xxii Introduction to the Makerstotjn Observations, 1843. 
scope are noted 18 seconds before the minute of observation, at the minute, and 
18 seconds after the minute ; the scale readings at these periods being a, 6, and c, 
. -. i i , -. a + 2 b + c 
the mean is deduced by the formula ~ — . 
22. All the observations of declination in this volume are absolute. They are 
rendered so as follows : — 
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 immediately 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 
23. If A' be the difference of the horizontal circle readings for the fixed tele- 
scope 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 P -A' + Z±D. 
The values of 180° — A' = A, obtained in 1843, are given in the following 
Table :— 
