with an Equatorial Micrometer. 
Say again, as 24' : 23' :: 6A2 : 6 7/ , the correction in de- 
clination. From the near equality of the lines L / and Ls, it 
is evident, that no correction of declination is neceflary on ac- 
count of the inclination of the wires, the whole difference fallino* 
£> 
in right afcenAon. As therefore Mercury pafled with z j z" 
more north declination than a. Orionis, and pafled through a 
part of the medium that lifted him up lefs ; it therefore gave 
him lefs north declination than it did to a, and therefore appa- 
rently diminifhed the real difference ; hence muft be added 
to the apparent difference 23' z'\ making it 23' 8" difference 
of declination : and as Mercury was lifted up lefs than a> 
he would not fo foon come to the middle wire by i 7/ .i as he 
fhould have done, he therefore came too late by 1". 1, which 
mufl: be fubtra&ed from the time of Mercury’s paflage the 2d 
of Sept, which will increafe the time in which he preceded 
u Orionis; that is, 18 h. 43' 42 /7 .4 increafed by 1.1 will be- 
come 18 h. 43 7 4 3". 5 difference of right afeenfion. 
I have been the more particular in the inveftigation of this 
obfervation, firft of all to afeertain the degree of dependance 
that may be formed on an inftrument of the kind ; and, fe- 
condly, to infer fuch eafy and Ample rules, that other Amilar 
obfervations may be the more eaAly reduced. Being therefore 
latisfledof the liability of the inflrument j if we had concluded 
the obfervation with that of Mercury in the morning, and of 
0 Tauri in the evening of the 23d, then the refult from Tab. 
111. fliould have been 
/ // 
Mercury pafled the wires at •* *- “ 22 34*9 
And 0 Tauri pafled at - 9 3 6 2 7-3 
Difference of right afeenfion 
i5 J 3 52.4 
which 
