Declinometer. xvii 



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 Thomas 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 Brisbane 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 

 cpgine 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 AND 1846. 



