'[ 104- ] 



"limbs of the Sun and Moon do not come to touch 

 one another at the wire farther"! from the plane of the 

 quadrant, the object end of the telefcope is inclined 

 towards the plane of the quadrant, and mull be 

 altered by the adjuftment accordingly. Let thefe 

 operations be repeated until the obfervation is the 

 'fame at both the parallel wires, and the axis of the 

 telefcope will be adjufled parallel to the plane of the 

 ^quadrant. In like manner, the axis of the telefcope 

 ■may be alfo adjufled parallel to the plane of the 

 .quadrant for the fore-obfervation. 



Second method. Set the index to (o) and hold the 

 ■plane of the quadrant parallel to the horizon of the 

 tea, w-ith the divided arch upwards, the two wires 

 being parallel to, and including both the direct fore- 

 horizon, and the reflected back-horizon, between 

 them. Raife or lower the plane of the quadrant 

 until the direct and reflected horizons coincide to- 

 gether : if the coincidence happens in the middle 

 between the two wires, or rather, to be more exact, 

 above the middle by fuch a part of the field of view 

 as anfwers to the number of minutes in the depreffion 

 of the horizon (which may be eafily eftimated if the 

 angular interval of the wires be firit found by experi- 

 ment, in manner hereafter mentioned) the axis of the 

 telefcope is parallel to the plane of the quadrant 5 

 but if it does not, the line of fight is inclined to the 

 plane of the quadrant, and mud be corrected as follows. 

 If the direct and reflected horizons, when they coin- 

 cide, appear higher above the middle between the 

 wires, than what the quantity of the depreffion of the 

 horizon amounts to, the object end of the telefcope is 

 inclined from the plane of the quadrant, and. muft 

 be altered by the adjuftment made, for that purpofe ; 



but 



I 



