TUNNELLING THROUGH VARIOUS STRATA. 155 



tend vory considerably under the collapsed portion of tlio 

 tunnel. 



This shows the nocossity, as was said before, when 

 tunnelling through colliery districts, of engineers making 

 themselves thoroughly acquainted with all the workings in 

 the district, so as, if possible, to avoid them or to take 

 precautionary measures for the future safety of the tunnel. 

 Railway Companies now usually purchase, if possible, the 

 mineral rights in colliery districts ; but it was not so 

 formerly, and hence the great trouble and expense they 

 have been ])ut to in many instances, not only with tunnels, 

 but also with open portions of their lines ; but, of course, in 

 no case is subsidence so disastrous as in that of tunnels. 



About Wolverhampton, and through the black country, 

 the bridges are built purposely with the view of subsidence 

 taking place, and the girders rest on wedges which can from 

 time to time be tightened ; and when the girders have been 

 lifted to the full extent of the wedges, the abutments can 

 be raised with brickwork or masonry as the case may be, 

 and the wedges again placed in position for future slight 

 raisings. Of course this cannot be done in the case of 

 tunnels, as they cannot be built on wedges ; and therefore 

 every precaution should be taken to secure their permanent 

 stability. The author believes that formerly in the black 

 country sudden depressions of the permanent way, of one or 

 two foot, would take place, which had to be instantly made 

 good with ballast in order to keep the traffic going ; but 

 whether such occur to the same extent now, he cannot say. 

 An instance of tunnelling tlirough the coal measures is 

 Swansea Tunnel on the South Wales Eailway of the Great 

 Western system. Brunei, who was the engineer, had at that 

 time a most unaccountable liking for flat-crowned arches in 

 his tunnels : and it is all the more unaccountable consider- 



