ALTAZIMUTH VERTICAL CIRCLE, RUNS OF MICROMETERS. 



411 



the lower with 60 divisions, to one minute of arc. The " Level Indication " 

 is one-fourth of the sum of the four readings, diminished by one-twentieth 

 of the sum of the readings of the upper level. Strictly, the diminution 

 should have been one-twenty-fourth of the sum for the upper level ; but the 

 error has no appreciable effect upon the final latitude, the vertical axis being 

 always within a few seconds of true perpendicularity. The level zero is 

 included in the zenith point. The vertical circles are 14 inches in diameter, 

 divided to 5' spaces. Both pivots are pierced, to enable the instrument to be 

 used with both positions of the horizontal axis in regard to its supports, 

 but this entails the reversal of the levels and micrometers to make the 

 direction of graduation accord with that on the circles. 



The correction applied to the reading of the vertical circle for runs of the 

 micrometers is shown in the following table : 



CORRECTION for RUNS of the MICROMETERS of the VERTICAL CIRCLE. 



The stars selected for the determination of co-latitude were such as had 

 been well observed in N.P.D. at Greenwich, Melbourne, or the Cape of Good 

 Hope. For stars South of the equator the N.P. Distances of the southern 



