l>ETERMDiDJO THE CENTER LINE 



13 



pillars or bench marks are placed normal to the center line on 



the edges of the shaft as shown by Fig. 4. Between the points 



.4 and B a wire is stretched, and from it two plumb bobs are 



suspended, as described in the 



preceding case ; these plumb 



bobs establish a vertical plane 



normal to the axis of the tun- 



nel. The excavation of the 



side tunnel is carried along the 



line BW until it intersects the 



line of the main tunnel, whose 



center line is determined by 



measuring off underground a 



distance equal to the distance 



BO on the surface. By setting 



the instrument over the under-ground point 0, and turnii: 



a right angle from the line BO, the center line of the tunnel is 



extended into the headings. 



Curvilinear Tunnels. There are various methods of locating 



the center line of curvilinear tunnels, but the method of tangent 

 offsets is the one most commonly employed. It 

 consists in finding the length of an ordinal** 

 DC\ Fig. 5, perpendit nlar to tin* tangent AX, 

 E at a point 7) taken at a known distance .!/> 

 = tl from tin* ]><>int of tangent .1. <> Ix-in^ the 

 center of tin* arc A /I and <>A Ix-ing the nidi us. 



! B 

 i 



~ 'Ctfitir " TC line 



__ 



4 



..r Tran^rrin K th* cnt*r 



Line down Side Shaft*. 



From the 



Pro. 5. DUgram From O dmw OZ parallel to the tangent .l.Y, 



1 and produce the perpendicular /><' until it in- 



Tangent offou tersects the line OZ at E. Join and <'. 



triangle O 1 '/'. 



(D 

 (2) 



