620 THE GREAT ALPINE TUNNELS. 



into the same vertical plane; hence, if they arc correct at "bank," or 

 surface, they must also be correct below ground. The engineers 

 below have to drive the galleries or headings so that only one wire is 

 visible from their instrument; so long as one wire exactly eclipses the 

 other wire, the gallery is being driven in the right direction. 



As regards accuracy in levels, this is done by ordinary leveling; but 

 it will be seen at once how much depends on care being devoted to 

 both these operations. 



Assume two shafts, L,000 yards apart, between which a gallery has 

 to be driven, and allowing a distance of 10 feet between the wires, 

 which are one-fortieth inch in diameter, an error of the diameter of 

 the wire at the shaft will cause a mistake of nearly -i inches at the point 

 of meeting, or of 7£ inches if a similar error occurs at the other shaft 

 in the opposite direction. The trickling of water down the wires 

 increases their diameter so appreciably, and therefore conduces to 

 further inaccuracy, that it is found necessary to fix a small shield or 

 umbrella on the wire to deflect the water. (This shield is to be seen 

 on the table.) 



Some years ago, a tunnel which had been commenced, but not fin- 



Fro. 3.— Plan. 



ished, had to be completed. The first thing to be done by the engi- 

 neers was to make an accurate survey of the then condition of the 

 work — this rough sketch (see tig. 3) indicates what was discovered. 

 The explanation given by the former ••ganger" was, that he found 

 the rock too hard, and he thought that by bearing round somewhat to 

 the right he might get into more easily excavated material! 



When the wires are hung down the shaft it is sometimes almost 

 impossible to prove that they are not touching, and consequently 

 being deflected from the true vertical line by some rope or pipe, stag- 

 ing or timber in the shaft. To overcome this, an electrical current 

 was passed down the wire — a galvanometer being in circuit. If the 

 wire proved absolutely silent, and no deflection was obtained in the 

 galvanometer, the conclusion could be safely drawn that the wire was 

 hanging freely and truly. 



In driving the necessary adit or heading for drainage purposes 

 beneath a subaqueous tunnel, a rising gradient from the shaft bottom 

 of 1 in 500 is allowed, to enable the water at the "face" to flow away 

 from the workmen to the pumps in the "sump" or shaft bottom (see 

 tig. 4). 



