On Land Surveys in the United States. 135 



some conceit of his own, or to suit his peculiar taste and sense of 

 propriety, but their improvements as tbey are termed, are gener- 

 ally nothing more than alterations ; and the reason is plain, for those 

 who make them have commonly little theoretical and no practical 

 knowledge of the subject. As instances of the imperfections alluded 

 to, the following may be mentioned. The radius of graduation is 

 frequently too small, thereby diminishing the effect of the force by 

 which the needle is brought to and retained in the direction of the 

 magnetic meridian, and likewise rendering the divisions upon the cir- 

 cle smaller than is proper for the correct reading and determination 

 of the fractional parts. The needles are not always finely pointed 

 and their extremes brought close enough to the edge of the gradua- 

 ted circle. 



Their shape instead of being the best, viz. that of exposing the 

 greatest surface, is often the reverse. They are likewise often inac- 

 curately poised, the point of support not being sufficiently near the 

 center of gravity of the needle, and in a line with its extremes. The 

 bearing point at the center of the needle is frequendy not formed of 

 the best material as it regards resistance from friction, and the instru- 

 ment is but seldom provided with verniers for obtaining fractions of 

 of degrees, or with spirit levels for regulating the perpendicularity 

 of the sights, circumstances of great importance as it respects the 

 accuracy of an observation. In the use likewise of this instrument 

 there are many sources of error which are not always understood 

 and of course not guarded against. The needle, from disuse or other 

 causes, not unfrequently loses much of its power, and becomes less 

 susceptible of the magnetic influence, from the resistance of a free 

 and easy motion occasioned by the accumulation of rust or other ob- 

 struction at its point of support. Precautions are not always taken 

 to ascertain whether the needle, in making an observation, is influen- 

 ced by local attractions of a magnetic character, and whether it is 

 not likewise influenced by causes confined to the instrument itself. 

 I allude now to the effect of the electrical fluid which sometimes col- 

 lects on the glass of the instrument, a circumstance which probably 

 more than any other, operates in diverting the needle from its proper 



direction. 



In addition to the preceding may be mentioned the want of skill 

 in the mode of effecting the adjustments, and the reliance which is 

 too often placed upon the trunks and tops of trees, he. as objects of 

 sight in the tracing of lines, together with the errors of the chain, 



