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VIII. Continuation of a former Paper on the Twenty-five Feet Zenith Telescope lately 

 erected at the Royal Observatory. By John Pond, Esq. A.R. F.R.S. 



Received March 11,— Read March 12, 1835. 



During the last summer I had the honour of submitting to this Society a short 

 paper on the subject of the large zenith telescope lately erected at this Observatory. 



It is now nearly twenty years since the erection of such an instrument was first 

 suggested to the President and Council of this Society ; at that time the Royal Ob- 

 servatory was in a very inefficient state compared to what it is at present. We had 

 only one circle ; and there existed doubts as to the excellence of this instrument, 

 though not any were ever entertained by me. The erection of a second circle put this 

 question at rest ; it has been abundantly shown in various volumes Of the Greenwich 

 Observations, by a series of more rigorous investigations than any instrument was 

 ever submitted to before, that both the circles may be considered as perfect, their 

 errors being less than their respective makers themselves assigned. 



This circumstance, though satisfactory to myself, a little diminished the importance 

 of the new zenith telescope. It was hardly to be expected that any new instrument 

 could throw light upon errors already reduced within such small limits ; this, how- 

 ever, has been done, and the object of this paper is to explain the process I have em- 

 ployed for the purpose. 



Whoever is acquainted with the method of constructing the Greenwich Catalogue, 

 must have perceived that the places of those stars which are observed by reflection 

 are, according to all probability, more exactly determined than those which have 

 been observed only by direct vision, y Draconis, a star which since the time of 

 Bradley has been of first-rate importance in the Greenwich Observations, cannot 

 be observed by reflection. The probability of error was therefore greater in the 

 place of this star than in that of any other. The new instrument has shown that this 

 error does not exceed a quarter of a second ; a degree of accuracy scarcely credible, 

 and no doubt requiring to be confirmed by future observations. 



The nature of the question to be determined in this case has happily produced a 

 competition for excellence among the observers with the different instruments, which 

 gives me an opportunity of showing the present state of practical astronomy at 

 Greenwich. 



The new instrument has been employed during the last summer under very un- 

 favourable circumstances, both the building and the instrument having been almost 

 constantly under repair. It is not requisite on this occasion to enter into the details 



MDCCCXXXV. u 



