RAILWAY CONSTRUCTION. 167 



the very slow travelling, or even the coming to a stand 

 from "slipping," may not produce unpleasant or alarming 

 consequences. 



In tunnels of any length it is usual, where possible, to con- 

 struct shafts extending from the surface of the ground overhead 

 down to the tunnel below. These shafts serve the double pur- 

 pose of enabling the excavation to be carried on at an increased 

 number of faces, and act as permanent ventilators after com- 

 pletion. In some cases the shafts are sunk exactly over the 

 centre line of the tunnel, in others a few yards away from the 

 centre line. The latter arrangement, if not quite so convenient 

 for hoisting material while carrying on the excavations, has 

 certainly the great after advantage that anything falling or 

 maliciously thrown down the shaft cannot strike a passing train. 

 The short side-gallery, or space between the tunnel and the shaft, 

 provides a good refuge for workmen employed in repairs, and a 

 convenient site for storing a few materials advisable to keep on 

 hand. 



Occasionally favourable opportunities present themselves for 

 making horizontal shafts. For a portion of its length the tunnel 

 may be located at no very great distance from the precipitous 

 sides of some deep mountain ravine, or run near to the cliffs on 

 the sea-coast, and advantage can be taken to drive a lateral head- 

 ing or gallery through which the material from the tunnel exca- 

 vation may be conveyed and thrown out into the gorge or sea- 

 shore below. 



In many cases the surface of the ground rises so abruptly 

 from the faces of the tunnel and ascends to so great a height, 

 that shafts of any kind are entirely out of the question, and the 

 whole of the work must be carried on from the two ends. The 

 rate of progress is consequently much slower, and the ventilation 

 more difficult. In a shaftless tunnel of considerable length, and 

 with a frequent train service, the question of providing suitable 

 appliances for promoting artificial ventilation is of the utmost 

 importance. 



When the centre line of the tunnel has been accurately set 

 out on the ground, and the levels of the different parts of the 

 work decided, the construction of the shafts and the driving of 

 the headings can be commenced. Working shafts intended to 

 serve for permanent ventilation are generally made nine or ten 

 feet or more in diameter, and are usually lined with substantial 



