33S Mr, Smeaton’s Observation of Mercury 
the didance run from the firil to the lad fuppofed place of the 
wire, we need only fay, as L.b= 30 : db — 5 1.6 :: lv — 1 i 7/ . 8 : /z 
r=20 // .3 ; and this will be its value when the declination Lc* 
fig, 2. is 24 / ; but then the declination L/ or L c ~, fig. 1. 
being greater than the perpendicular fide 1^0 (alTumed 30 7 ) in the 
proportion of L% : Lo, fay, by fimilarity of triangles converfely, 
as dL = 4i.7 : db~ 51.6 :: ho — 3c 7 : Lz — f&'.i ; but as the 
corredion before dated of 2o 7/ -3 is an angular error, taking place 
in proportion to the didance from the center, or the declination ; 
for the declination given of 2 f fay, as 38.2 : if :: 2o 7/ .3 : 13; 
to which adding 4 7/ . 4, we (hall have for the whole error 
in right afcenfion, fuppofing it in the equator, but mud be 
again increafed in the proportion in which a dar having decli- 
nation is dower than a dar in the equator ; that is, it mud be 
increafed in the proportion of any of the numbers in the 
fourth column of Tab. II. to the fimilar ones in col. 6. of 
the fame table; that is, as T 4b 77 : T 47" or as 106 : 107, 
:: 1 7 7/ .4 : 1 7 " 7 6 *. 
As all thefe errors, arifing from difference of refradion, are 
in proportion of the didance of theobjed from the center of the 
telefcope, they will take place in proportion to the difference of 
declination of the two objeds to be compared, whether they have 
paded the field on the fame, or on different fides of the center. 
Now the difference of declination of Mercury and u Orion is- 
being only 'if 2 " •> and the quantities being made out for 24 / 
fay (rejeding the 2 feconds), as 24 7 : 23 7 :: 17A4 : 
which turned into time in the run of the dar will be i 7/ .i 
in right afcenfion. 
* My friend Dr. Maskelyne obferves, that in JtriHneJs each liar ought to 
have its own proper reduftion, on account of difference of declination, which in 
mtrem cafes will amount to a fenlible quantity, 
3 
Say 
