1884.] on London (heloio bridge) North and South Communication. 497 



one of making a tunnel, or rather of making a covered way, be it 

 under tlie land or under the water. Moreover, engineering appliances 

 have become developed, I need hardly say, in the sixty years since 

 the Thames Tunnel was commenced, and better materials exist at 

 cheap rates. Brunei had a shield made of cast iron ; then steel 

 would have been an impossible metal for the purpose — it was sold 

 not by the ton, but by the pound ; in these days the shield would be 

 made of steel, at no greater cost than was then needed for one in 

 cast iron. Scre's^'jacks, demanding a great expenditure of manual power 

 to be exerted in a confined space, would now be replaced by hydraulic 

 presses worked by a steam-engine on the surface ; and the pumping of 

 the water which made its way in at the shield, instead of demanding 

 the labour of forty men, costing for the day and night work 150/. a 

 week, could be effected with ease by an hydraulic engine, by a com- 

 pressed-air engine, or even by electricity, which latter agent would 

 also afford the means of illumination without heat or contamination of 

 the air. 



These would be the means by which (even if the covered way were 

 truly a tunnel — that is to say, a "burrowing through the earth ") such a 

 work could be carried out with a cheapness, an expedition, and a cer- 

 tainty that were not possible sixty years ago. But, improved as the 

 means of tunnelling are, it is still desirable, that there should be some 

 considerable thickness of earth, between the bed of the river, and the 

 top of the hole, that is being tunnelled. Therefore, if the process of 

 timnelliug be followed, it is seen there would have to be allowed first, 

 the depth of the river, next the minimum thickness of earth over the 

 excavation, and then the depth, from the top of the excavation to the 

 surface of the roadway. If the minimum thickness were employed, 

 that ought to be allowed, the result would bo, with the height of 

 roadway it is proposed to give in the tunnel I am about to mention 

 to you, to put its floor 80 feet below Trinity high water. 



Now we have considered the question that we must have an ascent 

 to reach from the ground to the roadway of a high-level bridge, it is 

 equally clear we must have a descent to extend from the surface of 

 the ground down to the roadway of a tunnel ; and if that roadway 

 were 150 feet below the surface of the ground, the difficulties of the 

 descent would obviously be as great as the difficulties of the ascent. 

 But as has already been said, even if tunnelling be resorted to, the 

 depth of the roadway below high water need be no more than 80 feet ; 

 but by following a process in making this subaqueous road which is 

 pursued in corresponding cases on land, the depth can be so much 

 reduced, as to be only 60 feet. I mean the process technically known 

 as " cut and cover." The name expresses quite clearly what is done, 

 and tells us that the excavation is cut down from the surface, and 

 that then (the brickwork and covering arch being put in), earth is 

 filled over the top, and the surface is restored. This plan is one that 

 can be perfectly well followed for a subaqueous road, and can be 

 carried out in the following manner : — A coflerdam of a certain 



