THE SBVEBN TUNNEL. 77 



sketch of this Tunnel, the water pressures on the Tarious 

 parts of the work .are figured in lbs. on the square inch at 

 evoiy two foot in height. It will bo observed that while the 

 pressure on the crown of the arch is 52 lbs. on the square 

 inch, that under the invert is 64| lbs. 



These are the direct water pressures which have to bo 

 carried liy tho brickwork acting as an arch, whicli lias to 

 bear this converging pressure all round, and to convert the 

 vertical load into a thrust or hed-jprossure on the brickwork 

 a,ll tlio way round tlio Tunnel. 



Now these bed-pressures are many times greater than the 

 water pressures outside, so they are figured in cwts. per 

 square inch, and they vary directly in proportion to the 

 radius of curvature of tho ai'cli. The result, therefore, is 

 that while the water pressure on the crown of the arch is 

 52 lbs., the bed pressure on the arch is 3'15 cwts. on the 

 s(]uai'o inch ; but the invei't, being of a flatter radius, while 

 the water pressure was 64| lbs., the bed pressure is G'09 

 cwts., or just about double what it was on the arch. So, to 

 make them of e(]ual sti'ength against water pressure, the 

 invert should be twice as thick as the arcli. 



In fact, however, the 27 inch invert would have been quite 

 strong enough to have borne the thrust if the load had been 

 fairly distributed thi'ough the thickness of tlie work ; but as 

 it was, it did give way in some places in the wet part of the 

 Tunnel l)eforo alluded to. 



The cause of the failure of tho work to boar the great 

 watoi' pressure may probably be explained in this way : 

 In putting in the invert on a rocky bottom full of water 

 springs, after the rock has been got out, the bottom is of 

 course I'ough and broken, and the water wells up out of 

 every tiny crevice or crack, as well as out of tho wider joints 

 in the rock, and this water cannot all be got rid of by drains 



