5S2 WEBER OX A TRANSPORTABLE MAGNETOMETER. 



cross-section of the tube of the magnetometer telescope^ and 

 in B C the scale ; between A and B C is a transparent space ; 

 the theodoUte must be so placed that the observer may look with 

 the verification telescope through the space D towards the 

 mirror of the magnetometer needle, and perceive the image of 

 the scale attached to that telescope ; he must first observe the 

 position of the needle by this scale, and thence determine the 

 angle <f (fig. 12.), which the optical axis of the verification tele- 

 scope makes with the normal to the min'or of the magneto- 

 meter; he must then bisect objects of known azimuth with 

 the principal telescope of the theodolite, and thence find the 

 angle \I/ corresponding on the divided limb to the direction of 

 the principal telescope relatively to the north. 



These are all the obsei*vations required to be made in the 

 open air in determining the declination. The angle ;^, Fig. 12, 

 con-esponding, on the graduated limb, to the parallel position of 

 the optical axes of the two telescopes of the theodolite, can be 

 ascertained in a room ; as can also the angle § which the mag- 

 netic meridian makes Avith the normal of the mii'ror belonging 

 to the needle. Hence we obtain 



{X~i') the angle w hich the optical axis of the verification te- 

 lescope makes with the true meridian. 



ix~^) ~ *^' t^^ angle which the mirror-normal of the needle 

 makes with the true meridian. 



?~{(%~^)~'^} ^^^ angle which the magnetic meridian 

 makes with the true meridian. 



The angle ^ is found by placing a plane mirror before the 

 verification telescope, and viewing in the telescope the reflected 

 image of a vertical thread suspended over the middle of the 

 object glass; a vertical thread is also suspended over the 

 middle of the principal telescope, and the telescope adjusted to 

 its reflected image ; the reading on the circle gives the angle Xi 

 supposing the colhmation eiTor of the principal telescope to re- 

 main unaltered when the eye-piece is adjusted to distant ob- 

 jects ; otherwise the alteration must be sought by reversing the 

 telescope, and applied as a correction to the reading on the 

 circle. 



The angle g is determined by directing the pi'incipal tele- 

 scope of the theodolite from B (fig. 12.) to C, a second needle 

 suspended in the wooden case, as represented in fig. 3 ; the 



