In order that a sufficient number of hands may be employed together, and with perfect security, each perpendicular frame is divided into three small chambers, which may properly be denomi nated cells, (See Fig. 1-2). By this disposition thirty-three men may be brought to operate together with mechanical uniformity, and quite independent of each other. These cells, which are open at the back, present in front, against the ground, a com plete shield composed of small boards, which admit of being removed and replaced singly at pleasure.
"It is in these cells that the work of excavation is carried on. There each individual is to operate on the surface opposed to him, as a workman would cut out a recess in a wall for the purpose of letting in a piece of framing; with this difference only, that, instead of working upon the whole surface, he takes out one of the small boards at a time, cuts the ground to the depth of a few inches, and re places the board before he proceeds to the next. When he has thus gained from three to six inches over the whole surface (an operation which it is ex pected may be made in all the cells nearly in the same time), the frames are moved forward, and so much of the brick-work added to the body of the tunnel. Thus entrenched and secure, thirty-three men may carry on an excavation which is 630 feet superficial area, in regular order and uniform quan tities, with as much facility 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 these cells, and was excavated likewise by only one man, proceeded at the rate of from four to ten feet per day. In the plan now proposed, it is not intended that the pro gress should exceed the rate of three 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 ex amine 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 substrata, to the depth of 36 feet, are of a nature that present no obstacle to the progress of a tun nel; we are informed that no 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 twelve to seventeen feet in thickness quite undisturbed, (See Fig. 5.)
Admitting that, in descending to, or ascending from that line, we should come to a body of quick sand, 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 on a 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 in evitably be overwhelmed and lost.
Although we may encounter obstacles that may retard the daily progress of the work, it is with sa tisfaction we contemplate that every step we take tends to the performance and ultimate completion 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 under taking, it would still prove to be the most economi cal plan practicable for opening a land communica tion across a navigable river.
In order to execute the tunnel beneath the Thames by the method described in the preceding paper, it is proposed to raise a capital of £160,000, by trans ferable shares of X100 each. The following are the heads of expense: Our readers are already acquainted with the great disasters which have obstructed the comple tion of this great undertaking; disasters which af fect neither the judgment nor the foresight of the engineer, but which arose from circumstances which could neither be foreseen nor prevented. It could scarcely have been expected that private en terprise would again be embarked in this national work; but we anxiously hope that in better times this will be the first national work on which the public capital will be expended. The minister who lends his aid to its completion will acquire a repu tation which no other public measure could confer upon him.