354 TRANSIT OF VENUS, 1874. RODRIGUEZ. POINT VENUS. 



are the remains of an old fort, called on some cliarts Fort Duncan. This was 

 the site selected, foundations on solid rock being everywhere accessible. The 

 island, with the extensive coral formation surrounding it, was surveyed by 

 Commander W. H. L. Wharton, R.N. and the officers of H.M.S. Shearwater 

 in 1874 ; and on the Admiralty Chart of Rodriguez Island (Indian Ocean), 

 published 1876, February 16, the site is named Point Venus. 



From the Equatorial Stone the following true bearings were found : 



o / // 



Apex of Booby Island N. 74. 4. 38 AY. 



Apex of Diamond Island S. 76. 1 1. 27 W. 



Apex of Diamond Peak 8. 62. 25. 32 W. 



The Beacon on -the summit of Le Piton S. 3. 24. 41 E. 



Altazimuth Pier, 61 feet S. 77 E. 



Transit Pier, 84 feet N. 46 E. 



Photoheliograph Stone, 43 feet N. 46 E. 



The Transit Pier was 72 feet due North of the Altazimuth Pier. 



THE TRANSIT INSTRUMENT AND CLOCK. 



The Transit instrument and its mounting were in every respect exactly 

 similar to those used at Honolulu (see page 9). 



The great stone was laid upon a course of bricks, which rested upon the 

 levelled surface of the solid rock. The instrument was fairly in the Meridian 

 by August 31. In addition to the wooden observatory brought from Eng- 

 land, the instrument was protected by a high stone wall, enclosing the 

 wooden hut. 



The adopted value of one revolution of the transit-micrometer-screw, which 

 carried all five wires of the reticule, is 56" 30. It is the mean of a great 

 number of observations of close circumpolar stars at different times. The 

 integer revolutions of the screw were numbered in the observing books so 

 as to increase with motion of the wires towards the screw-head ; the center 

 wire coincided approximately with the optic axis when the reading was 

 20 r -4. 



The position of the transit-axis is always denoted by the record of the 

 micrometer-screw-head being on the east or west side of the telescope. 



