34° Afr. Smeaton’s Obfervation of Mercury 
which is the very fame as was before deduced from the 1 
mean of the whole : 
/ // 
And if to Mercury’s declination fouth of telefcope s center — i 8 
We add o Tauri’s north — 30 26 
We fnall have for the difference of declination — - 3 1 34 
the fame as before determined. Our obfervation would, there- 
fore, in this cafe limply have been, that Mercury preceded 
0 Tauri in right afeenfion 15b. if 5 2 ".4 mean time, and 
palled the wire with more fouth declination than 0 Tauri by 
0+ * 
After this, 0 Tauri would have required to be compared with 
fome Well rectified liar by meridian infhuments ; but in the 
prefent cafe « Orionis, one of Dr. Ma^kelyne’s Catalogue of 
34 principal liars happened to lie fufficiently near the fame pa- 
rallel of declination, to admit of 0 Tauri to be compared 
therewith by the fame inlfrument, while pointed to the fame 
place of the heavens. The operations which were fubfequent, 
therefore, mull be confidered as intended to fave thole of a 
meridian inllrument. 
Now had our obfervation concluded with the above, then 
the correction would have taken place upon the difference of 
declination of 0 Tauri with Mercury, inflead of the ultimate 
one with a Orionis; but it mull be obferved, that whatever 
quantity of correction the difference of declination would occa- 
lion, it would be compenfated in the difference of refraCtion of 
of 0 Tauri and « Orionis, when they came to be obferved on 
the meridian ; however, in the prefent cafe it happens to be 
more commodious, as both can be done under one. 
Preparatory then to the laying down the limple rule for the 
correction of refraction, it is proper to premife, that it is evi- 
dent, 
