C *ri 1 



diftance, found as above in the firft obfervation be- 

 tween the firft object and any other object feen by 

 reflection, will give the true angular diftance between 

 them reduced to the centre of the index-glafs. 



But if you fhould happen to be in a place where 

 you cannot command a convenient diftant object, the 

 following method may be ufed. 



The back-horizon-glafs being adjufted, find the 

 inftrumental angular diftance between' the objects; 

 this is to be corrected by means of the following 

 operations. Set up a mark at any convenient 

 diftance oppofite or nearly fo to the object which 

 has been taken as the direct object ; and looking at 

 the direct object move the index of the quadrant, 

 and bring the image of the mark to coincide with 

 the direct object, and read off the degrees and 

 minutes (landing on the arch of the quadrant., which 

 fubtract from 180 degrees, if (o) of the index falls 

 upon the quadrantal arch ; but add to 180 degrees, if 

 it falls upon the arch of excefs ; and you will have the 

 inftrumental angular diftance of the object and mark. 

 Invert the plane of the quadrant, taking care at the 

 fame time not to change the place of its centre, and 

 looking at the fame direct object as before, move the 

 index of the quadrant, and bring the image of the 

 mark to coincide again with the direct object, and 

 read off the degrees and minutes ftanding on the arch, 

 and thence alio find the inftrumental angular diftance 

 of the object and mark. Take the fum of this and the 

 former inftrumental angular diftance; half of its differ- 

 ence from 360 will be the correction, which added 

 to the inftrumental angular diftance firft found be- 

 tween the fame direct object and the other object feen 



Vol. LXII. R by 



