492 TRANSIT OF VENUS, 1874. NEW ZEALAND. BURNHAM. 



error again determined with the striding level, thus completing one 

 observation. 



Every observation of the Moon is accompanied by one of a star as near 

 the Moon as possible. The error of collimation and zero of azimuth are 

 deduced from the observations of the stars. The error of collimation is not 

 required to be known with great accuracy, but as this is an instrumental 

 quantity which cannot be subject to great changes, its determinations afford 

 a test of the general accuracy of the observations. The zero of azimuth will 

 of course be affected by any error that may exist in the local time obtained 

 by another observer with the Transit instrument and transferred to the 

 Altazimuth Clock ; but the adoption of the observed zero of azimuth on each 

 night practically makes the observations differential. 



To the mean of the clock times of transit over the five vertical wires, the 

 clock correction is to be applied ; it is taken from the Abstract of Tables IV. 

 and V. Then putting I for the error of level of the horizontal axis determined 

 by the striding spirit-level, the correction to the circle-reading for error of 

 level is 



+ 1. cot Z.D. 



The correction for error of collimation (c) is 



-f- c.cosec Z.D. 



By applying the zero of azimuth to the circle reading thus corrected, the 

 observed azimuth is obtained. The approximate apparent zenith distance is 

 then computed. 



The Greenwich Mean Solar Time corresponding to the local sidereal time 

 of each observation is then found on two assumptions of longitude, viz., 

 ll h . 29 m . s . and ll h . 30 m . s . East of Greenwich. The Apparent Eight 

 Ascension and North Polar Distance of the Moon's center are then inter- 

 polated with second differences from the hourly ephemeris in the Nautical 

 Almanac, and corrected for errors of the tables by the quantities given in 

 the Appendix. The semidiameter is also interpolated from the Nautical 

 Almanac. 



With the local sidereal time of observation and each of these sets of 

 elements, and with the latitude already found, the tabular azimuth of the 

 limb has been computed by the Normal-centric method, which is explained 

 and all the formulas given in the Introductions to recent volumes of the 

 Greenwich Observations. If the two tabular azimuths corresponding to the 

 one local time be nearly identical, the position of the Moon was not favour- 

 able, and the observation is of no value ; but if there be a difference con- 



