1900. J on the Great Alpi7ie Tunnels. 425 



buckets of water, or of tar, to bring tbeir oscillations to rest ; tbe 

 accurate direction 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 

 into the same vertical plane : hence, if they are 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 levelling ; 

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

 to both these operations. 



Assume two shafts, 1000 yards apart, between which a gallery has 

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

 which are ^th inch in diameter, an error of the diameter of the wire 

 at the shaft will cause a mistake of nearly 4 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 

 finished, had to be completed. The first thing to be done by the 

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

 the work — this rough sketch (see Fig. 3) indicates what was dis- 

 covered. 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 ! 



^. L 7 Ji& of TrmneL as 



\ 



Fig. 3.— Plan. 



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, 

 staging 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 



Vol. XVI. (No. 94.) 2 f 



