TRANSIT INSTRUMENT. MERIDIAN MARK. 13 



used. The mark (or the cross of the collimator) was bisected five or six 

 times with the center wire. The observations of the meridian mark are 

 given in Table III. 



32. As the whole system of wires moved with the micrometer-screw, the 

 error of collimation is always the difference between the actual reading of 

 the micrometer and the reading which corresponds to zero, or when the 

 center wire coincides with the optic axis. If M be the former, Z the latter, 

 and R the value of one revolution, the collimation-error in arc is given with 

 its proper sign for micrometer east by .the formula 



+ (Z - M) R. 



The readings for Z as determined by observation are shown in Table II. 

 The adopted reading and M are given each day with the transits of stars. 



33. The Azimuth-Error of the transit instrument was found in the usual 

 way by combining observations of Polaris, 8 Ursae Minoris, Cephei (Hev.) 51, 

 or x Ursa? Minoris, with those of clock stars. Some difficulty was experienced 

 in setting up a meridian mark at a proper distance. To the south the sea- 

 shore was too near ; to the north there was no choice ; the crest of a range 

 of hills four miles distant, and very difficult of access, was the only available 

 place. On November 4 Mr. Nichol, accompanied by Mr. C. J. Lyons, of the 

 Government Survey, who was well acquainted with the country, with a party 

 of native workmen, ascended the mountain to the height of 1,540 feet, and 

 erected the mark exactly on the crest of the hill, so that it showed out in 

 bold relief against the sky at an altitude of 3 40'. The mark itself consisted 

 principally of two vertical posts of timber, 16 feet long and 4 inches by 3 in 

 section, planted with 4r| feet of their length in a stiff red clay, and shored 

 up with four stout stays. The clear space between the inner surfaces of 

 the posts was 9 '94 inches, and subtended 8" '05 as seen from the transit 

 instrument ; its center was afterwards found to be about 10 inches east of 

 true north, and to be 21,218 feet from the transit instrument. 



34. The mark was observed for azimuth-error, after sunset, and as early as 

 possible every morning, the center wire being placed by estimation centrally 

 between the two posts. These observations showed a nearly regular diurnal 

 movement of the optic-axis in azimuth of about 3", the telescope pointing 

 more east in the morning than in the evening. It was never possible to 

 observe the mark during the heat of the day. In order to determine the 

 absolute azimuth of the mark and the law of the azimuthal change of position 

 of the instrument, the mark was illuminated through a hole in the center, 



c 2 



