per

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moved forward, and so much of the brick-work added to‘the body of the Tunnel. Thus intrenched and secure, 36 men may be made to carry on an excavation which is 630 feet superficial area, in regular order and uniform quantities, with as much fa- cility and safety as if one drift only of 19 feet square was to be opened by one man.

The drift carried under the Thames in 1809, which was about the size of one of these cells, proceeded at the rate of from 4 to 10 feet per day. In the plan now proposed, it is not in- tended the progress should exceed 3 feet per day, because the work should proceed with mechanical uniformity in all the points together.

With regard to the line of operation, if we examine the nature of the ground we have to go through, we observe under the third stratum, which has been found to resist infiltrations, that the sub- strata to the depth of 86 feet are of a nature that present no ob- stacle to the progress of a Tunnel; we are informed that little water was met there. It is therefore through these substrata that it is proposed to penetrate, and to carry the line that is to cross the deep and navigable part of the river, leaving over the crown of the Tunnel a head of earth of from 17 to 25 feet in thickness, quite undisturbed.

Admitting that in descending to or in ascending from that line we should come to a body of quicksand, such as that which was found within about 200 feet from the shore, it is then we should find in the combinations of the framing, before described, the means that are necessary for effecting, upon a large scale, what is practised, ona very small one, by miners when they meet with similar obstacles. Indeed, were it not for the means of security that are resorted to on many occasions, mines would inevitably be overwhelmed and lost.

Notwithstanding we may encounter obstacles that may retard the daily progress, it is with satisfaction we contemplate that every step we take tends to the performance and ultimate com- pletion of the object; and if we consider that the body of the Tunnel must exceed the length of Waterloo Bridge, it must be admitted that, if, instead of two years, three were necessary to complete the undertaking, it would still prove to be the most economical plan practicable for opening a land communication across a navigable river.

No notice is taken here of the mode of constructing the de- scents or approaches into the Tunnel; because whatever form or direction it may be found necessary to adopt, it is obvious that no difficulties oppose themselves to the accomplishment of that