for a Road-'-dcay wider the Thames. 141 



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 

 denominated cells. By this disposition, 33 men maybe brought 

 to operate together witli mechanical uniformity, and quite in- 

 dependent of each other. These cells, which are open at the 

 back, present in front agamst the ground a complete shield 

 composed of small boards, which admit of being removed 

 and replaced singly at pleasui*e. 



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 replaces the board before he pro- 

 ceed to the next. When he has thus gained from 3 to 6 

 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 intrenched and se- 

 cure, 33 men may be made to 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 4 to 10 feet per 

 day. In the plan now proposed, it is not intended that 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 na- 

 ture 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 86 feet are of a nature that 

 present no obstacle to the progress of a Tunnel ; we are in- 

 formed that no water was met there. It is therefore through 

 these substrata that it is proposed to penetrate, and to cai'ry 

 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 12 to 17 feet in thickness quite undisturbed. 



Admitting that in descending to or in ascending from that 

 line we should come to abody of(|uicksand, such as that which 

 was found within about 200 lect iVom the shore, it is then we 

 should find in the combinations of the framing, before dc- 



scribeil. 



