^78 Mr Brunei on a Nexo Plan of Tunnelling^ calculated 
are provided with such mechanism as may be necessary to move 
them forward, as well as to secure them against the brick-work, 
when they are stationary. It is to be observed, that six alter- 
nate frames are stationary, while the five intermediate ones are 
left free for the purpose of being moved forward when required ; 
these, in their turn, are made stationary for relieving the six al- 
ternate ones, and so on. 
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 de- 
nominated cells, (See Fig. 1-2). By this disposition, 33 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 ih a wall for the pur- 
pose 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 proceed to the 
next. When he has thus gained from 3 to 6 inches over the 
whole surface (an operation which it is expected 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, 33 men may carry on 
an excavation which is 630 feet superficial area, in regular order 
and uniform quantities, 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 the rate of 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 
