COMMENTATIO ASTRONOHICA. 201 



« slars, wliich Ihis astronomer observed , were such only as lay wilhin a few degrees 

 « of his zenith: and Ihough his observations do not seem to shew a sensible parallax in 

 « any of Ihem , yet we cannot thence absolutely conclude , Ihat, amongst the gi-eat 

 « number of visible stars, there are none in which it may be perceptible, tili they have 

 « all of them , especially ihose of the greatest luslre , been observed in proper places , 

 « near Ihe zenith, with the like care and accuracy, which he has used: for, as Dr. 

 «Bradley has himself remarked , where any stars are remote from the Zenilh, the 

 t tmeertainties of refraction, and the irregulär molions of the air, become so great, as 

 « lo take away from us all hopes of observing them to an equal degrce of exactness. " 



« The parlicular slar , which , I propose , should be carefully observed , wilh a view 

 (f of discovering its annual parallax , if sensible , is Sirius , the brightest of all the stars 

 « in the firmament, and which is therefore, probably, the nearest to us of them all. 

 « With US , this star passes Ihe meridian , at the altitude of 22° , where the refractions 

 « are too irregulär to admit of our discovering a very minule quantity , by Observation : 

 « but, at the island of St. Helena , Sirius passes only half a degree south of the zenilh; 

 « and, on this account, I have, for sorae time , looked upon that as the most proper 

 « place to make observations at, for this purpose. " 



« I shall now beg leave, great with deference, to lay before you a particular argu- 

 ft ment, which has, for these two years , induced me to think it probable that the an- 

 « nual parallax of Sirius is not so small, as to elude the nice discernment of our mo- 

 « dem astronomers ; and shall be glad to be determined by your opinion, wliether it 

 « affords a sufficient presumption to us , to undertake a careful and assiduous series of 

 « observations of the distances of Sirius , from the zenith of the island of St. Helena. 

 « This argument is drawn from an exaraination , which I have made of the observations 

 « of the zenith distances of Sirius taken at the Cape of Good Hope , in the years 1751 

 «and 1752, by that excellent astronomer, and diligent observer , the Ahb6 de la 

 « Caille, recitedin his Fund amenla Astronomiae, prinled about two years since, whe- 

 « rein also are contained all the observations, which he has taken of the principal 

 « fixed Stars; a work truly worthy of its name! Every particular Observation of the 

 « same star is reduced to one epoch , that of the beginning of the year 1750, by ap- 

 « plying the equations of aberration, precession, and devialion , to the observed places; 

 « so that the places corrected ought all to agree together , if the observations were per- 

 « fectly exact, and the star was affected with no sensible motion, that was unaccounted 

 n for. Bemg satisfied with the excellence of the observations , I was tempted to exami- 

 « ne those of some of the principal fixed stars , in hopes of discovering some sensible 

 « differences in the observations made at different times of the year, when a parallax, 

 « if there had been any , woul^ have had the greatest effect. But I found very few 



C c <( Stars , 



