OF THE UNITED STATES. 2\)5 



general scientific knowledge, he is acqiiaint^fl wifli the man- 

 ner in which the instiuinent lias l)een constiiictcd. and the 

 peculiar abilities of the artist wlio constructed it. Judi>;ing 

 tVoni these circumstances the possible and proportional ac- 

 curacy of the execution of the instrument, he w II be al)le to 

 direct his inquiry sooner to a satisfactory result. 



In an instrument which is as perfect as possible, the ad- 

 justments are of course only accurate within certain limits, 

 and he has to guard against tiie errors which he n)ay be lia- 

 i)Ie to in consequcjice of them, as well as against those ot 

 the instrument itself. 



It l)ecomes therefore the duty of an accurate o!)servcr in 

 no case to rely merely upon the accuracy of his instrument 

 and his own skill. I)ut to adopt such a metiiod of ol)serving 

 as will counteract, as far as possil)le, the errors ol' the instru- 

 ment and those to which he himself is liable in making his 

 observations. 



Without such a method, and a regular system in his obser- 

 vations, his mean results will be under the influence of ha- 

 zard, and may even be rendered useless by adding an ob- 

 servation, which would repeat an error already included in 

 another observation. 



It is possible to correct angles measured by an incorrect 

 or ill adjusted instrument, by mathematical formidiv, when 

 the data for the reduction are exactly known ; l)ut such data 

 aic always difficult to ascertain with sufficient accuracy. 

 The reductions rccpiire longer calculations than the obser- 

 vations themselves, or at least are more tedious than a re- 

 petition of the obsei vations. In a work of great extent, these 

 reductions occur so fiequently, and the calculations of the 

 ol)servations art- at the same time so mntierous, as to render 

 any method, m wliich it would be necessary to retain them, 

 extremely lalxtriuus. On the other harnl, the ol)servations 

 mav always be repeated in a way in wbicli these corrections 

 will compensate each other. 



As to the instrument intended for the survey, which is the 



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