Engineering Wonders of the World
Volume I

186 ENGINEERING WONDERS OF THE WORLD. ONE OF THE TWELVE FRAMES COMPOSING THE SHIELD.—P.S. = POLING SCREW. The outside of the two flank frames had a vertical protective casing, and between every two frames were rollers to allow a frame to be moved forward without setting up exces- sive friction against its neighbours. To sup- port the 11 working face ” of the ground, a number of “ poling boards,” 3 feet long, 6 inches wide, and 3 inches thick, rose in front of the frame. Each of these had at the ends small iron plates countersunk at the centre to give a purchase to “ poling screws,” extending from the front rail of the frame to the boards. To make the operation of the shield quite intelligible, let us suppose the frames to be all in line and the thirty-six workmen about to attack the face after an ad- vance of the shield. Each man Method releases the screws of the top Excavatjon poling board of his cell, re- moves the board, and excavates a strip of ground to the depth required. He then re- places the board, and forces it against the face by the screws. The other boards are removed and replaced in succession, working down- wards, until the whole of his particular por- tion of the face has been treated. When the time comes for an advance, alter- native frames only can be moved simulta- neously. It is impossible to force a frame through the ground with its boards and poling screws in Adv»n«ng their original position, so means Shield have been provided for sup- * porting the boards of a frame temporarily, independently of the frame. If frame 4, for example, has to be advanced, the miners re- lease the screws in succession and insert their rear ends into a second set of notches in the adjacent front rails of frames 3 and 5, and screw them hard down. The simple diagram on page 187 shows the frames, poling screws, and boards as they would appear from above after the rear ends of the screws, b &, of frame 4 have been transferred to the notches in frames 3 and 5, and frame 4 has been moved forward. The following method of advancing a frame was employed :— First, one leg of the frame was screwed up so as to raise its foot clear of the ground. The