486 TRANSIT OF VENUS, 1874. NEW ZEALAND. BURVHAM. 



of the micrometer-screw, taking the value of one revolution of this latter at 

 56"-40, as follows : 



October 28. November TO. December 4. 



Wire I + 427 '9 +428-1 + 428-0 



,, II + 2i5-g + 2i5'7 +210-6 



,, III + 0-9 + 0-8 + 0-8 



,, IV 210-4 215-4 2i5-o 



,, V 429-1 429-3 429-4 



The wires are numbered in the order in which a star above the pole will 

 cross them with the micrometer Bast. The reduction from the mean to the 



middle wire is therefore 0"'9 X 1 5 ' sin N.P.D.* which has been taken = 

 O06 for stars within 30 of the Equator. 



The value of one revolution of the micrometer-screw was determined on 

 25 nights from close circumpolar stars, the mean value being 56" - 40. 



The Gollimation of the center wire was found for the first few nights by 

 reversal on polar stars ; it was not steady. The meridian mark was then 

 used. Finding the readings irregular, the screws about the wire frame were 

 , carefully examined (on November 5), when two of them were found 1 to be a 

 little loose. This was rectified, and the collimation at the same time adjusted 

 nearly to zero (20 r '000). After this it was satisfactory. The collimation was 

 occasionally determined by means of a small reversed telescope mounted on a 

 pier about 12 feet on the north side of the instrument. It will be sufficient 

 to state here that, when observing stars for time, the reading of the micz-o- 

 meter-head was always kept at 20 r '000 throughout the whole series. [See 

 Table I.] 



The Error of Level of the transit axis was always determined with the 

 hanging spirit-level. The graduation of the glass bubble was re-examined 

 before the Expedition left England, when 30 divisions were found equal to 

 one minute of arc, which value has been used throughout. On October 28 

 the value of the level graduation was tested by means of the Bohnenberger 

 eye-piece and the known value of the micrometer-screw. One division was 

 found equal to 2"'005. The level-error was steady, and was kept small by 

 adjustment when necessary. It seldom exceeded 5". The usual course was 

 to apply the level at the beginning and end of each night's observations, 

 before and after polar stars, before and after the Moon's transit, and 

 generally at intervals of about an hour while observing. For the reduction 



