2tJ8> ASTRONOMICAL CIRCLE. 



exactly In a diameter, as they may then be used In two posi* 

 tlons. Beneath these should be formed the image of a luminous 

 point, according to Mr. Troughton'^ present method, by an 

 apparatus attached to the plumb-line tube ; when the two 

 points on the circle move away, by the necessary operation 

 in observing, the lower point will remain stationary, and indi- 

 cate any change of position in the whole instrument, if such 

 should accidentally take place, and which by the other method 

 alone would have passed unnoticed. 

 The second is The contrivance above described was executed for me at 

 crf°confidence. '^Y request by Mr. Troughton, and is represented in the 

 plate; but by some accident a part of the apparatus was 

 broken in putting it toother, so that 1 never was able to use 

 it. As each apparatus for this adjustement is quite independent 

 of the other, no possible inconvenience can attend their appli- 

 cation, as either may be employed alone, at the option of the 

 observer. But as any verification requiring many bisections 

 is objectionable, I would in general certainly prefer Mr. 

 Troughton's method, and only have recourse to the other, 

 when there was reason to suspect that some alteration had 

 taken place to render it necessary. 

 Errors of dlvi- One more circumstance respecting the instrument remains to 



sion m the m- be noticed : when the divisions were first examined by opposite 

 strument, ,. „ , ., , , . , ^ 



readmgs, J ,25 was the greatest possible error which was to 



be apprehended, andO'',7 the mean error ; but in its journey 

 it seemed to have suffered some very small derangement iii 

 its form : this was discernible both from examining the oppo- 

 site readings ; and by deducing the error of collimation by 

 zenith stars, and comparing it with that found by an horizon- 

 tal object, there was constantly perceived a difference of 3" 

 between the error of collimation deduced from y Draconis 

 with great ccr- ^"^ ^Y ^" horizontal object; and this quantity was very 

 tainty from the uniformly distributed through the intermediate arc. In what 

 ^^^^'" particular manner the observations would be affeeled by this 



derangement I willl not venture to decide, but 1 think it most 

 likely that it has only rendered the instrument rather less ac- 

 curate than it was originally, as is above stated. I have before 

 observed the great advantage the circle possesses of showing 

 the amount of its own error*;. These may be determined with 

 great certainty ^by examining the errors of collimation as 



deduced 



