ADJUSTING HADLET's QUADRANT. 157 



dex-error for the back-obfervation ; fuppofing the fore- 

 horizon-glafs to have been previoufly well adjufted. 

 This correction will be additive to all angles meafured bv 

 the back-obfervation, when the angle pointed out by the 

 index in the firfl of the above obfervations is greater 

 than the fupplement of the other, and vice verfa. 



For the fake of greater accuracy, you may repeat thefe 

 obferv'ations till you have taken two, four, or fix fets ; 

 obfersdng that if in your firfl fet you begin with the 

 fore-obfervation, as above direded, then in your fecond 

 fet you muft begin with the back-obfervation, and fo on. 

 A mean of the corrediions thus obtained may be taken 

 as the /r«^ coiTcdlion of adjuftment. 



I fliall conclude this paper with the following mifcel- 

 laneous remarks, relative to the i'ubjedt. 



1. If the arch of excefs beyond 90" be but fmall, as in 

 fome oftants is the cafe, then it may be neceffary to place 

 the index one or two degrees before the o, on the extra 

 arch, and adjuft the fore-hoi'izon-glafs to this polition ot 

 the index. You will thus obtain a greater range for the 

 index in adjufting the back-horizon-glafs. 



2. When the refle(3:ing glafs-plane is placed in the 

 pofition above mentioned, viz. at right angles to the plane 

 of the meridian paiTing through the fun, then the fun's 

 path in the heaA'ens will, for leveral minutes, be very 

 nearly parallel to the Iliid plane ; and theretorc no fen- 

 fible eiTor is to be apprehended from the increafe or 

 decreafe of the fun's altitude above this plane during 

 the interval between the firll and fecond obfervations in 

 the fame fet. But even if this fhculd be the cafe, from 

 the glals plane being placed confiderably out of the above 

 pofition, yet, by condudting the obfervations as above 

 diredled, the fmall errors arihng from this fource will na- 

 turally correal one another. 



3. When the polifh is taken oft' one furface of a piece 

 of glafs, then the image of the fun, being refledled only 



X from 



