Tl'NNELS THi:)l;H H.\l:D It >< K 99 



ated scale is noted. This is done four or five times, the aver- 

 age of these readings being taken as correct, and the plane is 

 clamped to that average. The instrument is then taken to the 

 first table and is placed quickly and accurately over the point 

 just found (by means of the sockets), and the lamp is carried 

 to the observatory. After first sighting back, a second point is 

 given on the second table, and so on. Tin-so points are marked 

 either temporarily in the roof of the heading ly a short piece 

 of cord hanging down, or permanently l>\ a Krass point held by 

 a small steel cylinder, 8 ins. long and : ins. in diameter, em- 

 bedded in concrete in the rock floor, and protected by a circular 

 casting, also sunk in cement concrete, holding an iron cover 

 resembling that of a small manhole. From time to time the 

 alinement is checked from these points by the engineers, ami 

 after each blast the general direction is given by the hand from 

 the temporary points. To check the results of the triangula- 

 tion survey, astronomical observations have been taken simul- 

 taneously at each end. With regard to the levels, those given 

 on the excellent Government surveys have lieen taken a> 

 rect, but they have also been checked over the pass. 



Details of Tunnels. In cross-section, tunnel No. 1 is 13 ft. 

 7 ins. wide at formation level, increasing to 10 ft. 5 ins., \\ilh 

 a total height of 18 ft above rail-level, and a cross-sectional 

 area of about 250 sq. ft. This large section will allow of 

 small repairs being executed in the roof without interruption 

 of the traffic, and will also allow of strengthening the walls by 

 additional masonry on the inside. The thickness of the lining. 

 never wholly absent, and the material of which it is composed, 

 depend upon the pressure to be rented, and only in the worst 

 case is an invert resorted to. The side drain, to which the rock 

 floor is made to slope, will be composed of half-pipes of 7 to 1 

 cement concrete. The roof is constructed of radial stones. 



Tunnel No. 2, being left as a heading, is driven on that side 

 nearest to No. 1, to minimize the length of the cross-headings, 

 and measures 10 ft 2 ins. wide by ' ft. 7 ins. high. Masonry 



