596 Royal Astronomical Society \ — Mr. Main on the 



of observations of the star in question at one station, and in one 

 position of the sector, into the least number for the same star at the 

 other station, the resulting mean amplitude from the stars north of 

 the zenith is 1° 13' 1"'173, and from the stars south of the zenith, 

 P 13'14"'961. 



By assigning to the amplitude found from each star a weight 

 equal to the quotient of the square of the number of observations 

 of the star by twice the sum of the squares of the errors at both 

 stations, the group to the north of the zenith gives 1° 13' 14"* 173, 

 and the group to the south 1° 13' 14"-953. The results of the 

 two methods of computation may be regarded as identical ; and the 

 stars north of the zenith give the amplitude less by 0"'78 than the 

 stars south of the zenith. 



It is not easy to assign the cause of this discrepancy. Mr. 

 Maclear inclines to ascribe it to the probable expansion of the tube 

 at Klyp Fonteyn from the high temperature. While the observa- 

 tions were made the thermometer was sometimes as high as 93°, 

 while at the Guardhouse the range was between 57° and 63°. 

 IjKc^iWe {Mem de I' Acad. 1752) states that his southern group of 

 stars gave the amplitude greater by 0"'8 than the northern. This 

 near coincidence with Mr. Maclear's result is remarkable. 



If the expansion of the tube be the cause of the discrepancy, the 

 mean between the north and south groups is as correct as if no ex- 

 pansion took place. This mean is 1° 13' 14"'56, with a probable 

 error not exceeding 0"'03. 



The axis of the sector on the corn-floor at Klyp Fonteyn was 

 216 feet (reduced to the meridian) south of the centre of the founda- 

 tion discovered on the 6th of April, and supposed to be Lacaille's 

 sector station. The axis of the sector in the Guardhouse was 45 

 feet on the meridian no7-th of Lacaille's sector station in Mrs. De 

 Witt's yard. Now 261 feet = 2"-56 ; which added to 1° 13' 14"-56 

 gives 1° 13' 17"' 12 for the amplitude of Lacaille's arc. Lacaille's 

 value is 1° 13' 17"-33*. 



The author remarks, in conclusion, " that although this work 

 does not clear up the anomaly of Lacaille's arc, it redounds to the 

 credit of that justly distinguished astronomer, that, with his means, 

 and in his day, his result from 1 5 stars is almost identical with 

 that from 1133 observations on 40 stars, made with a powerful and 

 celebrated instrument. Our field of inquiry is now limited to the 

 terrestrial measure, which every friend to science must wish to see 

 undertaken without delay, as a portion of a greater arc to extend so 

 far as to neutralise local attractions, and leave no doubt upon the 

 true curvature of this portion of the southern hemisphere." 



The Longitude of Madras, computed from Moon-culminating 

 Observations. By Edward Riddle, Esq. 



Mr. Riddle having undertaken, at the request of Mr. Baily, to 

 compute the longitude of the Madras Observatory from a mass of 



• In the Fundamenta AsironomicB (page 184), Lacaille states tliat the 

 amplitude deduced from the observations corrected for errors subsequently 

 discovered in the divisions of the sector, and recomputed, is 1° 13' ]7"'5. 



