98 TUNNELING 



subsequent correction of the alinement of the galleries. To 

 do this, a theodolite 24 ins. long and 2| ins. in diameter, 

 with a magnifying power of 40 times, was set up in the observ- 

 atory, and about 100 readings were taken of the angles between 

 the surrounding signals and the required sighting-points. In 

 this manner the error likely to occur was diminished to less 

 than 1'. Thus at the north end two points were found about 

 550 yds. before and behind the observatory, while on the south 

 side, owing to the narrowness of the gorge, the points could 

 only be placed at 82 yds. and 126 yds. in front. One of these 

 sighting-points consists of a fine sciatch ruled on a piece of glass 

 fixed in an iron frame, behind which is placed an acetylene 

 lamp, corrections of alinement are always done by night, 

 the whole being rigidly fixed into a niche cut in the rock and 

 protected from climatic and other disturbing agencies by an 

 iron plate. 



Method of Checking Alinement. The direction of heading 

 No. 1 is checked by experts from the Government Survey De- 

 partment at Lausanne about three times a year, and for this 

 purpose a transit instrument is set up in the observatory. A 

 number of three-legged iron tables are placed at intervals of 

 1 mile or 2 miles along the axis of tunnel No. 1, and upon 

 each of these is placed a horizontal plane, movable by means of 

 an adjusting screw, in a direction at right angles to the axis, 

 along a graduated scale. On this plane are small sockets, into 

 which the legs of an acetylene lamp and screen, or of the 

 transit instrument, can be quickly and accurately placed. The 

 screen has a vertical slit, 3 ins. in height, and variable between 

 }g in. and fa in. in breadth, according to the state of the atmos- 

 phere, and at a distance shows a fine thread of light. The 

 instrument, having first been sighted on to the illuminated 

 scratch of the sighting-point, is directed up the tunnel, where a 

 thread of light is shown from the first table. With the aid of 

 a telephone this light is adjusted so that its image is exactly 

 coincident Avith the cross hairs, and the reading on the gradu- 



