[ 10 5 ] 
but if the two horizons appear to coincide in a 
lower part of the field of the telefcope, the object end 
of the telefcope is inclined towards the plane of the 
quadrant, and muft be altered by the adjuftment 
accordingly. Repeat thefe operations till the two 
horizons appear to coincide above the middle between 
the two wires, by the quantity of the depreffion of 
the horizon, and the axis of the telefcope will be ad- 
jufted parallel to the plane of the quadrant. In 
order to find the angular interval between the wires, 
hold the quadrant perpendicular to the horizon, as 
in obferving altitudes j and turn about the eye-tube 
with the wires until they are parallel to, and include, 
the diredt fore-horizon and reflected back-horizon 
between them. Move the index from (o) along the 
divided arch, at the fame time railing or lowering the 
telefcope by the motion of the quadrant until the 
diredt horizon appears to coincide with the upper wire, 
and the reflected back-horizon with the lower wire j 
the number of degrees and minutes (hewn upon the 
arch, increafed by double the depreffion of the hori- 
zon, will be the angular interval of the wires j its 
proportion to the depreffion of the horizon will be 
therefore known ; and hence the fpace in the field of 
the telefcope anfwering to the depreffion of the hori- 
zon, may be eafily eftimated near enough for adjuftin^ 
the axis of the telefcope in the manner before-men- 
tioned. The firft of the two methods here given for 
adjufting the pofition of the telefcope will probably be 
found moft convenient j and the greater the diftance 
of the Sun and Moon is, the more nearly may the 
adjuftment be made, becaufe the fame deviation of 
the axis of the telefcope will caufe a greater error. 
Vol. LXII, P The 
