

The solai ifl mounteil \\itii a ^\\t\el adapter or 



tully centeied ' ulinarily covered \vithacenter- 



I ing tai^et lor o\ erhead plumb- 



The solar spirit le\el 

 insures parallelism of Collateral 

 ing, and the clamp-and- 

 tangent on the polar axis is 

 utili/ed to range the auxiliaiy 

 line intothe same veitical 

 plane with the main telescope. 

 The auxiliary sight-line 

 therefore, revolves in a vertical 

 plane that is normal to the ver- 

 tical axis and azimuth circle 

 so that no correction need be 

 made for angles observed on 

 the horizontal plates. These 

 conditions are guaranteed. 



To Correct Vertical Angles 



for Eccentricity of 



Collateral Sighting 



In Fig. 99 let the center 

 of the instrument, O, beset up 

 the upper level, and 

 let the auxiliary sight be taken 

 to a nail tied into a plumb line 

 a certain known distance be- 



Fi *- " low the hanging point, C. and 



above the point in the foot plate, B. 



The coordinates sought are OF, or AF, and FB. The vertical 

 angle read is HOM, which is too large. The angle sought is HOI 



Then HOE = HOM OED, or LOM, luu sin OLD 

 (ODI-; being a right-angle) = OD -: OF.. 



Assuming a concrete example: 



Let HOM (angle read) 71 



Let OK 'measured . ; ft. 



Let < >D (ei < entticitv ) = . > 1 ^ It. 



Then log. .315 

 a. c. /o%. 7'>. (| 



7.'.o-'^ ' 1L 



" > ;i3o i :> 



then 71 Ot 



i 







