234 PHILOSOPHICAL TRANSACTIONS. [aNNO 1787. 



the true altitude of the sun's centre ; and consequently the sun's true zenith dis- 

 tance 25° 22' 1 \". But the same was found from Dr. Bradley's observations, by my 

 tables of refractions and the sun's parallax, 28° O' 32'/.8. The difference 

 2° 38' 21''''.8, or 2° 38' 22", is the difference of latitude of the two observato- 

 ries, which added to 48" 50' 14^ the latitude of the Royal Observatory at Paris, 

 gives 51° 28' 36^ for the latitude of the Royal Observatory at Greenwich, or 

 only A" less than before stated from the Greenwich observations, the difference 

 lying the contrary way to that which the Abbe de la Caille carried the latitude 

 of Greenwich, by improperly applying his own table of refractions to the Green- 

 wich observations as well as to his own. 



The Abbe de la Caille having, in the sequel of his memoir, inferred the dif- 

 ference of latitude of Gottingen and the College of Mazarine, from 22 stars 

 observed by M. Mayer with a 6-feet mural quadrant of Bird's construction, 

 correspondent to the same observed by himself, I shall make some remarks on 

 this comparison, because it appears to afford a strong argument to show that the 

 Abbe de la Caille's refractions are too great ; and that Mayer's, which agree with 

 Dr. Bradley's, are just. The Abbe, after correcting the zenith distances of 22 

 stars observed at both places by his own table of refractions, finds the difference 

 of latitude of Gottingen and Paris, by a mean, to be 2° 40' 35''^.1 ; which 

 added to 48° 51' 29^.3, the latitude of the College of Mazarine, gives him the 

 latitude of M. Mayer's Observatory 51° 32' 4''.4. He adds, that some observa- 

 tions of the pole star sent to him by M. Mayer would give the latitude of Got- 

 tingen 19" less than he has established it, as just mentioned. Now I find, that 

 if M. Mayer's observations of the pole star, as well as of the stars to the south 

 of the zenith, be corrected by M. Mayer's table of refractions, and the Abbe 

 de la Caille's observations by his table of refractions, the latitude resulting from 

 M. Mayer's observations of the pole star will agree to 2" with that resulting 

 from the difference of latitude by the stars to the south; for subtracting 19'^ 

 from 51° 32' 4'^.4, the latitude which the Abbe de la Caille has assigned to Got- 

 tingen in the manner above-mentioned, there remains 51° 31' 45'''.4 the latitude 

 which he found by the pole star ; to which adding 52^8, the refraction at the 

 mean height of the pole star according to the Abbe de la Caille, the sum 

 51° 32' 38''.2, must be the apparent height of the pole by M. Mayer's observa- 

 tions ; which diminished by 45'''.6, M. Mayer's refraction, gives the true lati- 

 tude of M. Mayer's Observatory 51° 31' 52'^6. But by the difference of the 

 Abbe de la Caille's and M. Mayer's zenith distances of the 22 stars to the south, 

 corrected each by their own table of refractions, I find the difference of latitude 

 2°40'32''.0; 28'^9; 25''.!; 27^9; 21".Q; 28'''.3; 32'''.0; 26'/.7; 27''^2; 24^.2; 

 25'^4; 31''.3; 23'^2; 23.6; 28''.9; 18'U; l6''.7; 22''.0; 23'^8; 21^''.l; 27^''.3; 

 27^3; the mean of which is 2° 40' 25^^6; which added to 48° 51' 29/''.3, the 



