 " others, who will have belter mean's of following it up more in de- 

 " tail than I have been able to do with the present scanty material." 



II. The reading of Mr. Herschel's paper on the Orbits of Binary 

 Stars was continued; but as it could not be fipishcd this evening, the 

 conclusion was deferred till the next meeting. 



.After the meeting, Mr. Bate exhibited a machine for the illustration 



of the precession of the cquino.\es. It consists of a pair of flexible 



S 2 hoopi 



