Verification o^Lacaille's Arc of the Meridian. 595 



A cursory comparison of the observations having shown that 

 those made at the southern station did not deserve the confidence 

 required in a work of this kind, where the length of a few feet is a 

 quantity of importance, and experience having proved that good ob- 

 servations could not be obtained under a tent in Mrs. De Witt's j'^ard, 

 it was resolved to -look out for some solid building close to the sta- 

 tion, where the sector could have fair play. Mr. Maclear fixed upon 

 the Raggebay Guardhouse, and the necessary permission having been 

 obtained, a hole was made in the roof, the fioor taken up and sunk 

 to the requisite depth, and the sector erected on the 7th of May, 

 one week after the return of the party from Klyp Fonteyn. 



From the state of the weather, no observations could be obtained 

 until the 12th, and frequent interruptions afterwards occurred from 

 the same cause ; so' that six weeks were expended on a a work 

 which,i n favourable weather, might have been accomplished in two. 



The sector was dismounted on the 30th of June, and carried 

 back to the Observatory, where it was again set up in the sector- 

 room on the 2nd of July. On examination, it was found to be as 

 perfect as when first received, without the slightest mark of injury. 



The author next proceeds to give the reduction of the observa- 

 tions. The barometers employed at Klyp Fonteyn were made by 

 Mr. Thomas Jones, and from a comparison of their results with the 

 Journal kept at the Royal Observatory, it appears that the station 

 at Klyp Fonteyn is 485 feet above the mean level of the sea*. The 

 station in the Guardhouse is close to the sea beach ; the feet of 

 the sector tripod could not be more than two or three feet above 

 high-water mark. The chronometer employed at both stations 

 was by Arnold, and beats half-seconds. Its performance at Klyp 

 Fonteyn was good ; at the Guardhouse, the reverse. At the former 

 place, its rate was actually obtained by altitudes near the prime 

 vertical ; in Cape Town, by means of journeymen pocket-chrono- 

 meters, carried to and from the transit-clock at the Observatory. 



The collimation of the middle wire was deducted from the suc- 

 cessive transits of stars in the alternate position of the limb, east 

 and west. 



The corrections for aberration, precessions, and nutation were 

 calculated by means of the constant in the Royal Astronomical 

 Society's Catalogue, recomputed for 1838. 



The number of stars observed was 40 ; of which 20 were to the 

 north, and 20 to the south of the zenith at the Cape. The number 

 of observations at Klyp Fonte3m is 464, and at the Guardhouse G69 ; 

 in all, 1133. It may therefore be supposed that errors of observa- 

 tion are reduced to nothing. 



The final results are deduced as follows : — The amplitude being 

 found from the mean of the reduced zenith distances of each star 

 observed at both stations, and each result having a toeight assigned 

 to it equal to the product obtained by multiplying the least number 



* In the former communication the height of the station is stated to be 

 nearly 400 feet.— Monthly Notices, vol. iv. p. 194. [Phil. Mag. S. 3/vol. 

 xiv. p. 628,] 



2 Q2 



