44 ANNUAL OF SCIENTIFIC DISCOYEBY. 



in depth were out of the question, it was determined in the outset 

 that the tunnel should be worked from the extremities alone ; that is 

 to say, that four miles should be worked from one end and four from 

 the other. These conditions, however, involved two great difficulties : 

 one, the immense time which it would take to excavate so long a 

 tunnel from both faces, and the other the apparent impossibility of 

 ventilating it during the progress of the work. In order to obviate 

 these difficulties, various plans were proposed, but, after considerable 

 consultation on the part of the engineers intrusted with the matter, 

 the following arrangement was adopted. Air in the first instance is 

 forced into reservoirs, from whence it flows uniformly through a tube 

 into the interior of the tunnel or heading. This air is first used to 

 work the tools for drilling holes in the face of the rock, and then al- 

 lowed to escape in the tunnel-shaft, which last secures a perfectly 

 good ventilation. 



As this hydro-pneumatic machinery is the principal agent used in 

 carrying out the work, it is necessary, in the first instance, to get a 

 clear idea of the manner in which it acts. Some 30 or 40 yards above 

 the level of the plain there is a reservoir of water, filled by a canal 

 that is fed by a supply from a mountain torrent at some considerable 

 distance away. From this reservoir there are ten iron cylinders laid 

 on beds of masonry against the steep slope of the mountain, each of 

 which can be made to receive the water from the reservoir by open- 

 ing valves up above, in which case the water rushes down into iron res- 

 ervoirs for storing the air, one of these last being connected with each 

 tube that is laid against the side of the mountain. 



The way in which the air is forced into the iron reservoirs is as fol- 

 lows : Each tube that comes down the side of the mountain, and 

 which is about two feet in diameter, is continued on some ten or 

 twelve feet below the floor of the atelier, or building in which the 

 reservoirs are placed, after which it is bent and rises up perpendicu- 

 larly, or rather the main tutte communicates underneath with a hollow 

 vertical column from 12 to 15 feet high, in the top of which is a valve 

 opening downwards ; there is also another valve to separate the slop- 

 ing part of the tube from the horizontal portion. The whole of the 

 tube, including the sloping part between the water reservoir on the 

 side of the mountain and the flow of the atelier, the horizontal part 

 beneath the floor, and the vertical part which rises in the atelier some 

 12 or 16 feet, form altogether a siphon, in which not only the 

 weight of the water acts, but also the momentum of the water de- 

 scending all the way from the lofty mountain ridge whence it origi- 

 nally proceeds. We may suppose the whole of this siphon filled by 

 water, to begin with. The first operation then, in working, is to close 

 the valve at the foot of the sloping part, and afterwards to discharge 

 the water in the horizontal and vertical portions of the tube ; this 

 being done, the valve at the top of the vertical column falls down and 

 admits the air. The valve at the foot of the sloping tube is then re- 

 opened, and the one at the top of the vertical column closed ; the 

 water then rushes in and compresses the air in the shorter end of the 

 siphon to six atmospheres. This water afterwards is discharged as be- 

 fore, and fresh air admitted ; but it should be understood that when 

 the air is compressed in the upper portion of the vertical cylinder it 



