TRANSIT INSTRUMENT AND CLOCK. 355 



The system of .wires remained perfect during the series of observa- 

 tions. 



The Equatorial intervals were found, from 400 transits of time stars, as 

 follows : 



Wire I. (nearest to the screw-head) + 429*47 



,, II. ,, ,, + 2i3'6i 



,, HI. ,, ,, o-oo 



,, IV. ,, ,, - 212-83 



, , V. ,, ,, - 424-70 



The mean of the observed times of transit over the five wires is, therefore, 

 reduced to the center wire by applying the correction i5 gin N p D positive 



with micrometer W. The quantity used by Lieut. Neate was i5gi ' n] ^p D , and, 



as the instrument was habitually kept with the micrometer west, in 

 strictness the times of true transit of all the clock stars require to be 

 increased s '04. This minute correction, -however, has no importance, except 

 when transferred to the Altazimuth-Clock, in the reduction of the observed 

 vertical transits of the Moon. 



Collimation. The reading of the Micrometer for coincidence of the center 

 wire with the optic axis was found by observing temporary and fixed 

 collimators, or close circumpolar stars, with reversed positions of the 

 transit-axis (Tab. I.). 



The Error of Level was found with the hanging spirit-level, two or more 

 times each night (Tab. II.). The value of the graduations engraved on the 

 glass bubble was re-determined by the makers before the Expedition left 

 England. Thirty-nine divisions were equivalent to one minute of arc, which 

 value has been used throughout. 



The relative size of the pivots was examined by observing the level-error 

 50 times, the instrument being reversed between each two sets of five deter- 

 minations. No appreciable difference could be detected. 



For the Azimuth Error (Tab. III.) one or more of the stars in the following 

 list were observed on the center wire, the micrometer-screw being turned as 

 requisite to enable the star to transit the wire. 



z z 



