39 



ally differ, and thence the relative positions of the point of 

 suspension, and of the point below, be changed. To obviate 

 this inconvenience, which would be fatal to the accuracy of 

 the observations, the point of suspension is on a compound 

 bar formed of bars of brass and steel, and the point below 

 is also placed on a similar compound bar. By this the dis- 

 tance of the plumb line from the vertical axis remains always 

 the same. This contrivance appears to answer in a very satis- 

 factory manner. * 



The axis of the instrument being adjusted vertical, and 

 the plane of the circle in the meridian, and facing the east ; 

 let b, I, r, be the zenith distances of a star as shewn by the 

 bottom, left and right hand microscopes respectively. AVhen 

 the plane of the circle is in the meridian, and facing the 

 west, let U, I', r\ be the zenith distances of the same star as 

 shewn by the respective microscopes. Then the true zenith 



distance = g or-j-V— ^ — I 3— A And the cor- 

 rection of the mean of the three microscopes 



The accuracy of the result of an observation is affected 



* The circular instrument in plate 8 of Professor Vince's Practical Astronomy, may 

 be referred to for our circular instrument. Except that in our instrument there is np 

 azimuth circle. The plumb line is suspended in the position represented, but the com- 

 pound bars are not represented. Also the pillar F is perforated for the insertion of the 

 bottom microscope. The horizontal microscopes are not represented, and there is no 

 microscope at n. 



