HORIZONTAL SECTIONS. 75 



the telescope is swung to move vertically, and the angle 

 at which its line of sight coincides with the object to 

 which it is directed is indicated on the graduated arc 

 attached to and which moves with the telescope the 

 amount of inclination being read off with the greatest 

 accuracy by means of a vernier scale. 



Tor levelling by theodolite it is necessary that the 

 instrument be placed on the line of section (unless 

 correct heights at definite points only are required), and 

 that correct measurement be made of the distance be- 

 tween the instrument and the places at which observa- 

 tions are taken for it is evident that an inclined plane 

 gets to a greater or lesser height at every additional 

 portion of distance. 



The instrument is placed on the line by compass- 

 bearing, not necessarily ascertained every time it is set 

 up, for it will frequently happen that an object at 

 considerable distance is seen to be in the correct line, 

 and by chaining straight towards such object the desired 

 end may be attained with much less trouble. When, 

 to get the proper line of direction, a bearing has to be 

 taken, the needle beneath the telescope is set free and 

 the instrument revolved on its horizontal plates until 

 the needle comes to rest at the proper angle allowance 

 of course being made for magnetic variation.* In this, 

 and all other manipulations of the theodolite, the process 

 is commenced by bringing it nearly into the required 

 position and then fixing it by the " clamping screws," 

 the operation being completed to the greatest nicety by 

 means of finely-cut " tangent screws " provided for the 

 purpose. The divisions of the arc, plates, and verniers 

 Ante, p. 9. 



