with an Equatorial Micrometer. 341 
dent, the lines, fig. 2. L b, L c, ce, ef, being in continued 
proportion L£ will be to ef in triplicate proportion of L b to 
L c ; and that Lc will be to ef in duplicate proportion of 
L b : L c. The difference of declination, therefore, due to 
30' difference of elevation will be as L b to L c limply ; but the 
efteCt of difference of refraCtion in declination will be lefs than 
the difference of declination in the proportion of : Lc z ; 
and that the effeCt of difference of refraction in right afcenfion 
will be lefs than the difference of refraction in declination in 
the proportion of L b : cb limply. 
Now it has been remarked, that the elevation of the tele- 
fcope’s center above the horizon, and the horary angle VLP, 
will always be readily given near enough for the purpofe by 
the globe. A triangle given L bd can therefore be cfonltruCted, 
and the fide \J> being made 30' (or any convenient aliquot 
part of a degree) the other fides will be found by propor- 
tion : fay then, as in the prefent cafe, db= 5 1.6 : dL, = 4.1.7 :: 
1^ = 30 : Lr=24, for the difference of declination correfpond- 
ing to half a degree of altitude: fay then, as 51.6“ 41.7% 
that is, as 2663 : 1739 :: 24 : 15.7 = ^ But without trou- 
bling ourfelves with high numbers, if we take the proportion 
51.6 to 41.7 by the Hide-rule twice, we fhall arrive at 15.7, 
near enough for the value of the line ef : fay then, as L b~ 
30 : ef — 15.7 :: 1 T'.8 : 6". 2 for the refraCtion in' declination : 
and as ^ = 41.7 : U— 30 :: 6A2 : 4". 4 for the refraction in 
right alcenlion, according to the true pofition ot the wires : 
and, for the correction of right afcenfion in the pofition of 
the wires, lay, 
Fig. 2. Fig 1. 
t — A ^ / ' — ^ 
As L£z=30 : db ~ 5 1.6 :: h = 1 1 ".8 : lz— 20 " ; 
and again, dJL = 4 i-J ; <#1=51.6 :: L<?~30' ; Lz=?$' .2. 
Take 
