318 Lieut. -Col. A. Strange on a new Theodolite [May 2, 



The Zenith-Sector was, shortly after its exhibition to the Society, for- 

 warded to its destination, as have been more recently the other Zenith - 

 Sector, one of the 5 -feet Transit-Instruments, the two 12-inch Vertical 

 Circles, the two Galvanic Chronographs, and the three Astronomical Clocks. 

 Of these, the first forwarded Zenith-Sector alone has been used in actual 

 field-operations. It has been employed during two seasons in determining 

 latitudes in Southern India, by Capt. J. Herschel, R.E., F.R.S., of the 

 Great Trigonometrical Survey. One of my main objects in designing the 

 instrument, which is entirely unlike any of its class, was to qualify it for 

 more rapid observing than had hitherto been possible. In this respect it 

 has been successful. Capt. Herschel reports that he can observe with it a 

 series of stars differing only five minutes of time in Right Ascension. 

 Each star is observed twice in reversed positions of the telescope at the 

 same culmination, in the manner prescribed by the Astronomer Royal for 

 the Zenith-Sector designed by him for the British Ordnance Survey. 

 Each of these two reversed observations involves two settings of the 

 telescope in altitude, four microscoped, two level, and one micrometer 

 reading. To admit of all these operations being performed within five 

 minutes of time with the deliberation requisite for observations aiming at 

 fractions of a second, demands not only convenience of instrumental con- 

 struction, but the greatest system and efficiency in the observatory ar- 

 rangements, and a very high order of skill on the part of the observer. 



In the latest Report on the Survey, that for 1870-71 by Major Mont- 

 gomerie, R.E., Officiating Superintendent, that officer states that "Captain 

 Herschel's further experience with the Zenith-Sector has on the whole con- 

 firmed his first impressions as to its excellence." Captain Herschel reports 

 on an anomaly in the level readings, amounting, as I understand through 

 private sources, to about 0"*3, the cause of which he has not yet been able 

 to detect. It may possibly, as he suggests, be partly due to the rigidity 

 of the instrument as a whole not being absolute. I am myself inclined to 

 suspect that imperfections in the levels themselves may in some measure 

 contribute to the observed discrepancies. The levels have been reported, 

 and are known to me, as deficient both in uniformity of run and in sensi- 

 bility; they are certainly not on a par with the graduation, which is 

 superb, or with the optical power of the telescope and microscopes. The 

 accuracy of the instrument is thus limited by that of the levels, which are 

 its weakest member. I may here state incidentally that I have not been 

 ahle to procure levels in England which come up to the standard now re- 

 quired, and I must therefore have recourse to foreign makers, who have 

 paid more attention to these adjuncts than, as it seems, it is worth the 

 while of our English artists to do. 



Comparing the facility of working the Zenith-Sector and the former 

 Astronomical Circles, Captain Herschel says : — " The Sectors are com- 

 petent to turn out at least double the amount of work of the same order" 

 adding, " at this rate two or three years' work would equal in amount the 



