108 PHILOSOPHICAL TRANSACTIONS. [ANNO 1782. 



the observer, we will admit that if the parallax had amounted to 2 seconds he 

 would have perceived it. The star on which these observations were made, is 

 marked of the 3d magnitude in the catalogue of Ptolemy ; in Tycho Brahe's of 

 the 3d ; in the Prince of Hesse's of the 3d ; in Hevelius's between the 3d and 

 2d ; in Flamsteed's of the 2d ; and now appears as a very bright star of the 3d, 

 or a small star of the 2d magnitude ; therefore its parallax is probably consider- 

 ably less than that of a star of the first magnitude. Several authors, who have 

 touched on this subject, seem to have overlooked this distinction ; and from Dr. 

 Bradley's account of the parallax of y Draconis. have concluded the parallax of 

 the stars in general not to exceed 1 " ; but this appears by no means to follow from 

 the doctor's observations. It is rather evident that, for aught we know to the 

 contrary, the stars of the first magnitude may still have a parallax of several 

 seconds ; and probably this is as accurate a result as that method is capable of 

 giving, at least in latitudes where there is not a star of the first magnitude that 

 passes directly through the zenith.* 



* De La Lande, in his excellent book of Astronomy, says, that the parallax of the fixed stars has 



been proved to be absolutely insensible (liv. 16", § 2782.) He reports the observations of Tycho 



Brahe, Picard, Hook, and Flamsteed, and concludes (§ '27~>i) from die discovery of die aberration 



by Dr. Bradley (which it seems he also allows to be the most decisive on the subject) that now die 



question about parallax is resolved. In giving us the opinion which the doctor had of the result of 



his own observations with regard to the annual parallax, De La Lande only mentions " M. Bradley 



pense que si elle (la parallaxe) eut ete seulement de 1", il l'auroit appercue dans le grand nombre 



d'observations qu'il avoit faites, surtout de y du Dragon." But if we also take in those lines on which 



Dr. Bradley seems to lay the greatest stress, viz. " I believe 1 may venture to say, that in either of 



the two stars last mentioned it does not amount to 2 seconds ;" and if we allow for the magnitude of 



the stars on which die observations were made, I think I have fairly stated the full amount of all the 



actual proofs we have of the smallness of die annual parallax. Now, since it has escaped the finest 



observations of Bradley, it is not likely diat it should come up to the full quantity to which it might 



amount without being perceived ; and therefore the doctor might think it highly probable, " that it 



is not so great as one single second ;" and his opinion, as well as De La Lande's, who believes it to 



be absolutely insensible, are perfectly consistent with all the observations diat have hitherto been 



made ; though the actual proofs, which are die subject of our present inquiry, do not extend so far. 



Against die parallax of Sirius, De La Lande (§ 2731) mentions " forty-five meridian altitudes taken 



by Dr. Bevis,* with the eight-feet mural quadrant of the Royal Observatory at Greenwich, none of 



which differed 3 or 4 ' from the mean altitude." Now if they differed 3 or +" from die mean, we 



may suppose diey differed 6 or 8" from each other ; and that observations, subject to so many causes 



of error as I shall presently enumerate, and which differed so much from each other, cannot give the 



least evidence either for or against a parallax, will need no proof. Refraction alone, which is liable 



to such changes at the meridian altitude of Sirius, notwithstanding the most careful observations oi 



the barometer and diermometer should be made to ascertain its quantity, would, widi me, remain an 



unanswerable argument against die validity of such observations in a subject of this critical nicety. — 



Orig. 



« These observations were not made by Dr. Bevis, but extracted from the registers of the Royal Obs< rvatorj at my de- 

 sire and calculated by myselt, and sent in a letter by Dr. Bevis to Paris. — Ne\ il Maskelyne. 



