ADJUSTING HADLEY's qUADRANT. 16 i 



produdt by the diftance of the objed in yards, the quo- 

 tient will be the parallax of the inftrument, or cerrec- 

 tion of adjuftment, for that diftance in minutes. For 

 example, if the height of the index-fpeculum above the 

 line of vifion be three inches, and the diftance of the ob- 



jedt 150 yards: then ^ — ^ — r.9 will be the error of 



adjuftment. If therefore you place the index fo much 

 behind the o, on the limb of the inftrument and then 

 adjuft the horizon-glafs by an objedl at the above dif- 

 tance, the adjuftment will be true for an object con- 

 lidered as at an infinite diftance. 



From the above rule it is obvious that much exa£tnefs 

 in meafuring or eftimating the diftance of the objedt you 

 adjuft by is not necefTary, provided that diftance be not 

 very fmall ; for, in the above example, an error in the 

 diftance even of i o yards would have produced an error 

 of no more than about ^ of a minute in computing the 

 parallax of the inftrument. 



8. In meafuring angles by this inftrument, when the 

 objedl feen by diredt vifion is at no great diftance, the 

 parallax of the inftrument muft be taken into confidera- 

 tion. In fuch cafes it is commonly recommended, pre- 

 vioufly to adjuft the horizon-glafs by that objed: ; but 

 &his, in the back-obfervation, would be attended with 

 very great difficulty — it will therefore be beft always to 

 keep the back-horizon-glafs at the fame adjuftment, and 

 make the neceflary corredion, as above direded, for 

 the diftance of the objed. 



I am. Sir, with much efteera 

 Yours, &c. 



ROBERT PATTERSON. 



yi» 



