SURVEYING INSTRUMENTS. 

 Fig. 105. 



91 



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capstan-headed screw at the other, which sets it parallel to 

 the optical axis of the telescope ; at the opposite end is 

 another screw, e, to make it parallel in the direction side- 

 ways. One of the Y's is supported in a socket, and can be 

 raised or lowered by the screw B, to make the telescope 

 perpendicular to the vertical axis. Between the two sup- 

 ports is a compass-box, C, having a contrivance to throw 

 the magnetic needle off its centre when not in use ; it is 

 convenient for taking bearings, and is not necessarily con- 

 nected with the operations of levelling, but extends the use 

 of the instrument, making it a circumferentor. The whole 

 is mounted on parallel plates and three legs, the same as 

 the theodolite. 



It is evident, from the nature of this instrument, that 

 three adjustments are necessary. First, to place the inter- 

 section of the wires in the telescope, so that it shall coincide 

 with the axis of the cylindrical rings on which the telescope 

 turns ; secondly, to render the level parallel to this axis ; 

 and lastly, to set the telescope perpendicular to the vertical 

 axis, that the level may preserve its position while the 

 instrument is turned quite round upon the staves. 



To adjust the Line of Collimation. The eye-piece being 

 drawn out to see the wires distinctly, direct the telescope to 

 any distant object, and by the screw, A, adjust to distinct 



