TUNNELLING THROUGH VARIOUS STRATA. 167 



in rock, where springs of water axe often met with issuing 

 from fissures, to leave weepholes a little above the level 

 of the rails, so as to preserve the walls from any pressure 

 which may arise from tlio accumulation of water behind 

 them ; and they also tend to keep them dry, and prevent the 

 general dampness which is so evident in many tunnels, and 

 thus help to preserve the materials with which the tunnel 

 is lined from decay, owing to the action of frost forming 

 ice on the wet portions. The writer has seen tunnels of 

 considerable length one mass of icicles from beginning to 

 end, and it is a mistake to suppose that frost acts on them 

 a short distance from the ends only. 



Good drainage also keeps the rails dry, and free from that 

 general groasinoss which is the case in many tunnels, and, 

 where they are on steep gradients, materially retards the 

 tractive power of the engines. 



