38 Proposal for an Iron Tunnel, &c. 



vast weight and great length, the power of 67 screws at 

 each joining would be inadequate to hold the whole per- 

 fectly together; and that in case of accident the whole 

 must be infallibly lost, as it Would then be impossible to 

 remove it from the spot on which it wonld immediately 

 sink, or even to separate the different parts of it. But as 

 the tunnel formed in the manner proposed will be sub- 

 1 to no particular force whatsoever at it* launching, 

 but be altogether equally borne up by the rising tide ; as 

 the weights necessary to sink it may he all so gradually 

 applied as to ensure its regular descent, to which the 

 form of the whole when joined as above, viz. convex at 

 the top and rising at the ends, together with the greater 

 thickness of the metal at the bottom, are circumstances 

 particularly favourable; and as from the nature of the 

 bottom it is sure to rest on a soft and uniform bed of sand, 

 on which it cannot meet with any object to occasion any 

 partial bearing, — I conceive the danger of accident is very- 

 remote, and the strength of the entire sufficiently secured : 

 besides, trials may be made in a sale situation. 



The chief difficulty appears to me to be the excavation 

 of the bed of the river to the depth required. The best 

 mode of effecting this, or whether it would not be better 

 to choose another situation in which the existing depth 

 might be found sufficient, I leave to more able and ex- 

 perienced engineers to determine; stating merely, that as 

 the materials of which the tunnel is to be composed can 

 be procured for about ^44,000; allowing fifty per cent, 

 additional for all other charges incurred in its execution, 

 I do not conceive the expense would exceed the sum of 

 ^66,000. 



I beg leave further to add, that if it should be desired to 

 enlarge this tunnel so as to afford a foot-path in addition 

 to the space allowed for two carnages to pass, [ conceive 

 it may safely be done by giving it six more feet in width, 

 making altogether 24 feet between the interior flanches ; 

 and in order to afford it still greater strength, I would in 

 this case omit the interior lateral flanches, and in the room 

 thereof, applv plates of cast-iron of three or four in< hes 

 thick, the full height of the sides, to extend from the mid- 

 dle of one frame to that of the next, to be fastened bv a 

 number of the same kind of screws to the sides of the two 

 adjoining frames, with sheet-lead between and completely 

 covering the joint inside. This would give the tunnel 

 great additional strength without much increasing its 

 weight, besides that it would leave nearly a foot more of 



free 



