VOL. LXXVII.] PHILOSOPHICAL TRANSACTIONS. 293 



Deduction of the position of Mercury from the preceding observations. 



1st. In right ascension. 



• Tauri followed A Ceti Sept. 23 24"" 59' 



26 24 59.2 



— 30 24 59.1 



Oct. ]3 24 58.7 



On a mean of the four. . 24 59 



» Orionis followed o Tauri Sept, 30 2" 29 4.9-9 



__ Oct. 13 2 29 50.1 



at a mean 2 29 50 



Now Mercury preceded A Geti Sept. 23 15 48 53.4 



A Ceti preceded o Tauri by mean of four 24 59 



e Tauri preceded a Orionis by mean of two 2 29 50 



Mercury therefore preceded « Orionis by 18 43 42.4 



2d. In declination. 



Sept. 23, A.M. Mercury passed the middle horary wire, south of its centre . ^ . . 1' 8'' 



Same evening o Tauri passed the middle horary wire, north of it 30 26 



Therefore Mercury passed the middle horary wire more s. than o Tauri by 31 34 



But Sept. 26, A Ceti passed n. of centre 17' 11" 1 -p.-rr , „ _ 



» Tauri 30 18/^^"-^'^ ^ 



. 30, A Ceti 17 11 



B Tauri 30 17 



Oct. 13, A Ceti 17 11 



Tauri 30 15 



} 13 6 



} 13 4 



From the smallness of the above differences we may infer, that very little un- 

 certainty in declination had attended the passage of o Tauri on Sept. 23. 



On Sept. 30, Tauri passed n 30' irjsum54' 37" 



X Ononis s. . . . » 24 2o J ' 



On Oct. 13, Tauri n 30 15 1 



X Orionis s 24 20 / ^* '^^ 



tt Orionis then at a mean passed more south than o Tauri 54' 35" 



Mercury therefore on the 23d passed with more N. declination than a Orionis .... 23 2 



The preceding deductions and remarks show the consistency of the observa- 

 tions with themselves; yet, from the position of the telescope, it being only ele- 

 vated 1 1 ^ above the horizon, it is necessary to examine how far the deductions 

 above specified were capable of being affected by refraction.' And in this respect 

 it will appear, that if it be supposed, there is no difference in the quantity of 

 refraction of such objects as appear within the limits of the field of view of this 

 instrument, which is 1° 17^ then their relative positions to each other will not be 

 affected by it. On a calculation Mr. S. finds that the two stars, so altered by 

 refraction, will arrive together at the horary circle at the same time, and with 

 the same difference of declination, as if no refraction had taken place. It is 

 therefore only the difference of refraction which takes place in objects at different 

 heights in the same field, that can alter their relative situations; however, it 



