The tunneling process
From An explanation of the works of the tunnel under the Thames from Rotherhithe to Wapping
Complete caption: "The divisions of the shield are advanced separately and independently of each other, by the means pointed out in the foregoing sketch: each division, as is attempted to be shewn [shown] in the annexed design, has boards in front (known by the technical name of poling boards) supported and kept in position by means of jack screws, which are lodged against the front of the iron frame; these boards are in succession taken down while the earth in front of each is excavated, the first board being always replaced before a second is removed; thus forming a constant firm buttress."
NYPL Call Number: VDCP; Digital Image ID: 1158873
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