12 TRANSIT OF VENUS, 1874. HONOLULU. 



// 



1874, October 2, +0*042 with micrometer west. 



,, i5, +0-047 



November 20, +o'o85 ,, ,, 



,, +o - o52 ,, ,, 



The adopted correction for inequality of pivots is +0"'06 micrometer west. 

 The observed level-errors are given in Table I. The level-error adopted in 

 the reductions for a group of stars has always been the mean of the deter- 

 minations, by the same observer, accompanying the group. 



29. The Error of Gollimation of the transit instrument. There is no doubt 

 that, during the long outward voyage, the ring clamp, on which the rigidity 

 of the connection between the system of wires and the body of the instrument 

 depended, became slightly loose, and that this escaped detection until 

 November 6. Two or three times, during the month of October, parts of the 

 eye-end connected with the micrometer were removed under the impression 

 that they caused the uncertainty in the position of the optic axis. On 

 November 6 the ring clamp above mentioned was tightened, after which 

 there was no further trouble. 



30. At first the collimation-error was determined by observing a close 

 circumpolar star with reversed positions of the transit-axis. A few minutes 

 before the star came to the meridian, the observer removed the brass cover of 

 the micrometer-screw-head, set the center wire to some quarter of a revolution, 

 and allowed the star to transit the center wire. The micrometer was then 

 turned a quarter of a revolution or other known quantity, and another transit 

 of the center wire observed. Five or six such observations were taken before 

 the star came to the meridian ; the instrument was then reversed, and the 

 star observed in transit over the center wire with the micrometer successively 

 set to the same readings as before, but of course in reverse order. The 

 level-errors for these two sets of observations should differ by twice the 

 inequality of pivots ; but generally the level-error was determined before 

 and after, and applied as found. 



31. On 1874, October 5, a stout post with a T cross head was set up on 

 the north side of the transit-hut, to support a telescope of 30 inches focal 

 length with cross webs in the focus of the object glass. The post was 

 entirely protected from the direct rays of the sun by screens of rushes. The 

 observations of this collimator were generally made at night. The transit 

 instrument was always reversed twice because the collimator was not always 

 perfectly steady. After November 4 the meridian mark was more generally 



