10 TUNNELING 



let the centers of every stake ... be carefully verified. If this be carefully 

 done, and the centers be found correct, and thoroughly in one visual line as 

 seen through the telescope, there will be no fear but that a perfectly straight 

 line has been obtained. 



The center line which has thus been located on the ground 

 surface has to be transposed to the inside of the tunnel to 

 direct the excavation. To do this let A and B be the entrances 

 and a and b be the two distinct fixed points which have been 

 ranged in with the center line located on the ground surface 

 over the hill Af B, Fig. 1. The instrument is set up at V, 

 any point on the line A a produced, and a bearing secured by 

 observation on the center line marked on the surface. This 

 bearing is then carried into the tunnel by plunging the tele- 

 scope, and setting pegs in the roof of the heading. Lamps 



~B A 



FIG. 1. Diagram Showing Manner of Lining in Rectilinear Tunnels. 



hung from these pegs furnish the necessary sighting points. 

 This same operation is repeated on the opposite side of the 

 hill to direct the excavation from that end of the tunnel. 

 These operations serve to locate only the first few points inside 

 the tunnel. As the excavation penetrates farther into the hill, 

 it becomes impossible to continue to locate the line from the 

 outside point, and the line has to be run from the points 

 marked on the roof of the heading. Great accuracy is required 

 in all these observations, since a very small error at the begin- 

 ning becomes greater and greater as the excavation advances. 



In very long tunnels excavated under high mountains more 

 elaborate methods have to be adopted for locating the center 

 line. The theodolites employed must be of large size ; in ran- 

 ging the center line of the St. Gothard tunnel, the theodolite 

 used had an object glass eight inches in diameter.* Instead of 



* See also Simplon Tunnel, Chapter IX. 



