XX INTRODUCTION. 
vertical wire of the reading telescope ; the three verniers of the horizontal circle aré 
then read; the telescope is turned until its middle wire coincides with the vertical 
line on the north meridian mark of 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 is placed as nearly in the meridian as possible, 
and being accurately levelled, the time of the sun’s transit is observed by the mag- 
netic observatory clock ; the sun’s transit was also observed with the transit telescope 
in the astronomical observatory, and the clocks being compared, the true time of 
transit, by the magnetic observatory clock, is obtained. The difference, if any, be- 
tween the true and observed times of transit, is due to error of azimuth; the latter, 
being very small, is obtained from the former in multiplying by the factor 
cos. sun’s declin, 
cos. sun’s alt. 
21. If A be the difference of the horizontal circle readings for the fixed telescope 
and the north mark, Z the azimuth of the north mark, and D the angle contained 
by the line of collimation of the fixed telescope, and the magnetic axis of the decli- 
nation bar at any instant, D being equal to the observed reading, at that instant 
minus the scale reading at the magnetic axis of the bar in angular measure, the 
true declination will be 
180°—A + Z=+=D 
The following tables contain the observations for the value of Z and A :— 
