VOL. LXXVII.] PHILOSOPHICAL TRANSACTIONS. 233 



Dr. Bradley himself remarked) though his refractions made use of were so very 

 different. 



Having now shown that the Abbe de la Caille's refractions are too great, and 

 only fit to be applied to his own instrument, it will be easy, by a just calculation, 

 to reconcile the before-mentioned zenith distances of 14 stars observed at Green- 

 wich by Dr. Bradley and by the Abbe de la Caille at the College of Mazarine at 

 Paris, with the established latitudes of the two observatories, nearly ; in doing 

 which I shall claim the same right to correct Dr. Bradley's observations by his 

 table of refractions, as I have allowed the Abbe de la Caille to be entitled to cor- 

 rect his observations by his table of refractions ; which I think will be allowed 

 me, after what I have said of the manner in which the Greenwich refractions 

 were deduced and the instruments used. The difference of latitude of the 

 College of Mazarine, and the Royal Observatory at Greenwich will then come 

 out by the several stars, as follows : 1° 37' 12", 1 ; l6".0 ; 13^'.8 ; \3".7 ; IS'U ; 

 17^7; 19".7; 23'^2; 17''.7 ; 17".0\ \3".g\ 12" A; 5".6; iV'.g. The mean 

 is 2° 37' 15'^2 (or 8". 7 less than the Abbe de la Caille's result in his method of 

 calculation, which I have shown to be inadmissible) and added to 48° 5l' 2Q".3i 

 the latitude of the Abbe de la Caille's Observatory, give^ 51° 28' 44''''.5 for the 

 latitude of the Royal Observatory at Greenwich, only 4-^" more than established 

 by Dr. Bradley's observations and my own ; a sufficient agreement, especially 

 considering that many of the stars were at great distances from the zenith, and 

 that no account has been made of the temperature of the air at the times of the 

 observations. The proper method however, of settling the difference of lati- 

 titude of two observatories, is by stars near the zenith, as I observed before ; 

 and the difference of the latitudes of the two observatories of the College of 

 Mazarine and Greenwich, by the Abbe de la Caille's observations of j3 and y 

 Draconis compared with mine, was 2° 37' 10'''.4, and compared with Dr. Brad- 

 ley's 2° 37' 10'''.7 ; the first of which added to the Abbe de la Caille's latitude, 

 gives 51° 28' 39'^7, and the other 51° 28' 40'', for the latitude of the Royal 

 Observatory at Greenwich, exactly agreeing with that deduced immediately from 

 the observations made at this place. 



The same result nearly follows from M. Cassini de Thury's own observations of 

 the zenith distance of the sun at the summer solstice of 1755, contained in the 

 Memoires of the Royal Academy of Sciences for that year, compared with Dr. 

 Bradley's, which latter was communicated to me by the late John Howe, Esq. ; 

 for, by M. Cassini's observations, the solstitial altitude of the sun's upper limb 

 corrected by the difference of refraction and parallax, according to Dominico 

 Cassini's table, which happens to agree with the same difference by my tables at 

 this height, was 64° 53' 36" ; from which 15' 47'' being subtracted for the semi- 

 diameter of the sun according to Mayer's tables, there remains 64° 37' 49", 



VOL. XVI. H H 



