356 SURVEY OF THE COAST 



In making an observation, the needle box revolves by the 

 rims at n upon the plate I, I, and is clamped, and the telescope 

 pointed, by the clamping and tangent screw at p. If the sun 

 or a star is observed, the transits of the three wires are ob- 

 served as in any other azimuth, and the time accurately noted, 

 and determined by other observations. 



When a magnetic azimuth lias been observed, and read 

 off at both ends of the needle, with the telescope on one 

 side of the box, suppose to the right hand, then the needle 

 must be turned a semicircle in the azimuth, and the telescope 

 as much in the vertical, and the observation repeated again, 

 exactly as in the observations with the two feet theodolite. 



The indiscriminate mean of these two observations, with 

 their four readings, must be taken for the result, as it will 

 be corrected for the eccentricity of the needle, and the eccen- 

 tric position of the telescope. 



A more minute description of the operations will not be 

 necessary. The inversion just mentioned ought never to 

 be neglected in any use of the magnetic needle whatever, as 

 no reliance can be placed in the results without it. 



It may be proper to observe that it is necessary to pay 

 great attention to obtain what is called free brass for the con- 

 struction of all instruments in which the magnetic needle is 

 used. All castings from brass fiUngs or borings contain more 

 or less iron, which will act upon the needle. 



With a view to have the needle as little affected as possible, 

 I requested that the body of the azimuth compasses under 

 consideration should be made of pure copper. But such was 

 not to be obtained ; as in England the copper in commerce 

 is made by a precipitation from a copper solution by means 

 of iron. By this process, it is always mixed with iron, and 

 therefore rendered unfit for the use intended. I was there- 

 fore lead to the use of free brass, as just stated. 



