64 Proceeditigs of Philosophical Societies [Jan. 



of those heavenly bodies, it has as yet furnished no ohservations 

 formino- data for reasonino- on the distances of the fixed stars 

 from the sun. 



The other method, and that which has been most insisted 

 upon, seems hkewise to have originated with the illustrious 

 Florentine philosopher, namely, that of observing stars, about 

 the summer and v^'inter solstice, in or near the zenith, for the 

 purpose of avoiding the errors of refraction, by fixed instruments. 

 The celebrated Robert Ruoke, who erected, at Chelsea, a tele- 

 scope 36 feet long for examining y Draconis, imagined that he 

 had discovered a very considerable parallax for this star ; but 

 Hooke's observations were contradicted by Molyneux. 



Flamstead drew a similar conclusion from his experiments on 

 the pole star, but the results which he attributed to parallax 

 were explained by Bradley's great discoveries of the aberration 

 of light, and the nutation of the earth's axis ; and it is remark- 

 able that Hooke reasoned correctly on inaccurate observations, 

 whilst Flamstead formed wrong conclusions from exceedingly 

 correct results. 



James Cassini, in observing Sirius, attributed a parallax of 6'' 

 to this star ; and La Caille,from observatioiis made at the Cape 

 of Good Hope, supposed it to be 4''. 



Piazzi, whose conclusions are given with great diffidence, in 

 researches pursued from 1800 to 1806, supposes that several of 

 the fixed stars exhibit parallax. He assumes for Sirius nearly 

 the same parallax as La Caille, for Procyon 3'^, and for Capra 

 less than 1". in all these observations, nothing like southern 

 motion, it must be confessed, had ever been suspected. Dr. 

 Brinkley, in 1810, rated the parallax for a Lyrce at 2^''. The 

 general conclusions of the Astronomer Royal from observations 

 made both with a fixed instrument, and with the mural circle, 

 are unfavourable to the existence of sensible parallax for any of 

 the fixed stars, and he refers apparent parallax to imperfections 

 in the instruments used in the observations, and ofters as a proof, 

 the dindnutio)i of the indications in proportion as instruments 

 have become more delicate ; and estimating the Greenwich, as 

 superior to the Dublin Circle, thus accounts for the difference of 

 his results and those of Dr. Brinkley. 



Dr. Brinkley, in reply, does not allow the superiority of the 

 principle of the Greenwich instrument, and shows the consist- 

 ency of the Dublin instrument with itself, by numerous obser- 

 vations which place its permanent state beyond all doubt. The 

 results of 62 observations on « Lyra3, in 1811, give the mean 

 difference between the summer and winter zenith distances as 

 V'Z'Z ; and repeated observations, in the last ten years, give 

 sensible parallax, though with less consistency, for a Aquilse, 

 « Cygni, and Arcturus ; but none for y Draconis. Dr. Brinkley 

 seems entirely convinced of the accuracy of his general conclu- 



