40 



1. By the error of the mean of Uie six readings arising 

 from inaccuracy in the divisions and error of eccentricity. 



2. By inexact adjustment of tiie vertical axis. 

 S. By error of reading off, 



4. By error of bisection of the star. 



5. By error from change of temperature affecting the parts 

 of the instrument. 



From my examination of this instrument, I have reason 

 to conckide there is no sensible error of eccentricity, and that 

 as far as the divisions aro concerned, the mean of the six 

 readings can never occasion a greater error than 1", and an 

 error of this amount will take place ordy in very few parts of 

 the circle. A comparison of the results determined by the 

 bottom, and by the two horizontal microscopes : also a com- 

 parison of the corrections of the mean of the microscopes 

 determined by stars at different distances from the zenith, 

 seem to leave no doubt on this point. 



It may appear an imperfection in this instrument that we 

 cannot avail ourselves of a microscope at the highest part of 

 the circie. The reading off a microscope so placed would be 

 highly inconvenient, and on account of the circumstances of 

 the instrument, the use of it might be attended with some 

 danger, both to the observer and instrument. However it 

 does not appear that the accuracy of the results would be 

 materially affected by such an addition. 



By means of the plumb line the vertical axis can in ge- 

 neral be adjusted with great precision ; but this operation is 



