ASTRONOMICAL CIRCLE. SOS ' 



certainly not exceeding a second. The result I obtain by a j 

 very careful investigation b)* methods, entirely independent | 

 of the Greenwich quadrant, is 51°.28'.39",4. . i 

 I consider this comparison as interesting likewise on ano- ^^^ oTobserv' ■ 

 ther account ; it is an object deserving of curiosity to examine i 

 the present state of our best astronomical instruments, and to ; 

 ascertain vi^hat may reasonably be expected from them. The ;i 

 superiority of circular instruments is, I believe, too univer- I 

 sally admitted, to render it probable that quadrants will ever 1 

 again be substituted in their place. But the Greenwich j 

 quadrand is so intimately connected wih the history of astrono- 1 

 my, the observations that have been made with it, and the | 

 deductions from those observations, are of such infinite impor- 

 tance to the science, that every circumstance relating to it -i 

 cannot fail of being interesting Novi^, when it is considered j 

 that this instrument has been in constant use for upwards of : 

 half a century, and that the center error, trom constant friction, ; 

 would, during this time, have a regular tendency to increase, it ' 

 •will not appear at all surprising, if the former accuracy of this shows the pie- 

 instrument should be somewhat impaired. With a view, ?^"' ^^^"^ ^^ \ 

 therefore, of ascertaining more correctly the present stale of an 

 instrument on which so much depends, I have exhibited in 

 one view the polar distances as determined by circular iosfru- 

 ments alone; the respective co-latitu-'es being previously cor- 

 rected by the method above mentioned, and I have compared i 

 the mean result with the Greenwich Catalogue, that the na- ; 

 ture and amount of the deviations maybe seen^ and if it be i 

 judged necessary, corrected. 1 should add, that by some ' 5 

 observations of the sun at the winter solstice in 1800, the ! 

 difference between the Greenwich quadrant and the circle was ^ 

 10 or 12'', the quadrant ytill giving the zenith distance too [ 

 little. : 



General Description of the histriiment. \ 



The anexed plate represents the circle in its vertical position. Description of ^ 



It was originally made to be used likewise as an equatorial the aiuhoi's ] 

 instrument, a circumstance I need not have mentionned, but *^"^S"^y ^"*^''- 



as an apology tor slightness of its construction, which the Troughton. : 



artist, who made it, would not have recommended, had the j 



instrument been intended for the vertical position only. j 



The 



