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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 adjustment 

 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- 

 juiied parallel to the plane of the quadrant. In 

 order to find the angular interval between the wire?, 

 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 direct 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 

 direct horizon appears to coincide with the upper wire, 

 and the reflected back-horizon with the lower wire ; 

 the number of degrees and minutes fhewn 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 adjufting 

 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 

 adjustment be made, becaufe the fame deviation of 

 the axis of the telefcope will caufe a greater error. 

 Vol. LXII. P The 



