[ 5 66 ] 
add together Into one Turn the logarithm-tangent of 
double the firft arc, the co-fecant of double the fe- 
cond arc, and the conftant logarithm 2.0569 j the 
lum abating 20 from the index is the logarithm of 
the number of feconds required ; by which the dift- 
ance of the ftars, or of the Moon and liars, is con- 
trailed by refraction : which therefore, added to the 
obferved diftance, gives the true diftance, cleared from 
refradtion. 
N. B. This rule may be made univerfal, fo as to 
ferve with equal exaCtnefs almofl down to the 
horizon, if the apparent zenith diftances be di- 
minilhed by three times the refraCtion belong- 
ing to them, found from any common table of 
refraCtion, and the computation be made with 
the zenith diftances thus corrected. But if the 
altitudes of the Moon and ftar be not lefs than 
io°, this correction will not be neceffary. It 
will not be proper to make the obfervations, if 
the altitudes of the ftar and Moon are either of 
of them lefs than 4 0 or 5 0 , on account of the 
variablenefs of refraCtion near the horizon. 
Rule II. 
To find how much the diftance of the Moon and a 
ftar is increafed or diminifhed, on account of the 
Moon’s parallax. 
Add together into one fum the logarithm-tangents 
of half the fum, and half the difference of the zenith 
diftances, and the cotangent of half the diftance of 
the Moon and ftar, all corrected for refraCtion j the 
film, abating 20 from the index, is the tangent of arc 
the 
