40 REPORT ON 



was clamped, the time of transit of the sun's first limb over the vertical wire noted, 

 and the horizontal circle read. 5°. If the observation was made at a time of day 

 when the sun's azimuth was changing tolerably rapidly, the telescope was not 

 moved in azimuth at all, but, the reading of the horizontal circle remaining pre- 

 cisely as before, the sun was followed by moving the telescope in altitude, and the 

 transit of its second limb was waited for and noted. If, however, the sun was 

 changing its altitude much more rapidly than its azimuth then, in order to save 

 time, the horizontal circle was undamped, the telescope moved till its vertical wire 

 was just in advance of the sun's second limb, the horizontal circle clamped, the 

 time of transit of the sun's second limb over the vertical wire noted, and the hori- 

 zontal circle read. 6°. The telescope of the theodolite was reversed in its Y's. 

 7°. The transit of the sun's first limb over the vertical wire was observed, and the 

 horizontal circle read. 8°. The transit of the sun's second limb over the vertical 

 wire was observed, and the horizontal circle read. 9°. The colored glass was 

 removed from the eye-piece of the telescope, and a reading of the magnet scale 

 (which was still inverted) was taken. 10°. The magnet was revolved on its axis 

 through 180°, so as to place the scale erect, and another reading of the scale was 

 taken. 11°. The horizontal circle was read. 



Immediately before, and immediately after, going through with the operations 

 just described, the telescope should be pointed to some well-defined distant object, 

 and the reading of the horizontal circle noted. By so doing a check is afforded 

 against any accidental shift of the horizontal circle; and if the same station is 

 occupied at another time, absolute declinations may be determined without again 

 referring to the sun, thus rendering it possible to observe during cloudy weather. 



In the instruments under consideration the reading of the horizontal circle of 

 the theodolite increases from left to right; and in both the magnets, C. 32 and S. 8, 

 when the scale is erect an increase of scale reading indicates a motion of the north 

 end of the magnet towards the east. 



Let 

 p = reading of magnet, scale erect, 

 p' = reading of magnet, scale inverted. 

 _K'= reading of horizontal circle of theodolite at the time the readings p and p' 



were observed. 

 d = value, in minutes of arc, of one division of the magnet scale. 

 i2"= reading of horizontal circle of the theodolite at the time of transit of sun's 



first limb over the vertical wire. 

 B'"= reading of horizontal circle of the theodolite at the time of transit of sun's 



second limb over the vertical wire. 

 a = observed chronometer time of transit of sun's first limb over the vertical 



wire. 

 a = observed chronometer time of transit of sun's second limb over the vertical 



wire. 

 dt = correction of chronometer to reduce the reading of its face to local mean 



time. 

 <r = equation of time. 



