NO. 6.] INTRODUCTION. AZIMUTH. XVII 



may be inferred from the length of the level-bubble for the vertical circle of 

 the large altazimuth (List A of Observations) the length being about 20 for 

 and about 38 for 50 C. 



Determination of Azimuth. 



The astronomical foundation for the determination of magnetic declination 

 was furnished either by the altazimuth or by an azimuth-compass, or in 

 some cases by a magnetic theodolite. 



In the first case the telescope was first pointed to the magnetic observa- 

 tory (either a centered mark or the objective of the magnetic theodolite, illu- 

 minated from behind) and the horizontal circle read off on both microscopes ; 

 then to the Sun or a star (either one of the stars whose altitudes were meas- 

 ured for time and latitude, or, more frequently, a lower one) and the hori- 

 zontal circle and striding level read off after the noting of the time. Some- 

 times the observations were repeated in the other position of the instrument. 



If C r and Ci are the circle readings for a terrestrial mark in or near the 

 horizon, respectively with obj. right and obj. left, and the error of collimation 

 (c) is defined by the condition that the objective end of the optical axis forms 

 the angle 90 -}- c with the ocular end of the instrument's horizontal axis, then 



C = * (O r -C,) 



of course neglecting the difference of 180. 



If R and L are the readings of the right and left end of the striding 

 level, as seen by an observer facing the same way as the object glass, and if 

 further the inclination of the axis is defined as positive when the right end 

 is the higher, then 



p being the value of a division of the level. As remarked before, the sum 

 of the two differences R L, corresponding to the two positions of the level, 



will for this instrument give the inclination in seconds of arc. 



3 



