526 Astro7inmical Society. 



remaining undisturbed ; whereby the party were enabled to enjoy 

 the sight of one undeniable mark of the work of Lacaille. 



The author next proceeds to describe the operations for connect- 

 ing Lacaille's southern station with the Royd Observatory. With 

 these he included another position, " one which," he remarks, 

 " must ever excite the feelings and enthusiasm of the admirers of 

 genius, moral worth, and almost unlimited talent — namely, the scene 

 of Sir John Herschel's recent labours." This position being invi- 

 sible from the Observ'atorj', and also from Cape Town, it became ne- 

 cessary to choose a fourth station ; and, accordingly. King's Block- 

 house Battery was fixed upon, which commanded a view of the other 

 stations, and also of the base line. 



The site of the base is on the sandy plane to the north of the 

 Observatory. The east end is defined by the centre of the meridian 

 pillar of the transit room ; and the west, by a gun, let into a large 

 flag-stone, which was sunk in the ground to the depth of seven feet, 

 and firmly fixed by ramming down the soil. The line is so little 

 elevated above the sea, that a large portion of it is covered by water 

 at spring-tides. 



Almost the whole of the apparatus for the measurement of the 

 base was constructed at the Observatorj' ; and although the screws, 

 brass-work, &c. were homely in appearance, they sufficiently an- 

 swered the purpose. The twenty-feet measuring-rods, of which 

 three were used, were of well-seasoned white deal, on the model of 

 those employed by General Roy, on Hounslow Heath, excepting in 

 a few minor details. The square brass ferule covering each extre- 

 mity, was perforated with a hole three-eighths of an inch in diame- 

 ter; and a brass screw, one-fourth of an inch in diameter, and 

 nearly two inches long, was passed into the wood through the hole, 

 without touching the ferule, and ground Aown perfectly flat. The 

 head of this screw carried the division Avhich defined the length of 

 the rod ; and it is evident from the arrangement, that it could only 

 be afi^ected by the expansion of the wood. Eight trestles w^ere con- 

 structed on the model of General Roy's, having wood screws and 

 moveable teak- wood tables ; and the usual equipment of boruig-rods 

 and piquets prepared. 



A contrivance suggested by Sir John Herschel for measuring the 

 space between the divisions on the contiguous ends of the measuring- 

 rods, when laid in line, was adopted, and found to answer well in 

 practice ; and which obviated the possibility of any mistake in re- 

 gistering. This consisted in rendering the space a constant quan- 

 tity, which was effected in the following manner. A couple of 

 crosses being drawn with a fine point on a slip of mica, about 2*3 

 inches from each other, and the divided surface turned downwards 

 to prevent parallax, the crosses were seen through the transparent 

 mica, like spider lines ; and as they were placed, when used, over a 

 fine line on brass, a neat bisection of the cross was easily obtained, 

 with the assistance of a common magnifier. 



The twcnty.feet measuring-rods were compared with the four-fcct 



