DECLINOMETER. XVll 
The middle wire of the theodolite telescope is brought to coincide with the ver- 
tical wire of the fixed reading telescope (7.); the three verniers of the horizontal 
circle are then read; the theodolite telescope is turned (on the vertical axis of the 
circle) until its middle wire coincides with the vertical line on the north meridian 
mark of Sir THomMAS BRISBANE’s (the western) transit in the Astronomical Obser- 
vatory, and the verniers are again read. In order to obtain the reading of the 
horizontal circle for the astronomical meridian, the theodolite telescope was placed 
as nearly as possible in the meridian, and being accurately levelled, the time of the 
sun’s transit was observed by the Magnetic Observatory clock. The sun’s meridian 
passage was also observed by Sir THomas BrisBaNeE with his western transit in the 
Astronomical Observatory, and the clocks in the two observatories being imme- 
diately compared, the true time of transit by the clock in the Magnetic Observatory 
was obtained. The difference, if any, between the true and observed times, was 
due to error of azimuth ; the latter, being very small, was obtained from the former 
in multiplying by the factor, 
cosine sun’s declination 
cosine sun’s altitude 
16. If A’ be the difference of the horizontal circle readings for the wire of the 
fixed telescope and for the north mark, Z be the azimuth of the north mark, and 
D be the angle at any instant contained by the line of collimation of the reading 
telescope and the adopted zero scale reading, the true westerly declination at that 
time will be 
180° — A’ + Z+D. 
The values of 180°—A’=A, obtained in 1844, 1845, and 1846, are given in the 
following Table :— 
MAG. AND MET. obs. 1845 anp 1846. e 
