26 8 ] 
A RULE 
To compute the contraction or augmentation of the 
apparent diftance of the Moon from a ftar, on ac- 
count of the Moon’s parallax ; the zenith diftances 
of the Moon and ftar, and their diftance from 
each other being given nearly. 
Add together the tangents of half the fum, and 
half the difference of the zenith diftances of the 
Moon and ftar, and the cotangent of half the dift- 
ance of the Moon from the ftar ; the fum, abating 
20 from the index, is the tangent of an arch, which 
call A. Then, if the zenith diftance of the Moon 
is greater than that of the ftar, take the Sum of the 
arch A, juft found, and half the diftance of the 
Moon from the ftar; but, if the zenith diftance of 
the Moon be lefs than that of the ftar, take the 
difference of the faid arch A and half the diftance 
of the Moon from the ftar ; and the fum, or dif- 
ference call B. To the tangent of B, thus found, 
add the cofine of the Moon’s zenith diftance, and 
the logarithm of the Moon’s horizontal parallax, ex- 
prelf'ed in minutes and decimals ; the fum, abating 
20 from the index, is the logarithm of the effedl of 
parallax, tending always to augment the apparent 
diftance of the Moon from the ftar ; except the ze- 
nith diftance of the Moon be lefs than that of the 
ftar, and, at the fame time, the arch A be greater 
than half the diftance of the Moon from the ftar, 
in which cafe the effedt of parallax diminifhes the ap- 
parent diftance of the Moon from the ftar. 
2 DEMON- 
