TRANSIT INSTRUMENT. 407 



The system of wires remained perfect during the series of observations, 

 and the equatorial intervals from the central wire, determined from 400 transits, 

 are : 



The order of the wires refers to the order in which stars above the pole 

 transit them with the micrometer west. 



The center wire has always been considered to represent the mean of 

 wires. 



The Zero of Collimation, or the reading of the micrometer when the center 

 wire coincided with the optic axis, was determined nearly every day by 

 observations of a distant mark with reversed positions of the transit axis. 

 The distant mark was some sharply-defined feature of the rock (different in 

 different observations) seen projected against the northern sky. 



A Collimator, consisting of a telescope temporarily mounted on a tripod 

 stand, was also frequently used. All these observations, given in Table I., 

 indicate that some of the screws at the eye end of the instrument were loose, 

 probably the ring clamp-screw.* The cause, however, was not discovered 

 during the course of observations ; and consequently, throughout the entire 

 series, the Zero of Collimation is subject to an uncertainty, corresponding to 

 about 3" of arc ; and it is probable that the relation between the center wire 

 and the optic axis was liable to be disturbed every time that the micrometer- 

 screw was touched. The adopted zero for each night is enclosed in a bracket, 

 thus [20-550]. 



The Level Error was found with the hanging spirit-level ; the value of the 

 graduations engraved on the glass bubble was tested by the makers before 

 the Expedition left England, when it was found that 41 divisions were 

 equivalent to one minute of arc, which value has been used throughout. 

 (Table II.) 



Determinations of the wiequality of the pivots were made in 1873, November, 

 1874, November, and 1875, January. On all three occasions it was found 



* The ring-clamps of the Honolulu and New Zealand instruments worked loose during the 

 outward voyages, and caused similar trouble at first. 



3 o 



