Astronomical Society. 527 



brass scale belonging to the Observatory. This scale is by Dollond. 

 It is a thin brass bar, inclosed in a mahogany case, lined with baize, 

 and was brought to England by Sir John Herschel, on his return 

 from the Cape, for the purpose of comparison with the Royal Astro- 

 nomical Society's standard*. On making the proper reductions for 

 temperature, the standard measuring-rod was found to exceed 20 

 feet of the scale by about l-50th of an inch. 



The measurement of the base line was commenced on the 17th of 

 June, Sir J. Herschel, Lieut. Williams, and Mr. C. Piazzi Smyth, 

 taking part in the work ; but it had only proceeded a short distance, 

 when it was interrupted by an accident. While Mr. Maclear was 

 adjusting the 66th rod, a sudden gust of wind blew it and the next 

 off the trestles, whereby one of them was entirely destroyed, and the 

 other injured. This accident compelled him to postpone the mea- 

 surement until November, for before a new rod could be got ready 

 the winter had set in, and the floods spoiled the line. The author 

 remarks, that although the contrivances adopted for pi-eventing simi- 

 lar accidents proved effectual, rods of this description are unfitted 

 for a windy country like the Cape, being liable to short vibrations, 

 which no clamping can control. 



The measurement was recommenced from the meridian pillar on 

 the Sth of November, and proceeded without further interruption. 

 In the preceding June a pig of lead, weighing 56 lbs., was sunk in 

 the ground, at the end of the 27th rod, and a cross on its surface 

 adjusted to the plummet. On reaching the 27th rod, in the second 

 operation, the lead was uncovered, and the plummet fell short of the 

 cross upon it by nearly half an inch. As the place was frequently 

 covered with water during the winter, it is probable the lead had 

 shifted its position. 



The measurement was completed in four days. On each day the 

 rods were compared with the standard, and the proper reductions 

 made to obtain the length in terms of the standard rod, which also 

 was corrected to the temperature of 70°, this being the temperature 

 engraved on the brass scale. The final results gave the distance 

 between the centre of the meridian pillar and the centre of the gun 

 = 2919-364 feet. 



The angles were measured in the latter part of 1836, while the ap- 

 paratus for measuring the base was in progress. The instrument 

 employed for the purpose was the repeating instrument by Dollond, 

 described in the first volume of the Memoirs. The signals on the 

 Block-house Battery, the Observatory, and Base-line stations, were 

 tripods, surmounted by hoops, and covered witii white cloth. At 

 Mrs. De Witt's house the signal was a circular disc, painted on the 

 east chimney. . 



On computing the triangles, and making the jjropcr reductions, 

 the distance between Mrs. De Witt's chimney and the transit instru- 

 ment was found to be 17,096 feet, and its distance from the j)er- 



• The con)parison lias since been made by Sir John Ilcrsohcl atul Mr. 

 Daily. The mean of sixty-three comparisons gave its Icngtli = A^WiUb'd 

 standard inches of the Society's standard yard. 



I 



