THE GREAT ALPINE TUNNELS. 



619 



2 feet in width. :ukI ;^ feet in height, except at the probable point of 

 meeting, where the height is 4 feet 6 inches. The length is 1,708 feet, 

 and there is a fall of 1 foot in this distance. About the middle of its 

 course there are apparently two false cuts, as if a wrong direction had 

 been taken; but possibly these were intentional, and provided passing 

 places for the workmen and material. 



On the soffit of the tunnel is carved an inscription, of which the fol- 

 lowing is a translation: 



"Behold the excavation. Now this had been the history of the 

 excavation. While the workmen were still lifting up the pick, each 

 toward his neighl>or, and while 3 cubits (1 feet 6 inches) still remained 

 to cut through, each heard the voice of the other, who called to his 

 neighbor, since there was an excess of rock on the right hand and on 

 the left. And on the day of the excavation the workmen struck each 

 to meet his ncight)or pick against pick, and thei'e flowed the waters 

 from the spring to the pool for 1,200 cubits (1,820 feet), and 100 cubits 

 (151 feet) was the height of the rock over the head of the workmen." 



A Roman engineer gives an account of a tunnel which was being 

 driven under his directions for an aqueduct. And as he was only 



(/VoC Co Scait^ 

 Fir;. '2.— Plmi of Tuniu'l from Spring to Pool of Siloam. 



able to visit the work occasionally, he describes how on one of his 

 visits he found the two headings had missed each other, and he sa3's 

 that had his visit been deferred much longer there would have been 

 two tininels. 



The accurate meeting of the headings or driftwavs of a tunnel can 

 only be attained by the exercise of great care, both as regards direc- 

 tion as well as level. 



We need not go ver}' far to tind instances of such an error as inac- 

 curate meeting, but there is one well-known case on an important 

 main line in the Midland counties where the engineers failed to meet, 

 and to this day reverse curves exist in the tunnel to overcome the 

 difficulty. 



To attain this accurate meeting fine wires are hung down the shafts 

 of a tunnel, with heavy plumb bobs suspended from them in buckets 

 of water, or of tar, to bring their oscillations to rest, the accurate direc- 

 tion being given by means of a theodolite or transit instrument on the 

 surface. 



The wires are capable of side movement by means of a delicate 

 instrument (which is on the table), and are gradually brought exactly 



