44 ANNUAL OF SCIENTIFIC DISCOVERY. 



passes gradually through shades of gray, purple, and blue, into a 

 black, finely-grained rock, intensely hard and tough ; the lightest 

 and softest rock being at the top, and the densest and blackest at 

 the bottom. Regarded from an engineering point of view, the 

 work is considered eminently successful. 



Wherever difficulties have been met they hare been quickly and 

 successfully overcome. A siphon 600 yards long was employed 

 for the drainage of the upper half of the tunnel. The system of 

 ventilation has proved perfectly adapted to the requirements of 

 the cose, and has been not only effective, but simple and com- 

 paratively inexpensive. The engineers of the Mont Cenis Tunnel 

 have found it necessary to adopt similar means of ventilation in 

 that famous work. In the first instance air was driven in by fans 

 worked by horse-power, but this soon proved quite insufficient; 

 and when the works extended some distance, much time was lost 

 owing to the difficulty of getting rid of the smoke. To obviate 

 this on the Lyttleton side, the upper portion of the tunnel was 

 partitioned off by a floor or brattice, about 9 feet above rail level, 

 forming a smoke flue connected with one of the shafts, at the bot- 

 tom of which was placed a furnace, which, by rarefying the air, 

 caused a steady current up the shaft, and drew the smoke away 

 from the face of the workings. 



A similar plan was adopted at the north end, the chimney of a 

 forge being led into the shaft, and answering the purpose of a 

 furnace ; but the brattice was only continued for a short distance 

 beyond the upcast shaft. On the Lyttleton side this system 

 answered perfectly well, and the ventilation has continued good 

 ever since ; but on the Heathcote side, where the work for the last 

 quarter of a mile has been driven by a top heading (the tempo- 

 rary floor being left above the permanent rail-level for drainage 

 purposes), the ventilation at the close of the work became slug- 

 gish, and recourse was had to driving air on the face by means 

 of four fans driven by an eight-horse steam engine. This proved 

 perfectly successful. 



The system employed to secure the correctness of the alignment 

 of the two ends of the tunnel was very simple. A permanent 

 mark was fixed in the centre line of the tunnel, on a tower built 

 on the dividing range, nearly midway between the two ends. A 

 transit instrument being placed on the meridian of the tunnel, as 

 well as of the tower on the hill, it could be seen at once whether 

 the flame of a candle in the centre line of the work inside the tun- 

 nel was in a vertical plane with the mark on the tower. It was 

 also desirable, in case of error, to have the means, not only of 

 correcting, but of calculating, the amount of such error, and this 

 could be readily done. The permanent mark on the central tower 

 consisted of a batten 6 inches wide, with a black stripe 1 inch 

 wide down the centre. The eye-piece of the transit instrument 

 being furnished with 5 vertical wires placed at equal distances 

 apart, the value of the space between any two wires at a distance 

 equal to that of the mark on the tower could be ascertained by 

 reference to the width of the batten, which thus gave a scale by 

 which the error in the position of a. light placed in the tunnel 



