502 PHILOSOPHICAL TRANSACTIONS. [aNNO IjOO. 



gular motions of the air, become so gieat, as to take away from us all hopes 

 of observing them to an equal degree of exactness. 



The particular star which Mr. M. proposes should be carefully observed, 

 with a view of discovering its annual parallax, if sensible, is Sirius, the bright- 

 est of all the stars in the firmament, and which is therefore probably the nearest 

 to us of them all. With us, this star passes the meridian, at the altitude of 

 22°, where the refractions are too irregular to admit of our discovering a very 

 minute quantity, by observation: but, at the island of St. Helena, Sirius passes 

 only half a degree south of the zenith ; and, on this account, he has for some 

 time considered that as the most proper place to make observations at, for 

 this purpose. 



Mr. M. mentions a particular argument, which had for 2 years induced him 

 to think it probable, that the annual parallax of Sirius is not so small, as to 

 elude the nice discernment of our modem astronomers ; and he thinks it af- 

 fords a sufficient presumption 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 examination which he had made 

 of the observations of the zenith distances of Sirius, taken at the Cape of Good 

 Hope, in the years 1751 and 1752, by the Abbe de la Caille. Every particular 

 observation of the same star was reduced to one epoch, that of the beginning of 

 the year 1750, by applying the equations of aberration, precession; and de- 

 viation to the observed places ; so that the places corrected ought all to agree 

 together, if the observations were perfectly exact, and the star was affected 

 with no sensible motion that was unaccounted for. Being satisfied with the 

 excellence of the observations, Mr. M. was tempted to examine 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, would have had the greatest effect. But he found very few stars, 

 the observations of which were sufficient in number, or taken at proper seasons 

 of the year, to give room for any inference at all. Fortunately however the 

 observations of the zenith distances of Sirius were more in number, and, what 

 is still of more consequence in this case, made in various, and some in opposite 

 seasons of the year : and, on looking them over, he was agreeably surprized to 

 find a very sensible difference in the observations made at different times, agree- 

 ing in direction with what a parallax ought to produce, the zenith distance of 

 Sirius in July coming out no less than 8" greater than in the opposite season 

 of the year, viz. December and January; the zenith distance also in March and 

 April being of an intermediate quantity, as it ought to be : for Sirius being 

 in conjunction with the sun in June and July, it is evident he must be then 

 farther from the earth than in December and January, when he is in opposition 



