ALTAZIMUTH HORIZONTAL CIRCLE, RUNS OF MICROMETERS. 



413 



body of the instrument, consisting of the vertical axis, the horizontal circle, 

 and the tripod stand, was undisturbed during the series of observations, 

 except by occasional adjustments of the foot screws, each of which rested in 

 a radial groove cut on the surface of a brass plate, which was countersunk 

 in the circular slab of slate that was cemented to and formed the top of the 

 pier. The delicacy of the levels used with the instrument made it imperative 

 to preserve the vertical axis within a few seconds of true perpendicularity. 

 The pier seems to have undergone but little or no settlement. 



The runs of the micrometers of the horizontal circle were observed as 

 follows, chiefly by Mr. Sidgreaves : 



CORRECTION for RUNS of the MICROMETERS of the HORIZONTAL CIRCLE. 



The horizontal intervals of the vertical wires were determined by 

 Mr. Sidgreaves as follows : 



