254 Mr, Pond on the Changes in the [Oct. 



Then the observations of 1756 and 1822 being supposed perfect, 

 a catalogue for the year 1813 may be computed by interpola- 

 tion ; such a catalogue is annexed, and this (assumed to be cor- 

 rect), compared with the observed catalogue of 1813, will show 

 the errors of observations at that period. On this assumption 

 the Greenwich circle must, in 1813, have been in a very defect- 

 ive state ; and admitting the instrument to be now perfect, this 

 can be only attributed to the insufficiency of the braces which 

 then connected the telescope to the circle ; for this is the only 

 difference between the instrument in its former and in its present 

 state. The natural tendency of any such defect would be, I 

 think, continually to increase, and to give results every year 

 more and more distant from the truth : but this is contrary to 

 the known history of the Greenwich observations, which I have 

 found gradually for some time past approaching to those results 

 which are obtained at the present day, and which, according to 

 our present hypothesis, are supposed to be nearly perfect. If 

 the catalogue of 1813 were really so erroneous, as our present 

 hypothesis would compel us to regard it, then it would appear 

 that Dr. Brinkley's catalogue for the same period must have 

 been still more erroneous, as may be seen by inspection of the 

 annexed tables. Now admitting for a moment that there were 

 at that time certain imperfections in the Greenwich and Dublin 

 instruments, no person will believe them to have been so imper- 

 fect as our present hypothesis would tend to represent them. 



Let us now examme the second hypothesis, which presumes 

 the catalogue of 1813 to have been perfect, and consider what 

 confidence is due to the Greenwich observations of the present 

 day. This investigation is to be regarded as important, not 

 merely with a view to the discussion of the nature of the discord- 

 ances in question, but also from the circumstance, that instru- 

 ments of well-known celebrity are represented as giving very 

 different results ; for which reason I shall be excused for enter- 

 ing into considerable details on this particular question. As the 

 pnncipal rehance I place on the accuracy of the present cata- 

 logue, and on the superiority of the Greenwich circle over all 

 other instruments, with the history of which I am acquainted, 

 is derived from the coincidence of the results obtained by the 

 two independent methods ; the one of direct measurement of 

 polar distance, the other of observing the angular distance of 

 the direct and reflected image of the stars, it becomes oi' some 

 importance to consider in what way this coincidence is a proof 

 of the accuracy of either. The source of error the most to be 

 dreaded in every instrument whatever, quadrant or circle, is that 

 which will be caused by the flexure of the materials of which 

 the instrument is made. It is impossible in theory that any 

 instrument can be wholly free from this defect. In the Green- 

 wich circle the number of microscopes placed round its circum- 

 ference have an obvious tendency to diminish this error, though 



