336 Mr, Smeaton’s Obfervation of Mercury 
Suppofe 1 74 
db — 299 
d L =242 
and afifuming the fide L£ = 30 
the other Tides by proportion r db=$j.6 
as above will be i^ 7 L = 4i.y 
The triangles Lbc and dee are fimilar to dbL ; therefore fay,, 
as db — 5 1.6 : dL — 41.7 :: Lb — 30 : Lc = 2 4 ; and as L b~ 
30 : Lit — 24 :: ^=41.7 : = 33.5 ; and again, as db — 
51.6 : ^ — 33 5 :: L£ = 30 : r£ = 19.5 : but this will affeeff the 
declination, only in proportion of the line ef drawn parallel 
to LX ; and it will affect the right afcenfion according to the 
lineyt: but the triangle ecf being fimilar to the original one 
dbL , we fhall have db — 5 1 .6 : Lb — 30 ce = 19.5 :fc=: 1 1.3 
for the line affecting the right afcenfion; and alfo, as db — 
51.6 : dL =■ 41.7 :: ce— 19.5 : ef— 15.8 for the line affecting 
the declination. But the effect of difference of refraction upon 
the line Lb -=30' being only ii' 7 .8, the refpective effects of 
the lines fc and ef will be in proportion ; that is, 
/ / // // 
as 30 : 1 1.3 :: 1 18 : 4.4 for the effect in right afcenfion, 
and a3 30 : 15.8 :: 11.8 : 6.2 — declination ; 
but as it has been determined, that when the line L b is 30 
minutes, the line LC, or the correfponding declination, will be 
only 24 minutes ; the effets of refration above Bated will be 
therefore due to 24b 
Correction for the poftion of the wires. 
The above corretions take place on fuppofition that the 
feveral wires of the micrometer were ffritly parallel to the 
refpetive parts of the circles of declination, and horary circles 
in the heavens ; but in the pra&ical ufe of this inffrument it 
is 
1 
