TRANSIT INSTRUMENT. 487 



of the transits a level-error was obtained for each, star, or group of stars, by 

 combining the several determinations. [See Table II.] 



The correction to the level-error for inequality of the size of the pivots 

 was found on 1874, October 28, November 24, and December 3, to be 

 0"-05, 0"-15, and 0"-10 respectively, the sign referring to " Micrometer 

 East." The value 0"'05 was used up to November 23, 0"-15 to 

 December 2, and 0"'10 on and after December 3. 



The Error of Azimuth was determined from observations of the 8 close 

 southern circumpolar stars in Mr. Stone's list, 1874, February 28 (see 

 Eodriguez Observations, page 356). No other stars were used. On six 

 occasions the error of azimuth was obtained from the Meridian Mark, 

 assuming this latter to be 4"'65 Bast of true South. The changes of the 

 azimuthal position, if at times somewhat rapid, were pretty regular in one 

 direction during the first three weeks of November, namely, in that direction 

 which would correspond with a southerly movement of the east pier. After 

 this there were two or three sudden changes, from time to time, connected 

 apparently with heavy rain-fall, but no further steady progressive movement 

 in one direction. Rain seemed to act immediately on the azimuth without 

 affecting the level. The azimuth error was reduced by the adjusting screws 

 on November 2, 18, and December 10 ; on October 27 it was disturbed by 

 touching the level adjustment. [See Table III.] 



The Meridian Mark was erected October 26, about 6054 feet south of the 

 Transit instrument. It consisted of a small brick pier, about 2 feet square 

 and 7 feet high, to the north face of which was cemented an iron plate 2 feet 

 6 inches long, and 1 foot 6 inches broad, with top and bottom flanges entering 

 5 or 6 inches into the brickwork. The face of the plate was painted black, 

 with a diagonal cross of white lines half an inch wide. At the exact center 

 of this cross a circular hole one-third of an inch in diameter was drilled 

 through the plate, and behind it a chamber was left in the pier sufficiently 

 large to hold a railway signal lamp, which was trimmed and lighted at dusk 

 every night and which would burn for six hours. The zenith distance of the 

 mark as measured by the transit instrument was 90. 15'. At night, when 

 illuminated, it appeared like a star of the fifth or sixth magnitude. It has 

 not been removed. The iron plate was shifted 3^ inches to the Bast 

 on October 28. 



The following are the determinations of the absolute azimuth of tilt- 

 meridian mark, obtained by comparing the observations of it when illumi- 



3 B 



