xi 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 
TrouGuton ; 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. 
Ascale 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. 
113 
113 
113 
113 
113 
113 
113 
113 
113 
112 
112 
SPDT sss 5 
The mean value of 1 Scale division is 076715. 
