DETEKMIMN<; THK CKNTKU LINE 9 



CHAPTER II. 



METHODS OF DETERMINING THE CENTER 



LINE AND FORMS AND DIMENSIONS OF 



CROSS-SECTION. 



DETERMINING THE CENTER LINE. 



TUNNELS may be either curvilinear or rectilinear, lint the 

 latter form is the more common. In either case the first task 

 of the engineer, after the ends of the tunnel him- IK-CM definitely 

 fixed, is to locate the center line exactly. This is done on the 

 surface of the ground ; and its purpose is to find the exact 

 length of the tunnel, and to furnish a reference line by which 

 the excavation is directed. 



Rectilinear Tunnels. In short tunnels the center line may lc 

 accurately enough located for all practical purposes by means 

 of a common theodolite. The work is performed on a calm, 

 clear day, so as to have the instrument and observations sul>- 

 jected to as little atmospheric, disturbance as possible. Wooden 

 stakes are employed to mark the various located points of the 

 center line temporarily. The observations are usually related 

 once at least to check the errors, and the stakes arc altered as 

 the corrections dictate ; and after the line is finally decided to 

 be correctly fixed, they are replaced by jiennanent monu- 

 ments of stone accurately marked. The method of checking the 

 observations is descriled by Mr. W. 1 >. Ila.skoll as follows: 



" Let the theodolite be carefully rt up over OIL- ..f the RUkea, with tin- 

 nail driven int<> it, hi-liTtiimnin-th.it will numnand tln Ix-.-t po-iii.in KI AA to 

 range backwards and forwards over the whole length of line, and al<> ulitain a 

 view of the two distant point* that rau^i- with tin- -nt<-r lin<- ; this being done, 



" Practical Tunneling." by F. W. Sirnm*. 



