42 . 



Ill errors of observation I include errors of adjustment in 

 the vertical axis, errors of bisection of the star, errors of read- 

 ing off, and also errors arising from changes of temperature 

 in the instrument. 



From what has been said, the principal circumstances rela- 

 tive to the astronomical circle at our observatory will be 

 readily comprehended. 



The angle to be obtained by each observation is the exact 

 zenith distance of the object, from which, the zenith distance 

 of the pole having been previously determined, the polar dis- 

 tance or declination of the object is known. The zenith dis- 

 tance can only be had by the assistance of a plumb line or 

 spirit level. The inaccuracies and inconveniencies to which 

 both these instruments are liable have long been known, and 

 as the zenith distance is not necessary for finding the polar 

 distance, it has been, sometime ago, proposed to find the 

 polar distance without a reference to the zenith point, by 

 simply observing the arches of the meridian intercepted be- 

 tween the object, the polar distance of which is required, and 

 stars, the polar distances of which are known, or can be ob- 

 tained by help of circumpolar stars. 



Mr. Troughton, whose fame as an artist is justly so cele- 

 brated, has made several circles with this view, and has lately 

 made a mural circle of 6 feet in diameter for the Royal Ob- 

 servatory at Green\vich. The recent construction of this 

 instrument, in which Mr. Troughton has availed himself of 

 his long experience and the latest improvements ; the cir- 



