Xll 



Introduction. 



8. Between the Magnetometer and Telescope piers is the pier of the Theodolite, 

 used for determinations of the Absolute Declination. The Theodolite is by 

 Teoughton ; the circle is 15 inches in diameter, is divided to 5 minutes, and reads 

 with the verniers to 5 seconds. There are three Verniers ; by some accident, the 

 circle has probably been slightly flattened on one side, as there the verniers enter 

 rather too much upon the graduations. The error due to this, however, is small. 



The lines of collimation of the Theodolite and Reading Telescopes coincide 

 when the middle wire of the former is made to coincide with the vertical wire of the 

 latter. The circle is retained in the same position on its pier, but the telescope is 

 removed, except when required for Observations of Absolute Declination. 



9. Following are the determinations of the data requisite in reducing the ob- 

 servations of the Declinometer. 



10. Value of the Scale Divisions in angular measure. 



A scale of 300 divisions was used in 1841, and of 500 divisions afterwards. The 

 Magnet having been fixed on blocks, the middle wire of the Theodolite Telescope 

 was made to coincide with various scale divisions. At each coincidence, one 

 vernier of the circle was read. 



Table 1. — ^Value of the Divisions of the Short Scale in the Declinometer during 1841. 



Scale 



Theodolite 



Scale 



Theodolite 



Value of 100 



Division. 



Reading. 



Division. 



Reading. 



Scale Divisions. 



50 



115 4 5 



150 



O 1 II 



113 56 55 



1 7 10 



60 



114 57 20 



160 



113 50 10 



1 7 10 



70 



114 50 35 



170 



113 43 30 



1 7 5 



80 



114 43 55 



180 



113 36 45 



1 7 10 



90 



114 37 10 



190 



113 30 



1 7 10 



100 



114 30 30 



200 



113 23 20 



1 7 10 



110 



114 23 45 



210 



113 16 35 



1 7 10 



120 



114 17 5 



220 



113 9 55 



1 7 10 



130 



114 10 20 



230 



113 3 10 



1 7 10 



140 



114 3 40 



240 



112 56 30 



1 7 10 



150 



113 56 55 



250 



112 49 50 



1 7 5 



The mean value of 1 Scale division is 0'-6715. 



