41& Zach on Repeatmg Circles. 



M. Bessel finds that he differs not only from the great me- 

 ridian circle at Greenwich, but also from those by Ramsden 

 at Dublin, and at Palermo, and from a Keichenbach's 3-foot 

 repeating circle at Milan. M. Bessel says distinctly (p. 274), 

 *' that these differences did not surprise him, since his foi-mer 

 observations made with Gary's circle had already suggested 

 them to him." 



What are we to conclude from all this ? That we as yet 

 possess no instrument either- great or small with which we can 

 arrive at certainty within 2 or 3 seconds. Whatever can be 

 said most agi'eeable to reason and truth on this subject has 

 already been said by M. Gauss in a letter published in the 

 first volume of the Memoirs of the Astronomical Society of 

 London, p. 132. 



" A point," says he, " which has occupied the attention of 

 astronomers for some years, though it involves only a few 

 seconds, is yet of the highest importance, both in reference to 

 the art of astronomical observation, and on account of the 

 numerous astronomical elements, whose exact determination 

 depends on it; I mean the minute differences in the declinations 

 of stars, the obliquity of the ecliptic, and the altitude of the 

 pole, which appear in their determination by different though 

 very excellent instruments. There is no doubt these differences 

 arise from the action of gravity on the different parts of each 

 instrument, though hitherto the mode of this action has not 

 been clearly pointed out, nor is it possible to pronounce de- 

 cidedly which instrument has afforded the right and which 

 the wrong result. We know, in fact, veiy little of the extent 

 to which the yielding of the metals may go ; and it seems too 

 hazardous to deny the possibility of this cause exercising a 

 notable influence on the divisions, and in consequence on the 

 observations in any instrument, whatever be its construction, 

 without grounding such denial on sufficient proof. In our 

 meridian circle, the great artist has done every thing to ob- 

 viate the flexure of the telescope by a well-adapted system of 

 counterpoises: still a doubt may remain, whether all the flex- 

 ure be done away with by that means, or rendered quite insen- 

 sible; and the only direct means of ascertaining the point 

 seems to be, the combination of immediate observations of a 

 heavenly body, with those of its image reflected in an artificial 

 horizon." 



Thus, then, although we cannot decide which instrument 

 gives the true and which the false result in a question of two 

 or three seconds, whether the 18--inch circle, the 3-foot meri- 

 dian circle, or the 8-foot mural circle, like that at the Royal 

 Observatory at Greenwich ; and since tliis great 8-foot circle 



does 



