222 TRANSIT OF VENUS, 1874. KAILUA. 



Eeadings were now taken with the opposite edges of the images in contact. 

 Boiling of limbs very bad. 



Micrometer Eeadings. 



r 



26-872 

 26-993 

 26'ggo 

 26'g52 



G. FORBES. 



The Equatorial Telescope, its mounting and driving clock, which were 

 purchased by the Government from R. Hodgson, Esq., were almost the 

 counterpart of the " Bedford " Equatoreal, described by Admiral Smyth in 

 the Cycle of Celestial Objects. 



The object-glass, supposed to be by Cauchoix, was six inches in diameter, 

 but the effective aperture was 5'7 inches, and the focal length was 75 inches. 

 Professor Forbes describes its defining power as admirable. 



At Kailua the long polar axis was supported by a massive framework of 

 timber, which had been constructed at Greenwich, and adapted very nearly to 

 the requisite latitude. The instrument was used in the open air, and when 

 out of use was protected by a large tarpaulin. 



The double-image-micrometer and solar diagonal reflector were made by 

 Messrs. Troughton and Simms ; the former was kindly lent to the Astronomer 

 Royal by J. G. BARCLAY, Esq., of Leyton, Essex. It was accidentally injured 

 by a fall just before the transit of Venus ; the injury, however, appeared to 

 be confined to the outer portion of the screw which carried the milled head 

 (Plate III.). 



It is unfortunate that all Professor Forbes' micrometer readings for distance 

 of limbs were on one side of zero. He states also that the images could not 

 be made equally bright, by means of the adjusting screw, without rotating 

 the body of the micrometer; a circumstance which tends still farther to 

 diminish the value of the measures. 



The only clue to the zero reading of the micrometer, before it was rotated, 

 is the first set of three readings for the diameter of the planet, the mean of 

 which is 18 r '972. After rotation the next set of three gives 19 r- 029, 

 showing an apparent increase of the zero reading of '057. The zero obtained 



