1884.] on London (below bridge) North and South Communication. 489 



tion, and was divided into three storeys, so as to afford in the 

 whole shield working spaces for thirty-six men: in some cases it 

 appears that, under a dangerous state of things, as many as four men 

 were needed in one of these spaces, which were technically called 

 *' boxes." Each frame was in fact a girder, the top and bottom of 

 which were supported by powerful screws, abutting against the end 

 of the already completed brickwork of the tunnel, while the 

 weight was taken by a screw-leg, the foot being stepped into a shoe, 

 carried on a bottom plate, resting on 3-inch elm planks, which were 

 supported on the soil. Eadius bars were provided between the 

 frames, so that while one frame could be moved in advance of its 

 neighbours, it was prevented by the radius bars from rubbing against 

 them, and thus the friction was diminished. The tops of the frames 

 supported cast-iron plates, which at the end next the brickwork were 

 continued by wrought-iron plates, that extended over it so as to 

 exclude the earth. Plates of a generally similar construction were 

 carried by the sides of the shield. Each frame supported at its front 

 a number of horizontal boards about 6 inches deep and about 3 inches 

 thick ; these were called poling-boards ; they were latched one to 

 another, so as to make a kind of flexible wooden blind. Screws at the 

 ends of the poling-boards, supported them from their own frame, 

 except at the times when the frame was being pushed forward, and 

 then the supporting screws, abutted on the frames adjoining their own. 

 Assume, for example, the frames to be numbered from 1 on the 

 left hand to 12 on the right, and that numbers 1, 3, 5, &c., were 

 three inches more forward than numbers 2, 4, 6, &c. : the workmen 

 in these even-numbered frames, took down one poling-board in each 

 box, picked out the earth, which he found in front of it, for a distance 

 of 6 inches, then replaced the board, took down another board, and 

 repeated the operation, until the earth in front of these even-numbered 

 frames, had been excavated the 6 inches. Then (their poling-boards 

 being, as already said, supported by the screws abutting on the 

 neighbouring, the odd-numbered, frames) the even-numbered frames 

 were screwed forward the 6 inches, that had been excavated, and thus 

 became 3 inches in advance, of the odd-numbered frames. The odd- 

 numbered frames were then dealt with in the manner that had been 

 pursued with the even-numbered, and their poling-boards being sup- 

 ported from the even-numbered frames, the odd numbers were driven 

 on the 6 inches, so as once more to become the advanced frames. As 

 the shield went forward the brickwork was added to, so as to be 

 always about 9 feet behind the medium position of the face of the 

 poling-boards, and in this way the sheet-iron tailpieces of the roof 

 and sides were kept supported by the brickwork. The arches were 

 turned on narrow centerings, which travelled forward as the work 

 progressed. 



The openings in the intermediate wall between the two roadways 

 were not formed in this wall as it was built, but were cut through 

 afterwards. I have described the operations in the shield as going on 



2 K 2 



