96 Mr. Nixon on the instrumental Error of his Horizon-Sector. 



It is a good verification of the measui'ements to find the 

 reversing points of the proof-level the same on the cylinder 

 as on a plane made as nearly horizontal in every sense as pos- 

 sible. But it is to be remarked that unless the plates of the 

 proof-level are true planes, the reversing point may not be 

 the same on the cylinder as on the plane. For want of a per- 

 fect plane this satisfactory confirmation could not be obtained; 

 but the reversing points with the proof-level on the cylinder, 

 as may be seen from the statement below, agreed within the 

 fraction of a second of their mean value. 



April 13th, 1830. Temp. 50= 



April 16th, 1830. Temp, 58' 



April 16th, 1830. Temp. 56° 



(One division is equal to about half a second.) 



There is this serious objection to all the three methods, at 

 least in their application to the sector, — that the inclination of 

 the cylinder cannot be measured on that part of its surface 

 which comes in contact with the Ys during the process of ad- 

 justing the instrument. 



[To be continued.] 



IX. Notes 



