G8 



THE SEVERN TUNNEL. 



screws were well oiled ; tliey worked by a simple liand-wlieol 

 at one end. All was now ready below. The now transit 

 was fixed over the hole in the top stono, and adjusted in true 

 poHition and level. It was then turned upon a flagstafi: on 

 the other side of the Severn and clamped there, being now 

 in the true centre lino of the tunnel. 



Wlien this had all been accurately done, the telescope was 

 pointed down the shaft, where the screw and the wire pass- 

 ing over it could be plainly seen. A few turns of the screw 

 brought the wire into exact position. This being fixed, the 

 telescope was then directed to the farthest point of the wire 

 that could be seen before it entered the heading, and that 

 point was thcin brought into true line by means of the screw 

 in the heading two hundred and fifty feet away. This, of 

 course, took a little more time, because it had to bo done by 

 signalling. 



This having been accomplislied, the readings wore a second 

 time observed, to make sure there was no error of observa- 

 tion. Electric lamps had to be used below in order to get 

 a distinct view of the wire, and then the cross-liaii's of 

 the transit could be made to bisect the wire witli great 

 accuracy. 



The wire now ga,vo two hundred and (ifty feet of base line 

 very correctly sot out. The engineer below, with a smaller 

 transit, then fixed his instrument in the line, and about ton 

 yards further in than the end of the wire, and adjusted it 

 until he could exactly cut the wire correctly at both its ends ; 

 he was then in a position to fix correct ma-i'ks in, the rooC- 

 trees all along the heading so far as ho could see. 



These lines were checked throughout by the same process 

 several times afterwards ; and wlion tiio Jusadirig became 

 very long, the large transit was taken below to continue the 

 lines more correctly under the Severn. After they had been 



