SEVERN

See also:

• TUNNEL (Fr. tonne!, later tonneau, a diminutive from Low Lat. tonna, tunna, a tun, cask)

TUNNEL (4 M. 281 chains in length)  . Percentage of Wear per annum . lb per yard Down line, outside and quite clear of tunnel, % per annum . Bristol end, gradient falling t in loo . . . 0.280=0240 Up line, outside and quite clear of tunnel, Newport end, gradient falling I in 90 0440=0"390 At Bristol mouth, gradient falling I in too I 200 = I "020 33 chains from Bristol mouth, gradient falling I in too 2.160=1.860 3 m . 751 chains from Bristol mouth, gradient rising I in 90 1.900 =1.630 At Newport mouth 0310=0.270 Down and up line under main-shaft level 3200=2.750 It will be seen that the maximum wear and corrosion together reached the extraordinary weight of 21 lb per yard of rail per year—a very serious amount that involved great expenditure The wear occurred over the whole of the rail, but the top, over which the engine and train passed, wore at a greater rate, presumably on account of the surface being kept bright and the gases being able to act on it . The Great Western Company tried the experiment in the Severn tunnel of boxing up the rails, so that the ballast approached their surface within 1 in. or 11 in . It was found, however, that—in the case, at any rate, of the limestone ballast-the cure was almost worse than the disease, the result being a maximum wear of 21 lb and an average wear of just under 2 lb per yard of rail per year . The average on the open line would be about 0.25 lb in the same time . See Proc . Inst .

Cie . Eng.; also

works on tunnelling by Drinker, Simms, Stauffer and Prelini, and on tunnel shields, &c., by Copperthwaite . (H . A .